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18.1 Understanding Evolution By the end of this section, you will be able to do the following: • Describe how scientists developed the present-day theory of evolution • Define adaptation • Explain convergent and divergent evolution • Describe homologous and vestigial structures • Discuss misconceptions about the theory of evolution Evolution by natural selection describes a mechanism for how species change over time. Scientists, philosophers, researchers, and others had made suggestions and debated this topic well before Darwin began to explore this idea. Classical Greek philosopher Plato emphasized in his writings that species were static and unchanging, yet there were also ancient Greeks who expressed evolutionary ideas. In the eighteenth century, naturalist Georges-Louis Leclerc Comte de Buffon reintroduced ideas about the evolution of animals and observed that various geographic regions have different plant and animal populations, even when the environments are similar. Some at this time also accepted that there were extinct species. Also during the eighteenth century, James Hutton, a Scottish geologist and naturalist, proposed that geological change occurred gradually by accumulating small changes from processes operating like they are today over long periods of time. This contrasted with the predominant view that the planet's geology was a consequence of catastrophic events occurring during a relatively brief past. Nineteenth century geologist Charles Lyell popularized Hutton's view. A friend to Darwin. Lyell’s ideas were influential on Darwin’s thinking: Lyell’s notion of the greater age of Earth gave more time for gradual change in species, and the process of change provided an analogy for this change. In the early nineteenth century, Jean-Baptiste Lamarck published a book that detailed a mechanism for evolutionary change. We now refer to this mechanism as an inheritance of acquired characteristics by which the environment causes modifications in an individual, or offspring could use or disuse of a structure during its lifetime, and thus bring about change in a species. While many discredited this mechanism for evolutionary change, Lamarck’s ideas were an important influence on evolutionary thought. Charles Darwin and Natural Selection In the mid-nineteenth century, two naturalists, Charles Darwin and Alfred Russel Wallace, independently conceived and described the actual mechanism for evolution. Importantly, each naturalist spent time exploring the natural world on expeditions to the tropics. From 1831 to 1836, Darwin traveled around the world on H.M.S. Beagle, including stops in South America, Australia, and the southern tip of Africa. Wallace traveled to Brazil to collect insects in the Amazon rainforest from 1848 to 1852 and to the Malay Archipelago from 1854 to 1862. Darwin’s journey, like Wallace’s later journeys to the Malay Archipelago, included stops at several island chains, the last being the Galápagos Islands west of Ecuador. On these islands, Darwin observed species of organisms on different islands that were clearly similar, yet had distinct differences. For example, the ground finches inhabiting the Galápagos Islands comprised several species with a unique beak shape (Figure 18.2). The species on the islands had a graded series of beak sizes and shapes with very small differences between the most similar. He observed that these finches closely resembled another finch species on the South American mainland. Darwin imagined that the island species might be species modified from one of the original mainland species. Upon further study, he realized that each finch's varied beaks helped the birds acquire a specific type of food. For example, seed-eating finches had stronger, thicker beaks for breaking seeds, and insect-eating finches had spear-like beaks for stabbing their prey. 444 Chapter 18 • Evolution and the Origin of Species Access for free at openstax.org. Figure 18.2 Darwin observed that beak shape varies among finch species. He postulated that ancestral species' beaks had adapted over time to equip the finches to acquire different food sources. Wallace and Darwin both observed similar patterns in other organisms and they independently developed the same explanation for how and why such changes could take place. Darwin called this mechanism natural selection. Natural selection, or “survival of the fittest,” is the more prolific reproduction of individuals with favorable traits that survive environmental change because of those traits. This leads to evolutionary change. For example, Darwin observed a population of giant tortoises in the Galápagos Archipelago to have longer necks than those that lived on other islands with dry lowlands. These tortoises were “selected” because they could reach more leaves and access more food than those with short necks. In times of drought when fewer leaves would be available, those that could reach more leaves had a better chance to eat and survive than those that couldn’t reach the food source. Consequently, long-necked tortoises would be more likely to be reproductively successful and pass the long-necked trait to their offspring. Over time, only long-necked tortoises would be present in the population. Natural selection, Darwin argued, was an inevitable outcome of three principles that operated in nature. First, most characteristics of organisms are inherited, or passed from parent to offspring. Although no one, including Darwin and Wallace, knew how this happened at the time, it was a common understanding. Second, more offspring are produced than are able to survive, so resources for survival and reproduction are limited. The capacity for reproduction in all organisms outstrips the availability of resources to support their numbers. Thus, there is competition for those resources in each generation. Both Darwin and Wallace’s understanding of this principle came from reading economist Thomas Malthus' essay that explained this principle in relation to human populations. Third, offspring vary among each other in regard to their characteristics and those variations are inherited. Darwin and Wallace reasoned that offspring with inherited characteristics which allow them to best compete for limited resources will survive and have more offspring than those individuals with variations that are less able to compete. Because characteristics are inherited, these traits will be better represented in the next generation. This will lead to change in populations over generations in a process that Darwin called descent with modification. Ultimately, natural selection leads to greater adaptation of the population to its local environment. It is the only mechanism known for adaptive evolution. In 1858, Darwin and Wallace (Figure 18.3) presented papers at the Linnean Society in London that discussed the idea of natural selection. The following year Darwin’s book, On the Origin of Species, was published. His book outlined in considerable detail his arguments for evolution by natural selection. 18.1 • Understanding Evolution 445 Figure 18.3 Both (a) Charles Darwin and (b) Alfred Wallace wrote scientific papers on natural selection that they presented together at the Linnean Society in 1858. It is difficult and time-consuming to document and present examples of evolution by natural selection. The Galápagos finches are an excellent example. Peter and Rosemary Grant and their colleagues have studied Galápagos finch populations every year since 1976 and have provided important evidence of natural selection. The Grants found changes from one generation to the next in beak shape distribution with the medium ground finch on the Galápagos island of Daphne Major. The birds have inherited a variation in their bill shape with some having wide deep bills and others having thinner bills. During a period in which rainfall was higher than normal because of an El Niño, there was a lack of large hard seeds of which the large-billed birds ate; however, there was an abundance of the small soft seeds which the small-billed birds ate. Therefore, the small-billed birds were able to survive and reproduce. In the years following this El Niño, the Grants measured beak sizes in the population and found that the average bill size was smaller. Since bill size is an inherited trait, parents with smaller bills had more offspring and the bill evolved into a much smaller size. As conditions improved in 1987 and larger seeds became more available, the trend toward smaller average bill size ceased. CAREER CONNECTION Field Biologist Many people hike, explore caves, scuba dive, or climb mountains for recreation. People often participate in these activities hoping to see wildlife. Experiencing the outdoors can be incredibly enjoyable and invigorating. What if your job entailed working in the wilderness? Field biologists by definition work outdoors in the “field.” The term field in this case refers to any location outdoors, even under water. A field biologist typically focuses research on a certain species, group of organisms, or a single habitat (Figure 18.4). 446 Chapter 18 • Evolution and the Origin of Species Access for free at openstax.org. Figure 18.4 A field biologist tranquilizes a polar bear for study. (credit: Karen Rhode) One objective of many field biologists includes discovering new, unrecorded species. Not only do such findings expand our understanding of the natural world, but they also lead to important innovations in fields such as medicine and agriculture. Plant and microbial species, in particular, can reveal new medicinal and nutritive knowledge. Other organisms can play key roles in ecosystems or if rare require protection. When discovered, researchers can use these important species as evidence for environmental regulations and laws. Processes and Patterns of Evolution Natural selection can only take place if there is variation, or differences, among individuals in a population. Importantly, these differences must have some genetic basis; otherwise, the selection will not lead to change in the next generation. This is critical because nongenetic reasons can cause variation among individuals such as an individual's height because of better nutrition rather than different genes. Genetic diversity in a population comes from two main mechanisms: mutation and sexual reproduction. Mutation, a change in DNA, is the ultimate source of new alleles, or new genetic variation in any population. The genetic changes that mutation causes can have one of three outcomes on the phenotype. A mutation affects the organism's phenotype in a way that gives it reduced fitness—lower likelihood of survival or fewer offspring. A mutation may produce a phenotype with a beneficial effect on fitness. Many mutations will also have no effect on the phenotype's fitness. We call these neutral mutations. Mutations may also have a whole range of effect sizes on the organism's fitness that expresses them in their phenotype, from a small effect to a great effect. Sexual reproduction also leads to genetic diversity: when two parents reproduce, unique combinations of alleles assemble to produce the unique genotypes and thus phenotypes in each offspring. We call a heritable trait that helps an organism's survival and reproduction in its present environment an adaptation. Scientists describe groups of organisms adapting to their environment when a genetic variation occurs over time that increases or maintains the population's “fit” to its environment. A platypus's webbed feet are an adaptation for swimming. A snow leopard's thick fur is an adaptation for living in the cold. A cheetah's fast speed is an adaptation for catching prey. Whether or not a trait is favorable depends on the current environmental conditions. The same traits are not always selected because environmental conditions can change. For example, consider a plant species that grew in a moist climate and did not need to conserve water. Large leaves were selected because they allowed the plant to obtain more energy from the sun. Large leaves require more water to maintain than small leaves, and the moist environment provided favorable conditions to support large leaves. After thousands of years, the climate changed, and the area no longer had excess water. The direction of natural selection shifted so that plants with small leaves were selected because those populations were able to conserve water to survive the new environmental conditions. The evolution of species has resulted in enormous variation in form and function. Sometimes, evolution gives rise to groups of organisms that become tremendously different from each other. We call two species that evolve in diverse directions from a 18.1 • Understanding Evolution 447 common point divergent evolution. We can see such divergent evolution in the forms of the reproductive organs of flowering plants which share the same basic anatomies; however, they can look very different as a result of selection in different physical environments and adaptation to different kinds of pollinators (Figure 18.5). Figure 18.5 Flowering plants evolved from a common ancestor. Notice that the (a) dense blazing star (Liatrus spicata) and the (b) purple coneflower (Echinacea purpurea) vary in appearance, yet both share a similar basic morphology. (credit a: modification of work by Drew Avery; credit b: modification of work by Cory Zanker) In other cases, similar phenotypes evolve independently in distantly related species. For example, flight has evolved in both bats and insects, and they both have structures we refer to as wings, which are adaptations to flight. However, bat and insect wings have evolved from very different original structures. We call this phenomenon convergent evolution, where similar traits evolve independently in species that do not share a common ancestry. The two species came to the same function, flying, but did so separately from each other. These physical changes occur over enormous time spans and help explain how evolution occurs. Natural selection acts on individual organisms, which can then shape an entire species. Although natural selection may work in a single generation on an individual, it can take thousands or even millions of years for an entire species' genotype to evolve. It is over these large time spans that life on earth has changed and continues to change. Evidence of Evolution The evidence for evolution is compelling and extensive. Looking at every level of organization in living systems, biologists see the signature of past and present evolution. Darwin dedicated a large portion of his book, On the Origin of Species, to identifying patterns in nature that were consistent with evolution, and since Darwin, our understanding has become clearer and broader. Fossils Fossils provide solid evidence that organisms from the past are not the same as those today, and fossils show a progression of evolution. Scientists determine the age of fossils and categorize them from all over the world to determine when the organisms lived relative to each other. The resulting fossil record tells the story of the past and shows the evolution of form over millions of years (Figure 18.6). For example, scientists have recovered highly detailed records showing the evolution of humans and horses (Figure 18.6). The whale flipper shares a similar morphology to bird and mammal appendages (Figure 18.7) indicating that these species share a common ancestor. 448 Chapter 18 • Evolution and the Origin of Species Access for free at openstax.org. Figure 18.6 In this (a) display, fossil hominids are arranged from oldest (bottom) to newest (top). As hominids evolved, the skull's shape changed. An artist’s rendition of (b) extinct species of the genus Equus reveals that these ancient species resembled the modern horse (Equus ferus) but varied in size. Anatomy and Embryology Another type of evidence for evolution is the presence of structures in organisms that share the same basic form. For example, the bones in human, dog, bird, and whale appendages all share the same overall construction (Figure 18.7) resulting from their origin in a common ancestor's appendages. Over time, evolution led to changes in the bones' shapes and sizes different species, but they have maintained the same overall layout. Scientists call these synonymous parts homologous structures. Figure 18.7 The similar construction of these appendages indicates that these organisms share a common ancestor. Some structures exist in organisms that have no apparent function at all, and appear to be residual parts from a past common ancestor. We call these unused structures without function vestigial structures. Other examples of vestigial structures are wings on flightless birds, leaves on some cacti, and hind leg bones in whales. Not all similarities represent homologous structures. As explained in Determining Evolutionary Relationships, when similar characteristics occur because of environmental constraints and not due to a close evolutionary relationship, it is an analogy or homoplasy. For example, insects use wings to fly like bats and birds, but the wing structure and embryonic origin are completely different. These are analogous structures (Figure 20.8). LINK TO LEARNING Watch this video (http://openstax.org/l/bone_structures) exploring the bones in the human body. Another evidence of evolution is the convergence of form in organisms that share similar environments. For example, species of unrelated animals, such as the arctic fox and ptarmigan, living in the arctic region have been selected for seasonal white phenotypes during winter to blend with the snow and ice (Figure 18.8). These similarities occur not because of common ancestry, but because of similar selection pressures—the benefits of predators not seeing them. 18.1 • Understanding Evolution 449 Figure 18.8 The white winter coat of the (a) arctic fox and the (b) ptarmigan’s plumage are adaptations to their environments. (credit a: modification of work by Keith Morehouse) Embryology, the study of the anatomy of an organism's development to its adult form, also provides evidence of relatedness between now widely divergent groups of organisms. Mutational tweaking in the embryo can have such magnified consequences in the adult that tends to conserve embryo formation. As a result, structures that are absent in some groups often appear in their embryonic forms and disappear when they reach the adult or juvenile form. For example, all vertebrate embryos, including humans, exhibit gill slits and tails at some point in their early development. These disappear in the adults of terrestrial groups but adult forms of aquatic groups such as fish and some amphibians maintain them. Great ape embryos, including humans, have a tail structure during their development that they lose when they are born. Biogeography The geographic distribution of organisms on the planet follows patterns that we can explain best by evolution in conjunction with tectonic plate movement over geological time. Broad groups that evolved before the supercontinent Pangaea broke up (about 200 million years ago) are distributed worldwide. Groups that evolved since the breakup appear uniquely in regions of the planet, such as the unique flora and fauna of northern continents that formed from the supercontinent Laurasia and of the southern continents that formed from the supercontinent Gondwana. The presence of members of the plant family Proteaceae in Australia, southern Africa, and South America was most predominant prior to the southern supercontinent Gondwana breaking up. Marsupial diversification in Australia and the absence of other mammals reflect Australia’s long isolation. Australia has an abundance of endemic species—species found nowhere else—which is typical of islands whose isolation by expanses of water prevents species to migrate. Over time, these species diverge evolutionarily into new species that look very different from their ancestors that may exist on the mainland. Australia's marsupials, the Galápagos' finches, and many species on the Hawaiian Islands are all unique to their one point of origin, yet they display distant relationships to ancestral species on mainlands. Molecular Biology Like anatomical structures, the molecular structures of life reflect descent with modification. DNA's universality reflects evidence of a common ancestor for all of life. Fundamental divisions in life between the genetic code, DNA replication, and expression are reflected in major structural differences in otherwise conservative structures such as ribosome components and membrane structures. In general, the relatedness of groups of organisms is reflected in the similarity of their DNA sequences—exactly the pattern that we would expect from descent and diversification from a common ancestor. DNA sequences have also shed light on some of the mechanisms of evolution. For example, it is clear that the evolution of new functions for proteins commonly occurs after gene duplication events that allow freely modifying one copy by mutation, selection, or drift (changes in a population’s gene pool resulting from chance), while the second copy continues to produce a functional protein. Misconceptions of Evolution Although the theory of evolution generated some controversy when Darwin first proposed it, biologists almost universally 450 Chapter 18 • Evolution and the Origin of Species Access for free at openstax.org. accepted it, particularly younger biologists, within 20 years after publication of On the Origin of Species. Nevertheless, the theory of evolution is a difficult concept and misconceptions about how it works abound. LINK TO LEARNING This site (http://openstax.org/l/misconceptions) addresses some of the main misconceptions associated with the theory of evolution. Evolution Is Just a Theory Critics of the theory of evolution dismiss its importance by purposefully confounding the everyday usage of the word “theory” with the way scientists use the word. In science, we understand a “theory” to be a body of thoroughly tested and verified explanations for a set of observations of the natural world. Scientists have a theory of the atom, a theory of gravity, and the theory of relativity, each which describes understood facts about the world. In the same way, the theory of evolution describes facts about the living world. As such, a theory in science has survived significant efforts to discredit it by scientists. In contrast, a “theory” in common vernacular is a word meaning a guess or suggested explanation. This meaning is more akin to the scientific concept of “hypothesis.” When critics of evolution say it is “just a theory,” they are implying that there is little evidence supporting it and that it is still in the process of rigorous testing. This is a mischaracterization. Individuals Evolve Evolution is the change in a population's genetic composition over time, specifically over generations, resulting from differential reproduction of individuals with certain alleles. Individuals do change over their lifetime, obviously, but this is development and involves changes programmed by the set of genes the individual acquired at birth in coordination with the individual’s environment. When thinking about the evolution of a characteristic, it is probably best to think about the change of the average value of the characteristic in the population over time. For example, when natural selection leads to bill-size change in medium ground finches in the Galápagos, this does not mean that individual bills on the finches are changing. If one measures the average bill size among all individuals in the population at one time and then measures them in the population several years later, this average value will be different as a result of evolution. Although some individuals may survive from the first time to the second, they will still have the same bill size; however, there will be many new individuals who contribute to the shift in average bill size. Evolution Explains the Origin of Life It is a common misunderstanding that evolution includes an explanation of life’s origins. Conversely, some of the theory’s critics believe that it cannot explain the origin of life. The theory does not try to explain the origin of life. The theory of evolution explains how populations change over time and how life diversifies the origin of species. It does not shed light on the beginnings of life including the origins of the first cells, which define life. Importantly, biologists believe that the presence of life on Earth precludes the possibility that the events that led to life on Earth can repeat themselves because the intermediate stages would immediately become food for existing living things. However, once a mechanism of inheritance was in place in the form of a molecule like DNA either within a cell or pre-cell, these entities would be subject to the principle of natural selection. More effective reproducers would increase in frequency at the expense of inefficient reproducers. While evolution does not explain the origin of life, it may have something to say about some of the processes operating once pre-living entities acquired certain properties. Organisms Evolve on Purpose Statements such as “organisms evolve in response to a change in an environment” are quite common, but such statements can lead to two types of misunderstandings. First, do not interpret the statement to mean that individual organisms evolve. The statement is shorthand for “a population evolves in response to a changing environment.” However, a second misunderstanding may arise by interpreting the statement to mean that the evolution is somehow intentional. A changed environment results in some individuals in the population, those with particular phenotypes, benefiting and therefore producing proportionately more offspring than other phenotypes. This results in change in the population if the characteristics are genetically determined. It is also important to understand that the variation that natural selection works on is already in a population and does not arise in response to an environmental change. For example, applying antibiotics to a population of bacteria will, over time, select a population of bacteria that are resistant to antibiotics. The resistance, which a gene causes, did not arise by mutation because of applying the antibiotic. The gene for resistance was already present in the bacteria's gene pool, likely at a low frequency. The antibiotic, which kills the bacterial cells without the resistance gene, strongly selects individuals that are resistant, since these 18.1 • Understanding Evolution 451 would be the only ones that survived and divided. Experiments have demonstrated that mutations for antibiotic resistance do not arise as a result of antibiotic. In a larger sense, evolution is not goal directed. Species do not become “better” over time. They simply track their changing environment with adaptations that maximize their reproduction in a particular environment at a particular time. Evolution has no goal of making faster, bigger, more complex, or even smarter species, despite the commonness of this kind of language in popular discourse. What characteristics evolve in a species are a function of the variation present and the environment, both of which are constantly changing in a nondirectional way. A trait that fits in one environment at one time may well be fatal at some point in the future. This holds equally well for insect and human species. 18.2 Formation of New Species By the end of this section, you will be able to do the following: • Define species and describe how scientists identify species as different • Describe genetic variables that lead to speciation • Identify prezygotic and postzygotic reproductive barriers • Explain allopatric and sympatric speciation • Describe adaptive radiation Although all life on earth shares various genetic similarities, only certain organisms combine genetic information by sexual reproduction and have offspring that can then successfully reproduce. Scientists call such organisms members of the same biological species. Species and the Ability to Reproduce A species is a group of individual organisms that interbreed and produce fertile, viable offspring. According to this definition, one species is distinguished from another when, in nature, it is not possible for matings between individuals from each species to produce fertile offspring. Members of the same species share both external and internal characteristics, which develop from their DNA. The closer relationship two organisms share, the more DNA they have in common, just like people and their families. People’s DNA is likely to be more like their father or mother’s DNA than their cousin or grandparent’s DNA. Organisms of the same species have the highest level of DNA alignment and therefore share characteristics and behaviors that lead to successful reproduction. Species’ appearance can be misleading in suggesting an ability or inability to mate. For example, even though domestic dogs (Canis lupus familiaris) display phenotypic differences, such as size, build, and coat, most dogs can interbreed and produce viable puppies that can mature and sexually reproduce (Figure 18.9). Figure 18.9 The (a) poodle and (b) cocker spaniel can reproduce to produce a breed known as (c) the cockapoo. (credit a: modification of work by Sally Eller, Tom Reese; credit b: modification of work by Jeremy McWilliams; credit c: modification of work by Kathleen Conklin) In other cases, individuals may appear similar although they are not members of the same species. For example, even though bald eagles (Haliaeetus leucocephalus) and African fish eagles (Haliaeetus vocifer) are both birds and eagles, each belongs to a separate species group (Figure 18.10). If humans were to artificially intervene and fertilize a bald eagle's egg with an African fish eagle's sperm and a chick did hatch, that offspring, called a hybrid (a cross between two species), would probably be infertile—unable to successfully reproduce after it reached maturity. Different species may have different genes that are active in development; therefore, it may not be possible to develop a viable offspring with two different sets of directions. Thus, even 452 Chapter 18 • Evolution and the Origin of Species Access for free at openstax.org. though hybridization may take place, the two species still remain separate. Figure 18.10 The (a) African fish eagle is similar in appearance to the (b) bald eagle, but the two birds are members of different species. (credit a: modification of work by Nigel Wedge; credit b: modification of work by U.S. Fish and Wildlife Service) Populations of species share a gene pool: a collection of all the gene variants in the species. Again, the basis to any changes in a group or population of organisms must be genetic for this is the only way to share and pass on traits. When variations occur within a species, they can only pass to the next generation along two main pathways: asexual reproduction or sexual reproduction. The change will pass on asexually simply if the reproducing cell possesses the changed trait. For the changed trait to pass on by sexual reproduction, a gamete, such as a sperm or egg cell, must possess the changed trait. In other words, sexually-reproducing organisms can experience several genetic changes in their body cells, but if these changes do not occur in a sperm or egg cell, the changed trait will never reach the next generation. Only heritable traits can evolve. Therefore, reproduction plays a paramount role for genetic change to take root in a population or species. In short, organisms must be able to reproduce with each other to pass new traits to offspring. Speciation The biological definition of species, which works for sexually reproducing organisms, is a group of actual or potential interbreeding individuals. There are exceptions to this rule. Many species are similar enough that hybrid offspring are possible and may often occur in nature, but for the majority of species this rule generally holds. The presence in nature of hybrids between similar species suggests that they may have descended from a single interbreeding species, and the speciation process may not yet be completed. Given the extraordinary diversity of life on the planet there must be mechanisms for speciation: the formation of two species from one original species. Darwin envisioned this process as a branching event and diagrammed the process in the only illustration in On the Origin of Species (Figure 18.11a). Compare this illustration to the diagram of elephant evolution (Figure 18.11), which shows that as one species changes over time, it branches to form more than one new species, repeatedly, as long as the population survives or until the organism becomes extinct. Figure 18.11 The only illustration in Darwin's On the Origin of Species is (a) a diagram showing speciation events leading to biological diversity. The diagram shows similarities to phylogenetic charts that today illustrate the relationships of species. (b) Modern elephants 18.2 • Formation of New Species 453 evolved from the Palaeomastodon, a species that lived in Egypt 35–50 million years ago. For speciation to occur, two new populations must form from one original population and they must evolve in such a way that it becomes impossible for individuals from the two new populations to interbreed. Biologists have proposed mechanisms by which this could occur that fall into two broad categories. Allopatric speciation (allo- = "other"; -patric = "homeland") involves geographic separation of populations from a parent species and subsequent evolution. Sympatric speciation (sym- = "same"; -patric = "homeland") involves speciation occurring within a parent species remaining in one location. Biologists think of speciation events as the splitting of one ancestral species into two descendant species. There is no reason why more than two species might not form at one time except that it is less likely and we can conceptualize multiple events as single splits occurring close in time. Allopatric Speciation A geographically continuous population has a gene pool that is relatively homogeneous. Gene flow, the movement of alleles across a species' range, is relatively free because individuals can move and then mate with individuals in their new location. Thus, an allele's frequency at one end of a distribution will be similar to the allele's frequency at the other end. When populations become geographically discontinuous, it prevents alleles' free-flow. When that separation lasts for a period of time, the two populations are able to evolve along different trajectories. Thus, their allele frequencies at numerous genetic loci gradually become increasingly different as new alleles independently arise by mutation in each population. Typically, environmental conditions, such as climate, resources, predators, and competitors for the two populations will differ causing natural selection to favor divergent adaptations in each group. Isolation of populations leading to allopatric speciation can occur in a variety of ways: a river forming a new branch, erosion creating a new valley, a group of organisms traveling to a new location without the ability to return, or seeds floating over the ocean to an island. The nature of the geographic separation necessary to isolate populations depends entirely on the organism's biology and its potential for dispersal. If two flying insect populations took up residence in separate nearby valleys, chances are, individuals from each population would fly back and forth continuing gene flow. However, if a new lake divided two rodent populations continued gene flow would be unlikely; therefore, speciation would be more likely. Biologists group allopatric processes into two categories: dispersal and vicariance. Dispersal is when a few members of a species move to a new geographical area, and vicariance is when a natural situation arises to physically divide organisms. Scientists have documented numerous cases of allopatric speciation taking place. For example, along the west coast of the United States, two separate spotted owl subspecies exist. The northern spotted owl has genetic and phenotypic differences from its close relative: the Mexican spotted owl, which lives in the south (Figure 18.12). 454 Chapter 18 • Evolution and the Origin of Species Access for free at openstax.org. Figure 18.12 The northern spotted owl and the Mexican spotted owl inhabit geographically separate locations with different climates and ecosystems. The owl is an example of allopatric speciation. (credit "northern spotted owl": modification of work by John and Karen Hollingsworth; credit "Mexican spotted owl": modification of work by Bill Radke) Additionally, scientists have found that the further the distance between two groups that once were the same species, the more likely it is that speciation will occur. This seems logical because as the distance increases, the various environmental factors would likely have less in common than locations in close proximity. Consider the two owls: in the north, the climate is cooler than in the south. The types of organisms in each ecosystem differ, as do their behaviors and habits. Also, the hunting habits and prey choices of the southern owls vary from the northern owls. These variances can lead to evolved differences in the owls, and speciation likely will occur. Adaptive Radiation In some cases, a population of one species disperses throughout an area, and each finds a distinct niche or isolated habitat. Over time, the varied demands of their new lifestyles lead to multiple speciation events originating from a single species. We call this adaptive radiation because many adaptations evolve from a single point of origin; thus, causing the species to radiate into several new ones. Island archipelagos like the Hawaiian Islands provide an ideal context for adaptive radiation events because water surrounds each island which leads to geographical isolation for many organisms. The Hawaiian honeycreeper illustrates one example of adaptive radiation. From a single species, the founder species, numerous species have evolved, including the six in Figure 18.13. 18.2 • Formation of New Species 455 Figure 18.13 The honeycreeper birds illustrate adaptive radiation. From one original species of bird, multiple others evolved, each with its own distinctive characteristics. Notice the differences in the species’ beaks in Figure 18.13. Evolution in response to natural selection based on specific food sources in each new habitat led to evolution of a different beak suited to the specific food source. The seed-eating bird has a thicker, stronger beak which is suited to break hard nuts. The nectar-eating birds have long beaks to dip into flowers to reach the nectar. The insect-eating birds have beaks like swords, appropriate for stabbing and impaling insects. Darwin’s finches are another example of adaptive radiation in an archipelago. LINK TO LEARNING Watch this video (http://openstax.org/l/bird_evolution) to see how scientists use evidence to understand how birds evolved. Sympatric Speciation Can divergence occur if no physical barriers are in place to separate individuals who continue to live and reproduce in the same habitat? The answer is yes. We call the process of speciation within the same space sympatric. The prefix “sym” means same, so “sympatric” means “same homeland” in contrast to “allopatric” meaning “other homeland.” Scientists have proposed and studied many mechanisms. One form of sympatric speciation can begin with a serious chromosomal error during cell division. In a normal cell division event chromosomes replicate, pair up, and then separate so that each new cell has the same number of chromosomes. However, sometimes the pairs separate and the end cell product has too many or too few individual chromosomes in a condition that we call aneuploidy (Figure 18.14). 456 Chapter 18 • Evolution and the Origin of Species Access for free at openstax.org. VISUAL CONNECTION Figure 18.14 Aneuploidy results when the gametes have too many or too few chromosomes due to nondisjunction during meiosis. In this example, the resulting offspring will have 2n+1 or 2n-1 chromosomes. Which is most likely to survive, offspring with 2n+1 chromosomes or offspring with 2n-1 chromosomes? Polyploidy is a condition in which a cell or organism has an extra set, or sets, of chromosomes. Scientists have identified two main types of polyploidy that can lead to reproductive isolation of an individual in the polyploidy state. Reproductive isolation is the inability to interbreed. In some cases, a polyploid individual will have two or more complete sets of chromosomes from its own species in a condition that we call autopolyploidy (Figure 18.15). The prefix “auto-” means “self,” so the term means multiple chromosomes from one’s own species. Polyploidy results from an error in meiosis in which all of the chromosomes move into one cell instead of separating. Figure 18.15 Autopolyploidy results when mitosis is not followed by cytokinesis. For example, if a plant species with 2n = 6 produces autopolyploid gametes that are also diploid (2n = 6, when they should be n = 3), the gametes now have twice as many chromosomes as they should have. These new gametes will be incompatible with the normal gametes that this plant species produces. However, they could either self-pollinate or reproduce with other autopolyploid plants with gametes having the same diploid number. In this way, sympatric speciation can occur quickly by forming offspring with 4n that we call a tetraploid. These individuals would immediately be able to reproduce only with those of this new kind and not those of the ancestral species. The other form of polyploidy occurs when individuals of two different species reproduce to form a viable offspring that we call an allopolyploid. The prefix “allo-” means “other” (recall from allopatric): therefore, an allopolyploid occurs when gametes from two different species combine. Figure 18.16 illustrates one possible way an allopolyploid can form. Notice how it takes two 18.2 • Formation of New Species 457 generations, or two reproductive acts, before the viable fertile hybrid results. Figure 18.16 Alloploidy results when two species mate to produce viable offspring. In this example, a normal gamete from one species fuses with a polyploidy gamete from another. Two matings are necessary to produce viable offspring. The cultivated forms of wheat, cotton, and tobacco plants are all allopolyploids. Although polyploidy occurs occasionally in animals, it takes place most commonly in plants. (Animals with any of the types of chromosomal aberrations that we describe here are unlikely to survive and produce normal offspring.) Scientists have discovered more than half of all plant species studied relate back to a species evolved through polyploidy. With such a high rate of polyploidy in plants, some scientists hypothesize that this mechanism takes place more as an adaptation than as an error. Reproductive Isolation Given enough time, the genetic and phenotypic divergence between populations will affect characters that influence reproduction: if individuals of the two populations were brought together, mating would be less likely, but if mating occurred, offspring would be nonviable or infertile. Many types of diverging characters may affect the reproductive isolation, the ability to interbreed, of the two populations. Reproductive isolation can take place in a variety of ways. Scientists organize them into two groups: prezygotic barriers and postzygotic barriers. Recall that a zygote is a fertilized egg: the first cell of an organism's development that reproduces sexually. Therefore, a prezygotic barrier is a mechanism that blocks reproduction from taking place. This includes barriers that prevent fertilization when organisms attempt reproduction. A postzygotic barrier occurs after zygote formation. This includes organisms that don’t survive the embryonic stage and those that are born sterile. Some types of prezygotic barriers prevent reproduction entirely. Many organisms only reproduce at certain times of the year, often just annually. Differences in breeding schedules, which we call temporal isolation, can act as a form of reproductive isolation. For example, two frog species inhabit the same area, but one reproduces from January to March; whereas, the other reproduces from March to May (Figure 18.17). Figure 18.17 These two related frog species exhibit temporal reproductive isolation. (a) Rana aurora breeds earlier in the year than (b) Rana boylii. (credit a: modification of work by Mark R. Jennings, USFWS; credit b: modification of work by Alessandro Catenazzi) 458 Chapter 18 • Evolution and the Origin of Species Access for free at openstax.org. In some cases, populations of a species move or are moved to a new habitat and take up residence in a place that no longer overlaps with the same species' other populations. We call this situation habitat isolation. Reproduction with the parent species ceases, and a new group exists that is now reproductively and genetically independent. For example, a cricket population that was divided after a flood could no longer interact with each other. Over time, natural selection forces, mutation, and genetic drift will likely result in the two groups diverging (Figure 18.18). Figure 18.18 Speciation can occur when two populations occupy different habitats. The habitats need not be far apart. The cricket (a) Gryllus pennsylvanicus prefers sandy soil, and the cricket (b) Gryllus firmus prefers loamy soil. The two species can live in close proximity, but because of their different soil preferences, they became genetically isolated. Behavioral isolation occurs when the presence or absence of a specific behavior prevents reproduction. For example, male fireflies use specific light patterns to attract females. Various firefly species display their lights differently. If a male of one species tried to attract the female of another, she would not recognize the light pattern and would not mate with the male. Other prezygotic barriers work when differences in their gamete cells (eggs and sperm) prevent fertilization from taking place. We call this a gametic barrier. Similarly, in some cases closely related organisms try to mate, but their reproductive structures simply do not fit together. For example, damselfly males of different species have differently shaped reproductive organs. If one species tries to mate with the female of another, their body parts simply do not fit together. (Figure 18.19). Figure 18.19 The shape of the male reproductive organ varies among male damselfly species, and is only compatible with the female of that species. Reproductive organ incompatibility keeps the species reproductively isolated. In plants, certain structures aimed to attract one type of pollinator simultaneously prevent a different pollinator from accessing the pollen. The tunnel through which an animal must access nectar can vary widely in length and diameter, which prevents the plant from cross-pollinating with a different species (Figure 18.20). Figure 18.20 Some flowers have evolved to attract certain pollinators. The (a) wide foxglove flower is adapted for pollination by bees, while the (b) long, tube-shaped trumpet creeper flower is adapted for pollination by hummingbirds. 18.2 • Formation of New Species 459 When fertilization takes place and a zygote forms, postzygotic barriers can prevent reproduction. Hybrid individuals in many cases cannot form normally in the womb and simply do not survive past the embryonic stages. We call this hybrid inviability because the hybrid organisms simply are not viable. In another postzygotic situation, reproduction leads to hybrid birth and growth that is sterile. Therefore, the organisms are unable to reproduce offspring of their own. We call this hybrid sterility. Habitat Influence on Speciation Sympatric speciation may also take place in ways other than polyploidy. For example, consider a fish species that lives in a lake. As the population grows, competition for food increases. Under pressure to find food, suppose that a group of these fish had the genetic flexibility to discover and feed off another resource that other fish did not use. What if this new food source was located at a different depth of the lake? Over time, those feeding on the second food source would interact more with each other than the other fish; therefore, they would breed together as well. Offspring of these fish would likely behave as their parents: feeding and living in the same area and keeping separate from the original population. If this group of fish continued to remain separate from the first population, eventually sympatric speciation might occur as more genetic differences accumulated between them. This scenario does play out in nature, as do others that lead to reproductive isolation. One such place is Lake Victoria in Africa, famous for its sympatric speciation of cichlid fish. Researchers have found hundreds of sympatric speciation events in these fish, which have not only happened in great number, but also over a short period of time. Figure 18.21 shows this type of speciation among a cichlid fish population in Nicaragua. In this locale, two types of cichlids live in the same geographic location but have come to have different morphologies that allow them to eat various food sources. Figure 18.21 Cichlid fish from Lake Apoyeque, Nicaragua, show evidence of sympatric speciation. Lake Apoyeque, a crater lake, is 1800 years old, but genetic evidence indicates that a single population of cichlid fish populated the lake only 100 years ago. Nevertheless, two populations with distinct morphologies and diets now exist in the lake, and scientists believe these populations may be in an early stage of speciation.

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19.2 Population Genetics By the end of this section, you will be able to do the following: • Describe the different types of variation in a population • Explain why only natural selection can act upon heritable variation • Describe genetic drift and the bottleneck effect • Explain how each evolutionary force can influence a population's allele frequencies A population's individuals often display different phenotypes, or express different alleles of a particular gene, which scientists refer to as polymorphisms. We call populations with two or more variations of particular characteristics polymorphic. A number of factors, including the population’s genetic structure and the environment (Figure 19.3) influence population variation, the distribution of phenotypes among individuals. Understanding phenotypic variation sources in a population is important for determining how a population will evolve in response to different evolutionary pressures. Figure 19.3 The distribution of phenotypes in this litter of kittens illustrates population variation. (credit: Pieter Lanser) Genetic Variance Natural selection and some of the other evolutionary forces can only act on heritable traits, namely an organism’s genetic code. Because alleles are passed from parent to offspring, those that confer beneficial traits or behaviors may be selected, while deleterious alleles may not. Acquired traits, for the most part, are not heritable. For example, if an athlete works out in the gym every day, building up muscle strength, the athlete’s offspring will not necessarily grow up to be a body builder. If there is a genetic basis for the ability to run fast, on the other hand, a parent may pass this to a child. LINK TO LEARNING Before Darwinian evolution became the prevailing theory of the field, French naturalist Jean-Baptiste Lamarck theorized that organisms could inherit acquired traits. While the majority of scientists have not supported this hypothesis, some have recently begun to realize that Lamarck was not completely wrong. Visit this site (http://openstax.org/l/epigenetic) to learn more. Heritability is the fraction of phenotype variation that we can attribute to genetic differences, or genetic variance, among individuals in a population. The greater the heritability of a population’s phenotypic variation, the more susceptible it is to the evolutionary forces that act on heritable variation. We call the diversity of alleles and genotypes within a population genetic variance. When scientists are involved in the breeding of a species, such as with animals in zoos and nature preserves, they try to increase a population’s genetic variance to preserve as much of the phenotypic diversity as possible. This also helps reduce associated risks of inbreeding, the mating of closely related individuals, which can have the undesirable effect of bringing together deleterious recessive mutations that can cause 19.2 • Population Genetics 471 abnormalities and susceptibility to disease. For example, a disease that is caused by a rare, recessive allele might exist in a population, but it will only manifest itself when an individual carries two copies of the allele. Because the allele is rare in a normal, healthy population with unrestricted habitat, the chance that two carriers will mate is low, and even then, only 25 percent of their offspring will inherit the disease allele from both parents. While it is likely to happen at some point, it will not happen frequently enough for natural selection to be able to swiftly eliminate the allele from the population, and as a result, the allele maintains itself at low levels in the gene pool. However, if a family of carriers begins to interbreed with each other, this will dramatically increase the likelihood of two carriers mating and eventually producing diseased offspring, a phenomenon that scientists call inbreeding depression. Changes in allele frequencies that we identify in a population can shed light on how it is evolving. In addition to natural selection, there are other evolutionary forces that could be in play: genetic drift, gene flow, mutation, nonrandom mating, and environmental variances. Genetic Drift The theory of natural selection stems from the observation that some individuals in a population are more likely to survive longer and have more offspring than others; thus, they will pass on more of their genes to the next generation. A big, powerful male gorilla, for example, is much more likely than a smaller, weaker one to become the population’s silverback, the pack’s leader who mates far more than the other males of the group. The pack leader will father more offspring, who share half of his genes, and are likely to also grow bigger and stronger like their father. Over time, the genes for bigger size will increase in frequency in the population, and the population will, as a result, grow larger on average. That is, this would occur if this particular selection pressure, or driving selective force, were the only one acting on the population. In other examples, better camouflage or a stronger resistance to drought might pose a selection pressure. Another way a population’s allele and genotype frequencies can change is genetic drift (Figure 19.4), which is simply the effect of chance. By chance, some individuals will have more offspring than others—not due to an advantage conferred by some genetically-encoded trait, but just because one male happened to be in the right place at the right time (when the receptive female walked by) or because the other one happened to be in the wrong place at the wrong time (when a fox was hunting). 472 Chapter 19 • The Evolution of Populations Access for free at openstax.org. VISUAL CONNECTION Figure 19.4 Genetic drift in a population can lead to eliminating an allele from a population by chance. In this example, rabbits with the brown coat color allele (B) are dominant over rabbits with the white coat color allele (b). In the first generation, the two alleles occur with equal frequency in the population, resulting in p and q values of .5. Only half of the individuals reproduce, resulting in a second generation with p and q values of .7 and .3, respectively. Only two individuals in the second generation reproduce, and by chance these individuals are homozygous dominant for brown coat color. As a result, in the third generation the recessive b allele is lost. Do you think genetic drift would happen more quickly on an island or on the mainland? Small populations are more susceptible to the forces of genetic drift. Large populations, alternatively, are buffered against the effects of chance. If one individual of a population of 10 individuals happens to die at a young age before it leaves any offspring to the next generation, all of its genes—1/10 of the population’s gene pool—will be suddenly lost. In a population of 100, that’s 19.2 • Population Genetics 473 only 1 percent of the overall gene pool; therefore, it is much less impactful on the population’s genetic structure. LINK TO LEARNING Go to this site (http://openstax.org/l/genetic_drift) to watch an animation of random sampling and genetic drift in action. Natural events, such as an earthquake disaster that kills—at random—a large portion of the population, can magnify genetic drift. Known as the bottleneck effect, it results in suddenly wiping out a large portion of the genome (Figure 19.5). At once, the survivors' genetic structure becomes the entire population's genetic structure, which may be very different from the pre-disaster population. Figure 19.5 A chance event or catastrophe can reduce the genetic variability within a population. Another scenario in which populations might experience a strong influence of genetic drift is if some portion of the population leaves to start a new population in a new location or if a physical barrier divides a population. In this situation, those individuals are an unlikely representation of the entire population, which results in the founder effect. The founder effect occurs when the genetic structure changes to match that of the new population’s founding fathers and mothers. Researchers believe that the founder effect was a key factor in the genetic history of the Afrikaner population of Dutch settlers in South Africa, as evidenced by mutations that are common in Afrikaners but rare in most other populations. This is probably because a higher-than-normal proportion of the founding colonists carried these mutations. As a result, the population expresses unusually high incidences of Huntington’s disease (HD) and Fanconi anemia (FA), a genetic disorder known to cause blood marrow and congenital abnormalities—even cancer. 2 LINK TO LEARNING Watch this short video to learn more about the founder and bottleneck effects. Click to view content (https://www.openstax.org/l/founder_bottle) SCIENTIFIC METHOD CONNECTION Testing the Bottleneck Effect Question: How do natural disasters affect a population's genetic structure? 2A. J. Tipping et al., “Molecular and Genealogical Evidence for a Founder Effect in Fanconi Anemia Families of the Afrikaner Population of South Africa,” PNAS 98, no. 10 (2001): 5734-5739, doi: 10.1073/pnas.091402398. 474 Chapter 19 • The Evolution of Populations Access for free at openstax.org. Background: When an earthquake or hurricane suddenly wipes out much of a population, the surviving individuals are usually a random sampling of the original group. As a result, the population's genetic makeup can change dramatically. We call this phenomenon the bottleneck effect. Hypothesis: Repeated natural disasters will yield different population genetic structures; therefore, each time one runs this experiment the results will vary. Test the hypothesis: Count out the original population using different colored beads. For example, red, blue, and yellow beads might represent red, blue, and yellow individuals. After recording the number of each individual in the original population, place them all in a bottle with a narrow neck that will only allow a few beads out at a time. Then, pour 1/3 of the bottle’s contents into a bowl. This represents the surviving individuals after a natural disaster kills a majority of the population. Count the number of the different colored beads in the bowl, and record it. Then, place all of the beads back in the bottle and repeat the experiment four more times. Analyze the data: Compare the five populations that resulted from the experiment. Do the populations all contain the same number of different colored beads, or do they vary? Remember, these populations all came from the same exact parent population. Form a conclusion: Most likely, the five resulting populations will differ quite dramatically. This is because natural disasters are not selective—they kill and spare individuals at random. Now think about how this might affect a real population. What happens when a hurricane hits the Mississippi Gulf Coast? How do the seabirds that live on the beach fare? Gene Flow Another important evolutionary force is gene flow: the flow of alleles in and out of a population due to the migration of individuals or gametes (Figure 19.6). While some populations are fairly stable, others experience more flux. Many plants, for example, send their pollen far and wide, by wind or by bird, to pollinate other populations of the same species some distance away. Even a population that may initially appear to be stable, such as a pride of lions, can experience its fair share of immigration and emigration as developing males leave their mothers to seek out a new pride with genetically unrelated females. This variable flow of individuals in and out of the group not only changes the population's gene structure, but it can also introduce new genetic variation to populations in different geological locations and habitats. Figure 19.6 Gene flow can occur when an individual travels from one geographic location to another. Mutation Mutations are changes to an organism’s DNA and are an important driver of diversity in populations. Species evolve because of mutations accumulating over time. The appearance of new mutations is the most common way to introduce novel genotypic and phenotypic variance. Some mutations are unfavorable or harmful and are quickly eliminated from the population by natural selection. Others are beneficial and will spread through the population. Whether or not a mutation is beneficial or harmful is determined by whether it helps an organism survive to sexual maturity and reproduce. Some mutations do not do anything and can linger, unaffected by natural selection, in the genome. Some can have a dramatic effect on a gene and the resulting phenotype. 19.2 • Population Genetics 475 Nonrandom Mating If individuals nonrandomly mate with their peers, the result can be a changing population. There are many reasons nonrandom mating occurs. One reason is simple mate choice. For example, female peahens may prefer peacocks with bigger, brighter tails. Natural selection picks traits that lead to more mating selections for an individual. One common form of mate choice, called assortative mating, is an individual’s preference to mate with partners who are phenotypically similar to themselves. Another cause of nonrandom mating is physical location. This is especially true in large populations spread over vast geographic distances where not all individuals will have equal access to one another. Some might be miles apart through woods or over rough terrain, while others might live immediately nearby. Environmental Variance Genes are not the only players involved in determining population variation. Other factors, such as the environment (Figure 19.7) also influence phenotypes. A beachgoer is likely to have darker skin than a city dweller, for example, due to regular exposure to the sun, an environmental factor. For some species, the environment determines some major characteristics, such as gender. For example, some turtles and other reptiles have temperature-dependent sex determination (TSD). TSD means that individuals develop into males if their eggs are incubated within a certain temperature range, or females at a different temperature range. Figure 19.7 The temperature at which the eggs are incubated determine the American alligator's (Alligator mississippiensis) sex. Eggs incubated at 30°C produce females, and eggs incubated at 33°C produce males. (credit: Steve Hillebrand, USFWS) Geographic separation between populations can lead to differences in the phenotypic variation between those populations. We see such geographical variation between most populations and it can be significant. We can observe one type of geographic variation, a cline, as given species' populations vary gradually across an ecological gradient. Species of warm-blooded animals, for example, tend to have larger bodies in the cooler climates closer to the earth’s poles, allowing them to better conserve heat. This is a latitudinal cline. Alternatively, flowering plants tend to bloom at different times depending on where they are along a mountain slope. This is an altitudinal cline. If there is gene flow between the populations, the individuals will likely show gradual differences in phenotype along the cline. Restricted gene flow, alternatively can lead to abrupt differences, even speciation.

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20.1 Organizing Life on Earth By the end of this section, you will be able to do the following: • Discuss the need for a comprehensive classification system • List the different levels of the taxonomic classification system • Describe how systematics and taxonomy relate to phylogeny • Discuss a phylogenetic tree's components and purpose In scientific terms, phylogeny is the evolutionary history and relationship of an organism or group of organisms. A phylogeny describes the organisim's relationships, such as from which organisms it may have evolved, or to which species it is most closely related. Phylogenetic relationships provide information on shared ancestry but not necessarily on how organisms are similar or different. Phylogenetic Trees Scientists use a tool called a phylogenetic tree to show the evolutionary pathways and connections among organisms. A phylogenetic tree is a diagram used to reflect evolutionary relationships among organisms or groups of organisms. Scientists consider phylogenetic trees to be a hypothesis of the evolutionary past since one cannot go back to confirm the proposed relationships. In other words, we can construct a “tree of life” to illustrate when different organisms evolved and to show the relationships among different organisms (Figure 20.2). Unlike a taxonomic classification diagram, we can read a phylogenetic tree like a map of evolutionary history. Many phylogenetic trees have a single lineage at the base representing a common ancestor. Scientists call such trees rooted, which means there is a single ancestral lineage (typically drawn from the bottom or left) to which all organisms represented in the diagram relate. Notice in the rooted phylogenetic tree that the three domains— Bacteria, Archaea, and Eukarya—diverge from a single point and branch off. The small branch that plants and animals (including humans) occupy in this diagram shows how recent and miniscule these groups are compared with other organisms. Unrooted trees do not show a common ancestor but do show relationships among species. Figure 20.2 Both of these phylogenetic trees show the relationship of the three domains of life—Bacteria, Archaea, and Eukarya—but the (a) rooted tree attempts to identify when various species diverged from a common ancestor while the (b) unrooted tree does not. (credit a: modification of work by Eric Gaba) In a rooted tree, the branching indicates evolutionary relationships (Figure 20.3). The point where a split occurs, a branch point, represents where a single lineage evolved into a distinct new one. We call a lineage that evolved early from the root that remains unbranched a basal taxon. We call two lineages stemming from the same branch point sister taxa. A branch with more than two lineages is a polytomy and serves to illustrate where scientists have not definitively determined all of the relationships. Note that although sister taxa and polytomy do share an ancestor, it does not mean that the groups of organisms split or evolved from each other. Organisms in two taxa may have split at a specific branch point, but neither taxon gave rise to the other. 486 Chapter 20 • Phylogenies and the History of Life Access for free at openstax.org. Figure 20.3 A phylogenetic tree's root indicates that an ancestral lineage gave rise to all organisms on the tree. A branch point indicates where two lineages diverged. A lineage that evolved early and remains unbranched is a basal taxon. When two lineages stem from the same branch point, they are sister taxa. A branch with more than two lineages is a polytomy. The diagrams above can serve as a pathway to understanding evolutionary history. We can trace the pathway from the origin of life to any individual species by navigating through the evolutionary branches between the two points. Also, by starting with a single species and tracing back towards the "trunk" of the tree, one can discover species' ancestors, as well as where lineages share a common ancestry. In addition, we can use the tree to study entire groups of organisms. Another point to mention on phylogenetic tree structure is that rotation at branch points does not change the information. For example, if a branch point rotated and the taxon order changed, this would not alter the information because each taxon's evolution from the branch point was independent of the other. Many disciplines within the study of biology contribute to understanding how past and present life evolved over time; these disciplines together contribute to building, updating, and maintaining the “tree of life.” Systematics is the field that scientists use to organize and classify organisms based on evolutionary relationships. Researchers may use data from fossils, from studying the body part structures, or molecules that an organism uses, and DNA analysis. By combining data from many sources, scientists can construct an organism's phylogeny Since phylogenetic trees are hypotheses, they will continue to change as researchers discover new types of life and learn new information. Limitations of Phylogenetic Trees It may be easy to assume that more closely related organisms look more alike, and while this is often the case, it is not always true. If two closely related lineages evolved under significantly varied surroundings, it is possible for the two groups to appear more different than other groups that are not as closely related. For example, the phylogenetic tree in Figure 20.4 shows that lizards and rabbits both have amniotic eggs; whereas, frogs do not. Yet lizards and frogs appear more similar than lizards and rabbits. Figure 20.4 An organism that lacked a vertebral column roots this ladder-like phylogenetic tree of vertebrates. At each branch point, scientists place organisms with different characters in different groups based on shared characteristics. 20.1 • Organizing Life on Earth 487 Another aspect of phylogenetic trees is that, unless otherwise indicated, the branches do not account for length of time, only the evolutionary order. In other words, a branch's length does not typically mean more time passed, nor does a short branch mean less time passed— unless specified on the diagram. For example, in Figure 20.4, the tree does not indicate how much time passed between the evolution of amniotic eggs and hair. What the tree does show is the order in which things took place. Again using Figure 20.4, the tree shows that the oldest trait is the vertebral column, followed by hinged jaws, and so forth. Remember that any phylogenetic tree is a part of the greater whole, and like a real tree, it does not grow in only one direction after a new branch develops. Thus, for the organisms in Figure 20.4, just because a vertebral column evolved does not mean that invertebrate evolution ceased. It only means that a new branch formed. Also, groups that are not closely related, but evolve under similar conditions, may appear more phenotypically similar to each other than to a close relative. LINK TO LEARNING Head to this website (http://openstax.org/l/tree_of_life) to see interactive exercises that allow you to explore the evolutionary relationships among species. Classification Levels Taxonomy (which literally means “arrangement law”) is the science of classifying organisms to construct internationally shared classification systems with each organism placed into increasingly more inclusive groupings. Think about a grocery store's organization. One large space is divided into departments, such as produce, dairy, and meats. Then each department further divides into aisles, then each aisle into categories and brands, and then finally a single product. We call this organization from larger to smaller, more specific categories a hierarchical system. The taxonomic classification system (also called the Linnaean system after its inventor, Carl Linnaeus, a Swedish botanist, zoologist, and physician) uses a hierarchical model. Moving from the point of origin, the groups become more specific, until one branch ends as a single species. For example, after the common beginning of all life, scientists divide organisms into three large categories called domains: Bacteria, Archaea, and Eukarya. Within each domain is a second category called a kingdom. After kingdoms, the subsequent categories of increasing specificity are: phylum, class, order, family, genus, and species (Figure 20.5). Figure 20.5 The taxonomic classification system uses a hierarchical model to organize living organisms into increasingly specific categories. The common dog, Canis lupus familiaris, is a subspecies of Canis lupus, which also includes the wolf and dingo. (credit “dog”: modification of work by Janneke Vreugdenhil) The kingdom Animalia stems from the Eukarya domain. Figure 20.5 above shows the classification for the common dog. Therefore, the full name of an organism technically has eight terms. For the dog it is: Eukarya, Animalia, Chordata, Mammalia, Carnivora, Canidae, Canis, and lupus. Notice that each name is capitalized except for species, and the genus and species names are italicized. Scientists generally refer to an organism only by its genus and species, which is its two-word scientific name, or binomial nomenclature. Therefore, the scientific name of the dog is Canis lupus. The name at each level is also a taxon. In other words, dogs are in order Carnivora. Carnivora is the name of the taxon at the order level; Canidae is the taxon at the family level, and so forth. Organisms also have a common name that people typically use, in this case, dog. Note that the dog is additionally a subspecies: the “familiaris” in Canis lupus familiaris. Subspecies are members of the same species that are capable of mating and reproducing viable offspring, but they are separate subspecies due to geographic or behavioral isolation or other factors. Figure 20.6 shows how the levels move toward specificity with other organisms. Notice how the dog shares a domain with the widest diversity of organisms, including plants and butterflies. At each sublevel, the organisms become more similar because they are more closely related. Historically, scientists classified organisms using characteristics, but as DNA technology 488 Chapter 20 • Phylogenies and the History of Life Access for free at openstax.org. developed, they have determined more precise phylogenies. VISUAL CONNECTION Figure 20.6 At each sublevel in the taxonomic classification system, organisms become more similar. Dogs and wolves are the same species because they can breed and produce viable offspring, but they are different enough to be classified as different subspecies. (credit “plant”: modification of work by "berduchwal"/Flickr; credit “insect”: modification of work by Jon Sullivan; credit “fish”: modification of work by Christian Mehlführer; credit “rabbit”: modification of work by Aidan Wojtas; credit “cat”: modification of work by Jonathan Lidbeck; credit “fox”: modification of work by Kevin Bacher, NPS; credit “jackal”: modification of work by Thomas A. Hermann, NBII, USGS; credit “wolf”: modification of work by Robert Dewar; credit “dog”: modification of work by "digital_image_fan"/Flickr) At what levels are cats and dogs part of the same group? LINK TO LEARNING Visit this website (http://openstax.org/l/classify_life) to explore the classifications of thousands of organisms. This reference site contains about 10% of the described species on the planet. 20.1 • Organizing Life on Earth 489 Recent genetic analysis and other advancements have found that some earlier phylogenetic classifications do not align with the evolutionary past; therefore, researchers must make changes and updates as new discoveries occur. Recall that phylogenetic trees are hypotheses and are modified as data becomes available. In addition, classification historically has focused on grouping organisms mainly by shared characteristics and does not necessarily illustrate how the various groups relate to each other from an evolutionary perspective. For example, despite the fact that a hippopotamus resembles a pig more than a whale, the hippopotamus may be the whale's closest living relative. 20.2 Determining Evolutionary Relationships By the end of this section, you will be able to do the following: • Compare homologous and analogous traits • Discuss the purpose of cladistics • Describe maximum parsimony Scientists must collect accurate information that allows them to make evolutionary connections among organisms. Similar to detective work, scientists must use evidence to uncover the facts. In the case of phylogeny, evolutionary investigations focus on two types of evidence: morphologic (form and function) and genetic. Two Options for Similarities In general, organisms that share similar physical features and genomes are more closely related than those that do not. We refer to such features that overlap both morphologically (in form) and genetically as homologous structures. They stem from developmental similarities that are based on evolution. For example, the bones in bat and bird wings have homologous structures (Figure 20.7). Figure 20.7 Bat and bird wings are homologous structures, indicating that bats and birds share a common evolutionary past. (credit a: modification of work by Steve Hillebrand, USFWS; credit b: modification of work by U.S. DOI BLM) Notice it is not simply a single bone, but rather a grouping of several bones arranged in a similar way. The more complex the feature, the more likely any kind of overlap is due to a common evolutionary past. Imagine two people from different countries both inventing a car with all the same parts and in exactly the same arrangement without any previous or shared knowledge. That outcome would be highly improbable. However, if two people both invented a hammer, we can reasonably conclude that both could have the original idea without the help of the other. The same relationship between complexity and shared evolutionary history is true for homologous structures in organisms. Misleading Appearances Some organisms may be very closely related, even though a minor genetic change caused a major morphological difference to make them look quite different. Similarly, unrelated organisms may be distantly related, but appear very much alike. This usually happens because both organisms were in common adaptations that evolved within similar environmental conditions. When similar characteristics occur because of environmental constraints and not due to a close evolutionary relationship, it is an analogy or homoplasy. For example, insects use wings to fly like bats and birds, but the wing structure and embryonic origin is completely different. These are analogous structures (Figure 20.8). 490 Chapter 20 • Phylogenies and the History of Life Access for free at openstax.org. Similar traits can be either homologous or analogous. Homologous structures share a similar embryonic origin. Analogous organs have a similar function. For example, the bones in a whale's front flipper are homologous to the bones in the human arm. These structures are not analogous. A butterfly or bird's wings are analogous but not homologous. Some structures are both analogous and homologous: bird and bat wings are both homologous and analogous. Scientists must determine which type of similarity a feature exhibits to decipher the organisms' phylogeny. Figure 20.8 The (c) wing of a honeybee is similar in shape to a (b) bird wing and (a) bat wing, and it serves the same function. However, the honeybee wing is not composed of bones and has a distinctly different structure and embryonic origin. These wing types (insect versus bat and bird) illustrate an analogy—similar structures that do not share an evolutionary history. (credit a: modification of work by U.S. DOI BLM; credit b: modification of work by Steve Hillebrand, USFWS; credit c: modification of work by Jon Sullivan) LINK TO LEARNING This website (http://openstax.org/l/relationships) has several examples to show how appearances can be misleading in understanding organisms' phylogenetic relationships. Molecular Comparisons The advancement of DNA technology has given rise to molecular systematics, which is use of molecular data in taxonomy and biological geography (biogeography). New computer programs not only confirm many earlier classified organisms, but also uncover previously made errors. As with physical characteristics, even the DNA sequence can be tricky to read in some cases. For some situations, two very closely related organisms can appear unrelated if a mutation occurred that caused a shift in the genetic code. Inserting or deleting a mutation would move each nucleotide base over one place, causing two similar codes to appear unrelated. Sometimes two segments of DNA code in distantly related organisms randomly share a high percentage of bases in the same locations, causing these organisms to appear closely related when they are not. For both of these situations, computer technologies help identify the actual relationships, and, ultimately, the coupled use of both morphologic and molecular information is more effective in determining phylogeny. 20.2 • Determining Evolutionary Relationships 491 EVOLUTION CONNECTION Why Does Phylogeny Matter? Evolutionary biologists could list many reasons why understanding phylogeny is important to everyday life in human society. For botanists, phylogeny acts as a guide to discovering new plants that can be used to benefit people. Think of all the ways humans use plants—food, medicine, and clothing are a few examples. If a plant contains a compound that is effective in treating cancer, scientists might want to examine all of the compounds for other useful drugs. A research team in China identified a DNA segment that they thought to be common to some medicinal plants in the family Fabaceae (the legume family). They worked to identify which species had this segment (Figure 20.9). After testing plant species in this family, the team found a DNA marker (a known location on a chromosome that enabled them to identify the species) present. Then, using the DNA to uncover phylogenetic relationships, the team could identify whether a newly discovered plant was in this family and assess its potential medicinal properties. Figure 20.9 Dalbergia sissoo (D. sissoo) is in the Fabaceae, or legume family. Scientists found that D. sissoo shares a DNA marker with species within the Fabaceae family that have antifungal properties. Subsequently, researchers found that D. sissoo had fungicidal activity, supporting the idea that DNA markers are useful to screen plants with potential medicinal properties. 492 Chapter 20 • Phylogenies and the History of Life Access for free at openstax.org. Building Phylogenetic Trees How do scientists construct phylogenetic trees? After they sort the homologous and analogous traits, scientists often organize the homologous traits using cladistics. This system sorts organisms into clades: groups of organisms that descended from a single ancestor. For example, in Figure 20.10, all the organisms in the orange region evolved from a single ancestor that had amniotic eggs. Consequently, these organisms also have amniotic eggs and make a single clade, or a monophyletic group. Clades must include all descendants from a branch point. VISUAL CONNECTION Figure 20.10 Lizards, rabbits, and humans all descend from a common ancestor that had an amniotic egg. Thus, lizards, rabbits, and humans all belong to the clade Amniota. Vertebrata is a larger clade that also includes fish and lamprey. Which animals in this figure belong to a clade that includes animals with hair? Which evolved first, hair or the amniotic egg? Clades can vary in size depending on which branch point one references. The important factor is that all organisms in the clade or monophyletic group stem from a single point on the tree. You can remember this because monophyletic breaks down into “mono,” meaning one, and “phyletic,” meaning evolutionary relationship. Figure 20.11 shows various clade examples. Notice how each clade comes from a single point; whereas, the non-clade groups show branches that do not share a single point. 20.2 • Determining Evolutionary Relationships 493 VISUAL CONNECTION Figure 20.11 All the organisms within a clade stem from a single point on the tree. A clade may contain multiple groups, as in the case of animals, fungi and plants, or a single group, as in the case of flagellates. Groups that diverge at a different branch point, or that do not include all groups in a single branch point, are not clades. What is the largest clade in this diagram? Shared Characteristics Organisms evolve from common ancestors and then diversify. Scientists use the phrase “descent with modification” because even though related organisms have many of the same characteristics and genetic codes, changes occur. This pattern repeats as one goes through the phylogenetic tree of life: 1. A change in an organism's genetic makeup leads to a new trait which becomes prevalent in the group. 2. Many organisms descend from this point and have this trait. 3. New variations continue to arise: some are adaptive and persist, leading to new traits. 4. With new traits, a new branch point is determined (go back to step 1 and repeat). If a characteristic is found in the ancestor of a group, it is considered a shared ancestral character because all of the organisms in the taxon or clade have that trait. The vertebrate in Figure 20.10 is a shared ancestral character. Now consider the amniotic egg characteristic in the same figure. Only some of the organisms in Figure 20.10 have this trait, and to those that do, it is called a shared derived character because this trait derived at some point but does not include all of the ancestors in the tree. The tricky aspect to shared ancestral and shared derived characters is that these terms are relative. We can consider the same trait one or the other depending on the particular diagram that we use. Returning to Figure 20.10, note that the amniotic egg is a shared ancestral character for the Amniota clade, while having hair is a shared derived character for some organisms in this group. These terms help scientists distinguish between clades in building phylogenetic trees. Choosing the Right Relationships Imagine being the person responsible for organizing all department store items properly—an overwhelming task. Organizing the evolutionary relationships of all life on Earth proves much more difficult: scientists must span enormous blocks of time and work with information from long-extinct organisms. Trying to decipher the proper connections, especially given the presence of 494 Chapter 20 • Phylogenies and the History of Life Access for free at openstax.org. homologies and analogies, makes the task of building an accurate tree of life extraordinarily difficult. Add to that advancing DNA technology, which now provides large quantities of genetic sequences for researchers to use and analzye. Taxonomy is a subjective discipline: many organisms have more than one connection to each other, so each taxonomist will decide the order of connections. To aid in the tremendous task of describing phylogenies accurately, scientists often use the concept of maximum parsimony, which means that events occurred in the simplest, most obvious way. For example, if a group of people entered a forest preserve to hike, based on the principle of maximum parsimony, one could predict that most would hike on established trails rather than forge new ones. For scientists deciphering evolutionary pathways, the same idea is used: the pathway of evolution probably includes the fewest major events that coincide with the evidence at hand. Starting with all of the homologous traits in a group of organisms, scientists look for the most obvious and simple order of evolutionary events that led to the occurrence of those traits. LINK TO LEARNING Head to this website (http://openstax.org/l/using_parsimony) to learn how researchers use maximum parsimony to create phylogenetic trees. These tools and concepts are only a few strategies scientists use to tackle the task of revealing the evolutionary history of life on Earth. Recently, newer technologies have uncovered surprising discoveries with unexpected relationships, such as the fact that people seem to be more closely related to fungi than fungi are to plants. Sound unbelievable? As the information about DNA sequences grows, scientists will become closer to mapping the evolutionary history of all life on Earth. 

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33.1 Animal Form and Function By the end of this section, you will be able to do the following: • Describe the various types of body plans that occur in animals • Describe limits on animal size and shape • Relate bioenergetics to body size, levels of activity, and the environment Animals vary in form and function. From a sponge to a worm to a goat, an organism has a distinct body plan that limits its size and shape. Animals’ bodies are also designed to interact with their environments, whether in the deep sea, a rainforest canopy, or the desert. Therefore, a large amount of information about the structure of an organism's body (anatomy) and the function of its cells, tissues and organs (physiology) can be learned by studying that organism's environment. Figure 33.1 An arctic fox is a complex animal, well adapted to its environment. It changes coat color with the seasons, and has longer fur in winter to trap heat. (credit: modification of work by Keith Morehouse, USFWS) INTRODUCTION The arctic fox is an example of a complex animal that has adapted to its environment and illustrates the relationships between an animal’s form and function. The structures of animals consist of primary tissues that make up more complex organs and organ systems. Homeostasis allows an animal to maintain a balance between its internal and external environments. Chapter Outline 33.1 Animal Form and Function 33.2 Animal Primary Tissues 33.3 Homeostasis Body Plans Figure 33.2 Animals exhibit different types of body symmetry. The sponge is asymmetrical, the sea anemone has radial symmetry, and the goat has bilateral symmetry. Animal body plans follow set patterns related to symmetry. They are asymmetrical, radial, or bilateral in form as illustrated in Figure 33.2. Asymmetrical animals are animals with no pattern or symmetry; an example of an asymmetrical animal is a sponge. Radial symmetry, as illustrated in Figure 33.2, describes when an animal has an up-and-down orientation: any plane cut along its longitudinal axis through the organism produces equal halves, but not a definite right or left side. This plan is found mostly in aquatic animals, especially organisms that attach themselves to a base, like a rock or a boat, and extract their food from the surrounding water as it flows around the organism. Bilateral symmetry is illustrated in the same figure by a goat. The goat also has an upper and lower component to it, but a plane cut from front to back separates the animal into definite right and left sides. Additional terms used when describing positions in the body are anterior (front), posterior (rear), dorsal (toward the back), and ventral (toward the stomach). Bilateral symmetry is found in both land-based and aquatic animals; it enables a high level of mobility. Limits on Animal Size and Shape Animals with bilateral symmetry that live in water tend to have a fusiform shape: this is a tubular shaped body that is tapered at both ends. This shape decreases the drag on the body as it moves through water and allows the animal to swim at high speeds. Table 33.1 lists the maximum speed of various animals. Certain types of sharks can swim at fifty kilometers per hour and some dolphins at 32 to 40 kilometers per hour. Land animals frequently travel faster, although the tortoise and snail are significantly slower than cheetahs. Another difference in the adaptations of aquatic and land-dwelling organisms is that aquatic organisms are constrained in shape by the forces of drag in the water since water has higher viscosity than air. On the other hand, land-dwelling organisms are constrained mainly by gravity, and drag is relatively unimportant. For example, most adaptations in birds are for gravity not for drag. Maximum Speed of Assorted Land & Marine Animals Animal Speed (kmh) Speed (mph) Cheetah 113 70 Quarter horse 77 48 Fox 68 42 Table 33.1 922 Chapter 33 • The Animal Body: Basic Form and Function Access for free at openstax.org. Animal Speed (kmh) Speed (mph) Shortfin mako shark 50 31 Domestic house cat 48 30 Human 45 28 Dolphin 32–40 20–25 Mouse 13 8 Snail 0.05 0.03 Table 33.1 Most animals have an exoskeleton, including insects, spiders, scorpions, horseshoe crabs, centipedes, and crustaceans. Scientists estimate that, of insects alone, there are over 30 million species on our planet. The exoskeleton is a hard covering or shell that provides benefits to the animal, such as protection against damage from predators and from water loss (for land animals); it also provides for the attachments of muscles. As the tough and resistant outer cover of an arthropod, the exoskeleton may be constructed of a tough polymer such as chitin and is often biomineralized with materials such as calcium carbonate. This is fused to the animal’s epidermis. Ingrowths of the exoskeleton, called apodemes, function as attachment sites for muscles, similar to tendons in more advanced animals (Figure 33.3). In order to grow, the animal must first synthesize a new exoskeleton underneath the old one and then shed or molt the original covering. This limits the animal’s ability to grow continually, and may limit the individual’s ability to mature if molting does not occur at the proper time. The thickness of the exoskeleton must be increased significantly to accommodate any increase in weight. It is estimated that a doubling of body size increases body weight by a factor of eight. The increasing thickness of the chitin necessary to support this weight limits most animals with an exoskeleton to a relatively small size. The same principles apply to endoskeletons, but they are more efficient because muscles are attached on the outside, making it easier to compensate for increased mass. Figure 33.3 Apodemes are ingrowths on arthropod exoskeletons to which muscles attach. The apodemes on this crab leg are located above and below the fulcrum of the claw. Contraction of muscles attached to the apodemes pulls the claw closed. An animal with an endoskeleton has its size determined by the amount of skeletal system it needs in order to support the other tissues and the amount of muscle it needs for movement. As the body size increases, both bone and muscle mass increase. The speed achievable by the animal is a balance between its overall size and the bone and muscle that provide support and movement. Limiting Effects of Diffusion on Size and Development The exchange of nutrients and wastes between a cell and its watery environment occurs through the process of diffusion. All living cells are bathed in liquid, whether they are in a single-celled organism or a multicellular one. Diffusion is effective over a specific distance and limits the size that an individual cell can attain. If a cell is a single-celled microorganism, such as an 33.1 • Animal Form and Function 923 amoeba, it can satisfy all of its nutrient and waste needs through diffusion. If the cell is too large, then diffusion is ineffective and the center of the cell does not receive adequate nutrients nor is it able to effectively dispel its waste. An important concept in understanding how efficient diffusion is as a means of transport is the surface to volume ratio. Recall that any three-dimensional object has a surface area and volume; the ratio of these two quantities is the surface-to-volume ratio. Consider a cell shaped like a perfect sphere: it has a surface area of 4πr 2 , and a volume of (4/3)πr 3 . The surface-to-volume ratio of a sphere is 3/r; as the cell gets bigger, its surface to volume ratio decreases, making diffusion less efficient. The larger the size of the sphere, or animal, the less surface area for diffusion it possesses. The solution to producing larger organisms is for them to become multicellular. Specialization occurs in complex organisms, allowing cells to become more efficient at doing fewer tasks. For example, circulatory systems bring nutrients and remove waste, while respiratory systems provide oxygen for the cells and remove carbon dioxide from them. Other organ systems have developed further specialization of cells and tissues and efficiently control body functions. Moreover, surface-to-volume ratio applies to other areas of animal development, such as the relationship between muscle mass and cross-sectional surface area in supporting skeletons, and in the relationship between muscle mass and the generation and dissipation of heat. LINK TO LEARNING Visit this interactive site (http://openstax.org/l/nanoscopy) to see an entire animal (a zebrafish embryo) at the cellular and subcellular level. Use the zoom and navigation functions for a virtual nanoscopy exploration. Animal Bioenergetics All animals must obtain their energy from food they ingest or absorb. These nutrients are converted to adenosine triphosphate (ATP) for short-term storage and use by all cells. Some animals store energy for slightly longer times as glycogen, and others store energy for much longer times in the form of triglycerides housed in specialized adipose tissues. No energy system is one hundred percent efficient, and an animal’s metabolism produces waste energy in the form of heat. If an animal can conserve that heat and maintain a relatively constant body temperature, it is classified as a warm-blooded animal and called an endotherm. The insulation used to conserve the body heat comes in the forms of fur, fat, or feathers. The absence of insulation in ectothermic animals increases their dependence on the environment for body heat. The amount of energy expended by an animal over a specific time is called its metabolic rate. The rate is measured variously in joules, calories, or kilocalories (1000 calories). Carbohydrates and proteins contain about 4.5 to 5 kcal/g, and fat contains about 9 kcal/g. Metabolic rate is estimated as the basal metabolic rate (BMR) in endothermic animals at rest and as the standard metabolic rate (SMR) in ectotherms. Human males have a BMR of 1600 to 1800 kcal/day, and human females have a BMR of 1300 to 1500 kcal/day. Even with insulation, endothermal animals require extensive amounts of energy to maintain a constant body temperature. An ectotherm such as an alligator has an SMR of 60 kcal/day. Energy Requirements Related to Body Size Smaller endothermic animals have a greater surface area for their mass than larger ones (Figure 33.4). Therefore, smaller animals lose heat at a faster rate than larger animals and require more energy to maintain a constant internal temperature. This results in a smaller endothermic animal having a higher BMR, per body weight, than a larger endothermic animal. Figure 33.4 The mouse has a much higher metabolic rate than the elephant. (credit “mouse”: modification of work by Magnus Kjaergaard; 924 Chapter 33 • The Animal Body: Basic Form and Function Access for free at openstax.org. credit “elephant”: modification of work by “TheLizardQueen”/Flickr) Energy Requirements Related to Levels of Activity The more active an animal is, the more energy is needed to maintain that activity, and the higher its BMR or SMR. The average daily rate of energy consumption is about two to four times an animal’s BMR or SMR. Humans are more sedentary than most animals and have an average daily rate of only 1.5 times the BMR. The diet of an endothermic animal is determined by its BMR. For example: the type of grasses, leaves, or shrubs that an herbivore eats affects the number of calories that it takes in. The relative caloric content of herbivore foods, in descending order, is tall grasses > legumes > short grasses > forbs (any broadleaved plant, not a grass) > subshrubs > annuals/biennials. Energy Requirements Related to Environment Animals adapt to extremes of temperature or food availability through torpor. Torpor is a process that leads to a decrease in activity and metabolism and allows animals to survive adverse conditions. Torpor can be used by animals for long periods, such as entering a state of hibernation during the winter months, in which case it enables them to maintain a reduced body temperature. During hibernation, ground squirrels can achieve an abdominal temperature of 0° C (32° F), while a bear’s internal temperature is maintained higher at about 37° C (99° F). If torpor occurs during the summer months with high temperatures and little water, it is called estivation. Some desert animals use this to survive the harshest months of the year. Torpor can occur on a daily basis; this is seen in bats and hummingbirds. While endothermy is limited in smaller animals by surface to volume ratio, some organisms can be smaller and still be endotherms because they employ daily torpor during the part of the day that is coldest. This allows them to conserve energy during the colder parts of the day, when they consume more energy to maintain their body temperature. Animal Body Planes and Cavities A standing vertebrate animal can be divided by several planes. A sagittal plane divides the body into right and left portions. A midsagittal plane divides the body exactly in the middle, making two equal right and left halves. A frontal plane (also called a coronal plane) separates the front from the back. A transverse plane (or, horizontal plane) divides the animal into upper and lower portions. This is sometimes called a cross section, and, if the transverse cut is at an angle, it is called an oblique plane. Figure 33.5 illustrates these planes on a goat (a four-legged animal) and a human being. Figure 33.5 Shown are the planes of a quadrupedal goat and a bipedal human. The midsagittal plane divides the body exactly in half, into right and left portions. The frontal plane divides the front and back, and the transverse plane divides the body into upper and lower portions. Vertebrate animals have a number of defined body cavities, as illustrated in Figure 33.6. Two of these are major cavities that contain smaller cavities within them. The dorsal cavity contains the cranial and the vertebral (or spinal) cavities. The ventral cavity contains the thoracic cavity, which in turn contains the pleural cavity around the lungs and the pericardial cavity, which 33.1 • Animal Form and Function 925 surrounds the heart. The ventral cavity also contains the abdominopelvic cavity, which can be separated into the abdominal and the pelvic cavities. Figure 33.6 Vertebrate animals have two major body cavities. The dorsal cavity contains the cranial and the spinal cavity. The ventral cavity contains the thoracic cavity and the abdominopelvic cavity. The thoracic cavity is separated from the abdominopelvic cavity by the diaphragm. The abdominopelvic cavity is separated into the abdominal cavity and the pelvic cavity by an imaginary line parallel to the pelvis bones. (credit: modification of work by NCI) CAREER CONNECTION Physical Anthropologist Physical anthropologists study the adaption, variability, and evolution of human beings, plus their living and fossil relatives. They can work in a variety of settings, although most will have an academic appointment at a university, usually in an anthropology department or a biology, genetics, or zoology department. Nonacademic positions are available in the automotive and aerospace industries where the focus is on human size, shape, and anatomy. Research by these professionals might range from studies of how the human body reacts to car crashes to exploring how to make seats more comfortable. Other nonacademic positions can be obtained in museums of natural history, anthropology, archaeology, or science and technology. These positions involve educating students from grade school through graduate school. Physical anthropologists serve as education coordinators, collection managers, writers for museum publications, and as administrators. Zoos employ these professionals, especially if they have an expertise in primate biology; they work in collection management and captive breeding programs for endangered species. Forensic science utilizes physical anthropology expertise in identifying human and animal remains, assisting in determining the cause of death, and for expert testimony in trials.   33.2 Animal Primary Tissues By the end of this section, you will be able to do the following: • Describe epithelial tissues • Discuss the different types of connective tissues in animals • Describe three types of muscle tissues • Describe nervous tissue The tissues of multicellular, complex animals are four primary types: epithelial, connective, muscle, and nervous. Recall that tissues are groups of similar cells (cells carrying out related functions). These tissues combine to form organs—like the skin or kidney—that have specific, specialized functions within the body. Organs are organized into organ systems to perform functions; examples include the circulatory system, which consists of the heart and blood vessels, and the digestive system, consisting of several organs, including the stomach, intestines, liver, and pancreas. Organ systems come together to create an entire organism. Epithelial Tissues Epithelial tissues cover the outside of organs and structures in the body and line the lumens of organs in a single layer or multiple layers of cells. The types of epithelia are classified by the shapes of cells present and the number of layers of cells. Epithelia composed of a single layer of cells is called simple epithelia; epithelial tissue composed of multiple layers is called stratified epithelia. Table 33.2 summarizes the different types of epithelial tissues. Different Types of Epithelial Tissues Cell shape Description Location squamous flat, irregular round shape simple: lung alveoli, capillaries; stratified: skin, mouth, vagina cuboidal cube shaped, central nucleus glands, renal tubules columnar tall, narrow, nucleus toward base; tall, narrow, nucleus along cell simple: digestive tract; pseudostratified: respiratory tract transitional round, simple but appear stratified urinary bladder Table 33.2 Squamous Epithelia Squamous epithelial cells are generally round, flat, and have a small, centrally located nucleus. The cell outline is slightly irregular, and cells fit together to form a covering or lining. When the cells are arranged in a single layer (simple epithelia), they facilitate diffusion in tissues, such as the areas of gas exchange in the lungs and the exchange of nutrients and waste at blood capillaries. 33.2 • Animal Primary Tissues 927 Figure 33.7 Squamous epithelia cells (a) have a slightly irregular shape, and a small, centrally located nucleus. These cells can be stratified into layers, as in (b) this human cervix specimen. (credit b: modification of work by Ed Uthman; scale-bar data from Matt Russell) Figure 33.7a illustrates a layer of squamous cells with their membranes joined together to form an epithelium. Image Figure 33.7b illustrates squamous epithelial cells arranged in stratified layers, where protection is needed on the body from outside abrasion and damage. This is called a stratified squamous epithelium and occurs in the skin and in tissues lining the mouth and vagina. Cuboidal Epithelia Cuboidal epithelial cells, shown in Figure 33.8, are cube-shaped with a single, central nucleus. They are most commonly found in a single layer representing a simple epithelia in glandular tissues throughout the body where they prepare and secrete glandular material. They are also found in the walls of tubules and in the ducts of the kidney and liver. Figure 33.8 Simple cuboidal epithelial cells line tubules in the mammalian kidney, where they are involved in filtering the blood. Columnar Epithelia Columnar epithelial cells are taller than they are wide: they resemble a stack of columns in an epithelial layer, and are most commonly found in a single-layer arrangement. The nuclei of columnar epithelial cells in the digestive tract appear to be lined up at the base of the cells, as illustrated in Figure 33.9. These cells absorb material from the lumen of the digestive tract and prepare it for entry into the body through the circulatory and lymphatic systems. 928 Chapter 33 • The Animal Body: Basic Form and Function Access for free at openstax.org. Figure 33.9 Simple columnar epithelial cells absorb material from the digestive tract. Goblet cells secrete mucus into the digestive tract lumen. Columnar epithelial cells lining the respiratory tract appear to be stratified. However, each cell is attached to the base membrane of the tissue and, therefore, they are simple tissues. The nuclei are arranged at different levels in the layer of cells, making it appear as though there is more than one layer, as seen in Figure 33.10. This is called pseudostratified, columnar epithelia. This cellular covering has cilia at the apical, or free, surface of the cells. The cilia enhance the movement of mucus and trapped particles out of the respiratory tract, helping to protect the system from invasive microorganisms and harmful material that has been breathed into the body. Goblet cells are interspersed in some tissues (such as the lining of the trachea). The goblet cells contain mucus that traps irritants, which in the case of the trachea keep these irritants from getting into the lungs. Figure 33.10 Pseudostratified columnar epithelia line the respiratory tract. They exist in one layer, but the arrangement of nuclei at different levels makes it appear that there is more than one layer. Goblet cells interspersed between the columnar epithelial cells secrete mucus into the respiratory tract. Transitional Epithelia Transitional or uroepithelial cells appear only in the urinary system, primarily in the bladder and ureter. These cells are arranged in a stratified layer, but they have the capability of appearing to pile up on top of each other in a relaxed, empty bladder, as illustrated in Figure 33.11. As the urinary bladder fills, the epithelial layer unfolds and expands to hold the volume of urine introduced into it. As the bladder fills, it expands and the lining becomes thinner. In other words, the tissue transitions 33.2 • Animal Primary Tissues 929 from thick to thin. VISUAL CONNECTION Figure 33.11 Transitional epithelia of the urinary bladder undergo changes in thickness depending on how full the bladder is. Which of the following statements about types of epithelial cells is false? a. Simple columnar epithelial cells line the tissue of the lung. b. Simple cuboidal epithelial cells are involved in the filtering of blood in the kidney. c. Pseudostratisfied columnar epithilia occur in a single layer, but the arrangement of nuclei makes it appear that more than one layer is present. d. Transitional epithelia change in thickness depending on how full the bladder is. Connective Tissues Connective tissues are made up of a matrix consisting of living cells and a nonliving substance, called the ground substance. The ground substance is made of an organic substance (usually a protein) and an inorganic substance (usually a mineral or water). The principal cell of connective tissues is the fibroblast. This cell makes the fibers found in nearly all of the connective tissues. Fibroblasts are motile, able to carry out mitosis, and can synthesize whichever connective tissue is needed. Macrophages, lymphocytes, and, occasionally, leukocytes can be found in some of the tissues. Some tissues have specialized cells that are not found in the others. The matrix in connective tissues gives the tissue its density. When a connective tissue has a high concentration of cells or fibers, it has proportionally a less dense matrix. The organic portion or protein fibers found in connective tissues are either collagen, elastic, or reticular fibers. Collagen fibers provide strength to the tissue, preventing it from being torn or separated from the surrounding tissues. Elastic fibers are made of the protein elastin; this fiber can stretch to one and one half of its length and return to its original size and shape. Elastic fibers provide flexibility to the tissues. Reticular fibers are the third type of protein fiber found in connective tissues. This fiber consists of thin strands of collagen that form a network of fibers to support the tissue and other organs to which it is connected. The various types of connective tissues, the types of cells and fibers they are made of, and sample locations of the tissues is summarized in Table 33.3. Connective Tissues Tissue Cells Fibers Location loose/areolar fibroblasts, macrophages, some lymphocytes, some neutrophils few: collagen, elastic, reticular around blood vessels; anchors epithelia Table 33.3 930 Chapter 33 • The Animal Body: Basic Form and Function Access for free at openstax.org. Tissue Cells Fibers Location dense, fibrous connective tissue fibroblasts, macrophages mostly collagen irregular: skin; regular: tendons, ligaments cartilage chondrocytes, chondroblasts hyaline: few: collagen fibrocartilage: large amount of collagen shark skeleton, fetal bones, human ears, intervertebral discs bone osteoblasts, osteocytes, osteoclasts some: collagen, elastic vertebrate skeletons adipose adipocytes few adipose (fat) blood red blood cells, white blood cells none blood Table 33.3 Loose/Areolar Connective Tissue Loose connective tissue, also called areolar connective tissue, has a sampling of all of the components of a connective tissue. As illustrated in Figure 33.12, loose connective tissue has some fibroblasts; macrophages are present as well. Collagen fibers are relatively wide and stain a light pink, while elastic fibers are thin and stain dark blue to black. The space between the formed elements of the tissue is filled with the matrix. The material in the connective tissue gives it a loose consistency similar to a cotton ball that has been pulled apart. Loose connective tissue is found around every blood vessel and helps to keep the vessel in place. The tissue is also found around and between most body organs. In summary, areolar tissue is tough, yet flexible, and comprises membranes. Figure 33.12 Loose connective tissue is composed of loosely woven collagen and elastic fibers. The fibers and other components of the connective tissue matrix are secreted by fibroblasts. Fibrous Connective Tissue Fibrous connective tissues contain large amounts of collagen fibers and few cells or matrix material. The fibers can be arranged irregularly or regularly with the strands lined up in parallel. Irregularly arranged fibrous connective tissues are found in areas of the body where stress occurs from all directions, such as the dermis of the skin. Regular fibrous connective tissue, shown in Figure 33.13, is found in tendons (which connect muscles to bones) and ligaments (which connect bones to bones). 33.2 • Animal Primary Tissues 931 Figure 33.13 Fibrous connective tissue from the tendon has strands of collagen fibers lined up in parallel. Cartilage Cartilage is a connective tissue with a large amount of the matrix and variable amounts of fibers. The cells, called chondrocytes, make the matrix and fibers of the tissue. Chondrocytes are found in spaces within the tissue called lacunae. A cartilage with few collagen and elastic fibers is hyaline cartilage, illustrated in Figure 33.14. The lacunae are randomly scattered throughout the tissue and the matrix takes on a milky or scrubbed appearance with routine histological stains. Sharks have cartilaginous skeletons, as does nearly the entire human skeleton during a specific pre-birth developmental stage. A remnant of this cartilage persists in the outer portion of the human nose. Hyaline cartilage is also found at the ends of long bones, reducing friction and cushioning the articulations of these bones. Figure 33.14 Hyaline cartilage consists of a matrix with cells called chondrocytes embedded in it. The chondrocytes exist in cavities in the matrix called lacunae. Elastic cartilage has a large amount of elastic fibers, giving it tremendous flexibility. The ears of most vertebrate animals contain this cartilage as do portions of the larynx, or voice box. Fibrocartilage contains a large amount of collagen fibers, giving the tissue tremendous strength. Fibrocartilage comprises the intervertebral discs in vertebrate animals. Hyaline cartilage found in movable joints such as the knee and shoulder becomes damaged as a result of age or trauma. Damaged hyaline cartilage is replaced by fibrocartilage and results in the joints becoming “stiff.” Bone Bone, or osseous tissue, is a connective tissue that has a large amount of two different types of matrix material. The organic matrix is similar to the matrix material found in other connective tissues, including some amount of collagen and elastic fibers. This gives strength and flexibility to the tissue. The inorganic matrix consists of mineral salts—mostly calcium salts—that give 932 Chapter 33 • The Animal Body: Basic Form and Function Access for free at openstax.org. the tissue hardness. Without adequate organic material in the matrix, the tissue breaks; without adequate inorganic material in the matrix, the tissue bends. There are three types of cells in bone: osteoblasts, osteocytes, and osteoclasts. Osteoblasts are active in making bone for growth and remodeling. Osteoblasts deposit bone material into the matrix and, after the matrix surrounds them, they continue to live, but in a reduced metabolic state as osteocytes. Osteocytes are found in lacunae of the bone. Osteoclasts are active in breaking down bone for bone remodeling, and they provide access to calcium stored in tissues. Osteoclasts are usually found on the surface of the tissue. Bone can be divided into two types: compact and spongy. Compact bone is found in the shaft (or diaphysis) of a long bone and the surface of the flat bones, while spongy bone is found in the end (or epiphysis) of a long bone. Compact bone is organized into subunits called osteons, as illustrated in Figure 33.15. A blood vessel and a nerve are found in the center of the structure within the Haversian canal, with radiating circles of lacunae around it known as lamellae. The wavy lines seen between the lacunae are microchannels called canaliculi; they connect the lacunae to aid diffusion between the cells. Spongy bone is made of tiny plates called trabeculae; these plates serve as struts to give the spongy bone strength. Over time, these plates can break causing the bone to become less resilient. Bone tissue forms the internal skeleton of vertebrate animals, providing structure to the animal and points of attachment for tendons. 33.2 • Animal Primary Tissues 933 Figure 33.15 (a) Compact bone is a dense matrix on the outer surface of bone. Spongy bone, inside the compact bone, is porous with weblike trabeculae. (b) Compact bone is organized into rings called osteons. Blood vessels, nerves, and lymphatic vessels are found in the central Haversian canal. Rings of lamellae surround the Haversian canal. Between the lamellae are cavities called lacunae. Canaliculi are microchannels connecting the lacunae together. (c) Osteoblasts surround the exterior of the bone. Osteoclasts bore tunnels into the bone and osteocytes are found in the lacunae. Adipose Tissue Adipose tissue, or fat tissue, is considered a connective tissue even though it does not have fibroblasts or a real matrix and only has a few fibers. Adipose tissue is made up of cells called adipocytes that collect and store fat in the form of triglycerides, for energy metabolism. Adipose tissues additionally serve as insulation to help maintain body temperatures, allowing animals to be endothermic, and they function as cushioning against damage to body organs. Under a microscope, adipose tissue cells appear empty due to the extraction of fat during the processing of the material for viewing, as seen in Figure 33.16. The thin lines in the image are the cell membranes, and the nuclei are the small, black dots at the edges of the cells. 934 Chapter 33 • The Animal Body: Basic Form and Function Access for free at openstax.org. Figure 33.16 Adipose is a connective tissue is made up of cells called adipocytes. Adipocytes have small nuclei localized at the cell edge. Blood Blood is considered a connective tissue because it has a matrix, as shown in Figure 33.17. The living cell types are red blood cells (RBC), also called erythrocytes, and white blood cells (WBC), also called leukocytes. The fluid portion of whole blood, its matrix, is commonly called plasma. Figure 33.17 Blood is a connective tissue that has a fluid matrix, called plasma, and no fibers. Erythrocytes (red blood cells), the predominant cell type, are involved in the transport of oxygen and carbon dioxide. Also present are various leukocytes (white blood cells) involved in immune response. The cell found in greatest abundance in blood is the erythrocyte. Erythrocytes are counted in millions in a blood sample: the average number of red blood cells in primates is 4.7 to 5.5 million cells per microliter. Erythrocytes are consistently the same size in a species, but vary in size between species. For example, the average diameter of a primate red blood cell is 7.5 µl, a dog is close at 7.0 µl, but a cat’s RBC diameter is 5.9 µl. Sheep erythrocytes are even smaller at 4.6 µl. Mammalian erythrocytes lose their nuclei and mitochondria when they are released from the bone marrow where they are made. Fish, amphibian, and avian red blood cells maintain their nuclei and mitochondria throughout the cell’s life. The principal job of an erythrocyte is to carry and deliver oxygen to the tissues. Leukocytes are the predominant white blood cells found in the peripheral blood. Leukocytes are counted in the thousands in the blood with measurements expressed as ranges: primate counts range from 4,800 to 10,800 cells per µl, dogs from 5,600 to 19,200 cells per µl, cats from 8,000 to 25,000 cells per µl, cattle from 4,000 to 12,000 cells per µl, and pigs from 11,000 to 22,000 cells per µl. Lymphocytes function primarily in the immune response to foreign antigens or material. Different types of lymphocytes make antibodies tailored to the foreign antigens and control the production of those antibodies. Neutrophils are phagocytic cells and they participate in one of the early lines of defense against microbial invaders, aiding in the removal of bacteria that has entered 33.2 • Animal Primary Tissues 935 the body. Another leukocyte that is found in the peripheral blood is the monocyte. Monocytes give rise to phagocytic macrophages that clean up dead and damaged cells in the body, whether they are foreign or from the host animal. Two additional leukocytes in the blood are eosinophils and basophils—both help to facilitate the inflammatory response. The slightly granular material among the cells is a cytoplasmic fragment of a cell in the bone marrow. This is called a platelet or thrombocyte. Platelets participate in the stages leading up to coagulation of the blood to stop bleeding through damaged blood vessels. Blood has a number of functions, but primarily it transports material through the body to bring nutrients to cells and remove waste material from them. Muscle Tissues There are three types of muscle in animal bodies: smooth, skeletal, and cardiac. They differ by the presence or absence of striations or bands, the number and location of nuclei, whether they are voluntarily or involuntarily controlled, and their location within the body. Table 33.4 summarizes these differences. Types of Muscles Type of Muscle Striations Nuclei Control Location smooth no single, in center involuntary visceral organs skeletal yes many, at periphery voluntary skeletal muscles cardiac yes single, in center involuntary heart Table 33.4 Smooth Muscle Smooth muscle does not have striations in its cells. It has a single, centrally located nucleus, as shown in Figure 33.18. Constriction of smooth muscle occurs under involuntary, autonomic nervous control and in response to local conditions in the tissues. Smooth muscle tissue is also called non-striated as it lacks the banded appearance of skeletal and cardiac muscle. The walls of blood vessels, the tubes of the digestive system, and the tubes of the reproductive systems are composed of mostly smooth muscle. Figure 33.18 Smooth muscle cells do not have striations, while skeletal muscle cells do. Cardiac muscle cells have striations, but, unlike the multinucleate skeletal cells, they have only one nucleus. Cardiac muscle tissue also has intercalated discs, specialized regions running along the plasma membrane that join adjacent cardiac muscle cells and assist in passing an electrical impulse from cell to cell. Skeletal Muscle Skeletal muscle has striations across its cells caused by the arrangement of the contractile proteins actin and myosin. These muscle cells are relatively long and have multiple nuclei along the edge of the cell. Skeletal muscle is under voluntary, somatic nervous system control and is found in the muscles that move bones. Figure 33.18 illustrates the histology of skeletal muscle. Cardiac Muscle Cardiac muscle, shown in Figure 33.18, is found only in the heart. Like skeletal muscle, it has cross striations in its cells, but cardiac muscle has a single, centrally located nucleus. Cardiac muscle is not under voluntary control but can be influenced by the autonomic nervous system to speed up or slow down. An added feature to cardiac muscle cells is a line than extends along the end of the cell as it abuts the next cardiac cell in the row. This line is called an intercalated disc: it assists in passing electrical impulse efficiently from one cell to the next and maintains the strong connection between neighboring cardiac cells. 936 Chapter 33 • The Animal Body: Basic Form and Function Access for free at openstax.org. Nervous Tissues Nervous tissues are made of cells specialized to receive and transmit electrical impulses from specific areas of the body and to send them to specific locations in the body. The main cell of the nervous system is the neuron, illustrated in Figure 33.19. The large structure with a central nucleus is the cell body of the neuron. Projections from the cell body are either dendrites specialized in receiving input or a single axon specialized in transmitting impulses. Some glial cells are also shown. Astrocytes regulate the chemical environment of the nerve cell, and oligodendrocytes insulate the axon so the electrical nerve impulse is transferred more efficiently. Other glial cells that are not shown support the nutritional and waste requirements of the neuron. Some of the glial cells are phagocytic and remove debris or damaged cells from the tissue. A nerve consists of neurons and glial cells. Figure 33.19 The neuron has projections called dendrites that receive signals and projections called axons that send signals. Also shown are two types of glial cells: astrocytes regulate the chemical environment of the nerve cell, and oligodendrocytes insulate the axon so the electrical nerve impulse is transferred more efficiently. LINK TO LEARNING Click through the interactive review (http://openstax.org/l/tissues) to learn more about epithelial tissues. CAREER CONNECTION Pathologist A pathologist is a medical doctor or veterinarian who has specialized in the laboratory detection of disease in animals, including humans. These professionals complete medical school education and follow it with an extensive post-graduate residency at a medical center. A pathologist may oversee clinical laboratories for the evaluation of body tissue and blood samples for the detection of disease or infection. They examine tissue specimens through a microscope to identify cancers and other diseases. Some pathologists perform autopsies to determine the cause of death and the progression of disease. 33.3 Homeostasis By the end of this section, you will be able to do the following: • Define homeostasis • Describe the factors affecting homeostasis • Discuss positive and negative feedback mechanisms used in homeostasis • Describe thermoregulation of endothermic and ectothermic animals Animal organs and organ systems constantly adjust to internal and external changes through a process called homeostasis (“steady state”). These changes might be in the level of glucose or calcium in blood or in external temperatures. Homeostasis 33.3 • Homeostasis 937 means to maintain dynamic equilibrium in the body. It is dynamic because it is constantly adjusting to the changes that the body’s systems encounter. It is equilibrium because body functions are kept within specific ranges. Even an animal that is apparently inactive is maintaining this homeostatic equilibrium. Homeostatic Process The goal of homeostasis is the maintenance of equilibrium around a point or value called a set point. While there are normal fluctuations from the set point, the body’s systems will usually attempt to go back to this point. A change in the internal or external environment is called a stimulus and is detected by a receptor; the response of the system is to adjust the deviation parameter toward the set point. For instance, if the body becomes too warm, adjustments are made to cool the animal. If the blood’s glucose rises after a meal, adjustments are made to lower the blood glucose level by getting the nutrient into tissues that need it or to store it for later use. Control of Homeostasis When a change occurs in an animal’s environment, an adjustment must be made. The receptor senses the change in the environment, then sends a signal to the control center (in most cases, the brain) which in turn generates a response that is signaled to an effector. The effector is a muscle (that contracts or relaxes) or a gland that secretes. Homeostatsis is maintained by negative feedback loops. Positive feedback loops actually push the organism further out of homeostasis, but may be necessary for life to occur. Homeostasis is controlled by the nervous and endocrine system of mammals. Negative Feedback Mechanisms Any homeostatic process that changes the direction of the stimulus is a negative feedback loop. It may either increase or decrease the stimulus, but the stimulus is not allowed to continue as it did before the receptor sensed it. In other words, if a level is too high, the body does something to bring it down, and conversely, if a level is too low, the body does something to make it go up. Hence the term negative feedback. An example is animal maintenance of blood glucose levels. When an animal has eaten, blood glucose levels rise. This is sensed by the nervous system. Specialized cells in the pancreas sense this, and the hormone insulin is released by the endocrine system. Insulin causes blood glucose levels to decrease, as would be expected in a negative feedback system, as illustrated in Figure 33.20. However, if an animal has not eaten and blood glucose levels decrease, this is sensed in another group of cells in the pancreas, and the hormone glucagon is released causing glucose levels to increase. This is still a negative feedback loop, but not in the direction expected by the use of the term “negative.” Another example of an increase as a result of the feedback loop is the control of blood calcium. If calcium levels decrease, specialized cells in the parathyroid gland sense this and release parathyroid hormone (PTH), causing an increased absorption of calcium through the intestines and kidneys and, possibly, the breakdown of bone in order to liberate calcium. The effects of PTH are to raise blood levels of the element. Negative feedback loops are the predominant mechanism used in homeostasis. Figure 33.20 Blood sugar levels are controlled by a negative feedback loop. (credit: modification of work by Jon Sullivan) Positive Feedback Loop A positive feedback loop maintains the direction of the stimulus, possibly accelerating it. Few examples of positive feedback loops exist in animal bodies, but one is found in the cascade of chemical reactions that result in blood clotting, or coagulation. 938 Chapter 33 • The Animal Body: Basic Form and Function Access for free at openstax.org. As one clotting factor is activated, it activates the next factor in sequence until a fibrin clot is achieved. The direction is maintained, not changed, so this is positive feedback. Another example of positive feedback is uterine contractions during childbirth, as illustrated in Figure 33.21. The hormone oxytocin, made by the endocrine system, stimulates the contraction of the uterus. This produces pain sensed by the nervous system. Instead of lowering the oxytocin and causing the pain to subside, more oxytocin is produced until the contractions are powerful enough to produce childbirth. VISUAL CONNECTION Figure 33.21 The birth of a human infant is the result of positive feedback. State whether each of the following processes is regulated by a positive feedback loop or a negative feedback loop. a. A person feels satiated after eating a large meal. b. The blood has plenty of red blood cells. As a result, erythropoietin, a hormone that stimulates the production of new red blood cells, is no longer released from the kidney. Set Point It is possible to adjust a system’s set point. When this happens, the feedback loop works to maintain the new setting. An example of this is blood pressure: over time, the normal or set point for blood pressure can increase as a result of continued increases in blood pressure. The body no longer recognizes the elevation as abnormal and no attempt is made to return to the lower set point. The result is the maintenance of an elevated blood pressure that can have harmful effects on the body. Medication can lower blood pressure and lower the set point in the system to a more healthy level. This is called a process of alteration of the set point in a feedback loop. Changes can be made in a group of body organ systems in order to maintain a set point in another system. This is called acclimatization. This occurs, for instance, when an animal migrates to a higher altitude than that to which it is accustomed. In order to adjust to the lower oxygen levels at the new altitude, the body increases the number of red blood cells circulating in the blood to ensure adequate oxygen delivery to the tissues. Another example of acclimatization is animals that have seasonal changes in their coats: a heavier coat in the winter ensures adequate heat retention, and a light coat in summer assists in keeping body temperature from rising to harmful levels. LINK TO LEARNING Feedback mechanisms can be understood in terms of driving a race car along a track: watch a short video lesson on positive and negative feedback loops. Click to view content (https://www.openstax.org/l/feedback_loops) 33.3 • Homeostasis 939 Homeostasis: Thermoregulation Body temperature affects body activities. Generally, as body temperature rises, enzyme activity rises as well. For every ten degree centigrade rise in temperature, enzyme activity doubles, up to a point. Body proteins, including enzymes, begin to denature and lose their function with high heat (around 50oC for mammals). Enzyme activity will decrease by half for every ten degree centigrade drop in temperature, to the point of freezing, with a few exceptions. Some fish can withstand freezing solid and return to normal with thawing. LINK TO LEARNING Watch this Discovery Channel video on thermoregulation to see illustrations of this process in a variety of animals. Click to view content (https://www.openstax.org/l/thermoregulate) Endotherms and Ectotherms Animals can be divided into two groups: some maintain a constant body temperature in the face of differing environmental temperatures, while others have a body temperature that is the same as their environment and thus varies with the environment. Animals that rely on external temperatures to set their body temperature are ectotherms. This group has been called cold-blooded, but the term may not apply to an animal in the desert with a very warm body temperature. In contrast to ectotherms, poikilotherms are animals with constantly varying internal temperatures. An animal that maintains a constant body temperature in the face of environmental changes is called a homeotherm. Endotherms are animals that rely on internal sources for maintenance of relatively constant body temperature in varying environmental temperatures. These animals are able to maintain a level of metabolic activity at cooler temperature, which an ectotherm cannot due to differing enzyme levels of activity. It is worth mentioning that some ectotherms and poikilotherms have relatively constant body temperatures due to the constant environmental temperatures in their habitats. These animals are so-called ectothermic homeotherms, like some deep sea fish species. Heat can be exchanged between an animal and its environment through four mechanisms: radiation, evaporation, convection, and conduction (Figure 33.22). Radiation is the emission of electromagnetic “heat” waves. Heat comes from the sun in this manner and radiates from dry skin the same way. Heat can be removed with liquid from a surface during evaporation. This occurs when a mammal sweats. Convection currents of air remove heat from the surface of dry skin as the air passes over it. Heat will be conducted from one surface to another during direct contact with the surfaces, such as an animal resting on a warm rock. 940 Chapter 33 • The Animal Body: Basic Form and Function Access for free at openstax.org. Figure 33.22 Heat can be exchanged by four mechanisms: (a) radiation, (b) evaporation, (c) convection, or (d) conduction. (credit b: modification of work by “Kullez”/Flickr; credit c: modification of work by Chad Rosenthal; credit d: modification of work by “stacey.d”/Flickr) Heat Conservation and Dissipation Animals conserve or dissipate heat in a variety of ways. In certain climates, endothermic animals have some form of insulation, such as fur, fat, feathers, or some combination thereof. Animals with thick fur or feathers create an insulating layer of air between their skin and internal organs. Polar bears and seals live and swim in a subfreezing environment and yet maintain a constant, warm, body temperature. The arctic fox, for example, uses its fluffy tail as extra insulation when it curls up to sleep in cold weather. Mammals have a residual effect from shivering and increased muscle activity: arrector pili muscles cause “goose bumps,” causing small hairs to stand up when the individual is cold; this has the intended effect of increasing body temperature. Mammals use layers of fat to achieve the same end. Loss of significant amounts of body fat will compromise an individual’s ability to conserve heat. Endotherms use their circulatory systems to help maintain body temperature. Vasodilation brings more blood and heat to the body surface, facilitating radiation and evaporative heat loss, which helps to cool the body. Vasoconstriction reduces blood flow in peripheral blood vessels, forcing blood toward the core and the vital organs found there, and conserving heat. Some animals have adaptions to their circulatory system that enable them to transfer heat from arteries to veins, warming blood returning to the heart. This is called a countercurrent heat exchange; it prevents the cold venous blood from cooling the heart and other internal organs. This adaption can be shut down in some animals to prevent overheating the internal organs. The countercurrent adaption is found in many animals, including dolphins, sharks, bony fish, bees, and hummingbirds. In contrast, similar adaptations can help cool endotherms when needed, such as dolphin flukes and elephant ears. Some ectothermic animals use changes in their behavior to help regulate body temperature. For example, a desert ectothermic animal may simply seek cooler areas during the hottest part of the day in the desert to keep from getting too warm. The same animals may climb onto rocks to capture heat during a cold desert night. Some animals seek water to aid evaporation in cooling them, as seen with reptiles. Other ectotherms use group activity such as the activity of bees to warm a hive to survive winter. Many animals, especially mammals, use metabolic waste heat as a heat source. When muscles are contracted, most of the energy from the ATP used in muscle actions is wasted energy that translates into heat. Severe cold elicits a shivering reflex that generates heat for the body. Many species also have a type of adipose tissue called brown fat that specializes in generating heat. 33.3 • Homeostasis 941 Neural Control of Thermoregulation The nervous system is important to thermoregulation, as illustrated in Figure 33.22. The processes of homeostasis and temperature control are centered in the hypothalamus of the advanced animal brain. VISUAL CONNECTION Figure 33.23 The body is able to regulate temperature in response to signals from the nervous system. When bacteria are destroyed by leuckocytes, pyrogens are released into the blood. Pyrogens reset the body’s thermostat to a higher temperature, resulting in fever. How might pyrogens cause the body temperature to rise? The hypothalamus maintains the set point for body temperature through reflexes that cause vasodilation and sweating when the body is too warm, or vasoconstriction and shivering when the body is too cold. It responds to chemicals from the body. When a bacterium is destroyed by phagocytic leukocytes, chemicals called endogenous pyrogens are released into the blood. These pyrogens circulate to the hypothalamus and reset the thermostat. This allows the body’s temperature to increase in what is commonly called a fever. An increase in body temperature causes iron to be conserved, which reduces a nutrient needed by bacteria. An increase in body heat also increases the activity of the animal’s enzymes and protective cells while inhibiting the enzymes and activity of the invading microorganisms. Finally, heat itself may also kill the pathogen. A fever that was once thought to be a complication of an infection is now understood to be a normal defense mechanism.

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39.1 Systems of Gas Exchange By the end of this section, you will be able to do the following: • Describe the passage of air from the outside environment to the lungs • Explain how the lungs are protected from particulate matter The primary function of the respiratory system is to deliver oxygen to the cells of the body’s tissues and remove carbon dioxide, a cell waste product. The main structures of the human respiratory system are the nasal cavity, the trachea, and lungs. All aerobic organisms require oxygen to carry out their metabolic functions. Along the evolutionary tree, different organisms have devised different means of obtaining oxygen from the surrounding atmosphere. The environment in which the animal lives greatly determines how an animal respires. The complexity of the respiratory system is correlated with the size of the organism. As animal size increases, diffusion distances increase and the ratio of surface area to volume drops. In unicellular organisms, diffusion across the cell membrane is sufficient for supplying oxygen to the cell (Figure 39.2). Diffusion is a slow, passive transport process. In order for diffusion to be a feasible means of providing oxygen to the cell, the rate of oxygen uptake must match the rate of diffusion across the membrane. In other words, if the cell were very large or thick, diffusion would not be able to provide oxygen quickly enough to the inside of the cell. Therefore, dependence on diffusion as a means of obtaining oxygen and removing carbon dioxide remains feasible only for small organisms or those with highly-flattened bodies, such as many flatworms (Platyhelminthes). Larger organisms had to evolve specialized respiratory tissues, such as gills, lungs, and respiratory passages accompanied by complex circulatory systems, to transport oxygen throughout their entire body. Figure 39.2 The cell of the unicellular alga Ventricaria ventricosa is one of the largest known, reaching one to five centimeters in diameter. Like all single-celled organisms, V. ventricosa exchanges gases across the cell membrane. Direct Diffusion For small multicellular organisms, diffusion across the outer membrane is sufficient to meet their oxygen needs. Gas exchange by direct diffusion across surface membranes is efficient for organisms less than 1 mm in diameter. In simple organisms, such as cnidarians and flatworms, every cell in the body is close to the external environment. Their cells are kept moist and gases diffuse quickly via direct diffusion. Flatworms are small, literally flat worms, which ‘breathe’ through diffusion across the outer membrane (Figure 39.3). The flat shape of these organisms increases the surface area for diffusion, ensuring that each cell within the body is close to the outer membrane surface and has access to oxygen. If the flatworm had a cylindrical body, then the cells in the center would not be able to get oxygen. 1124 Chapter 39 • The Respiratory System Access for free at openstax.org. Figure 39.3 This flatworm’s process of respiration works by diffusion across the outer membrane. (credit: Stephen Childs) Skin and Gills Earthworms and amphibians use their skin (integument) as a respiratory organ. A dense network of capillaries lies just below the skin and facilitates gas exchange between the external environment and the circulatory system. The respiratory surface must be kept moist in order for the gases to dissolve and diffuse across cell membranes. Organisms that live in water need to obtain oxygen from the water. Oxygen dissolves in water but at a lower concentration than in the atmosphere. The atmosphere has roughly 21 percent oxygen. In water, the oxygen concentration is much lower than that. Fish and many other aquatic organisms have evolved gills to take up the dissolved oxygen from water (Figure 39.4). Gills are thin tissue filaments that are highly branched and folded. When water passes over the gills, the dissolved oxygen in water rapidly diffuses across the gills into the bloodstream. The circulatory system can then carry the oxygenated blood to the other parts of the body. In animals that contain coelomic fluid instead of blood, oxygen diffuses across the gill surfaces into the coelomic fluid. Gills are found in mollusks, annelids, and crustaceans. Figure 39.4 This common carp, like many other aquatic organisms, has gills that allow it to obtain oxygen from water. (credit: "Guitardude012"/Wikimedia Commons) The folded surfaces of the gills provide a large surface area to ensure that the fish gets sufficient oxygen. Diffusion is a process in which material travels from regions of high concentration to low concentration until equilibrium is reached. In this case, blood with a low concentration of oxygen molecules circulates through the gills. The concentration of oxygen molecules in water is higher than the concentration of oxygen molecules in gills. As a result, oxygen molecules diffuse from water (high concentration) to blood (low concentration), as shown in Figure 39.5. Similarly, carbon dioxide molecules in the blood diffuse from the blood (high concentration) to water (low concentration). 39.1 • Systems of Gas Exchange 1125 Figure 39.5 As water flows over the gills, oxygen is transferred to blood via the veins. (credit "fish": modification of work by Duane Raver, NOAA) Tracheal Systems Insect respiration is independent of its circulatory system; therefore, the blood does not play a direct role in oxygen transport. Insects have a highly specialized type of respiratory system called the tracheal system, which consists of a network of small tubes that carries oxygen to the entire body. The tracheal system is the most direct and efficient respiratory system in active animals. The tubes in the tracheal system are made of a polymeric material called chitin. Insect bodies have openings, called spiracles, along the thorax and abdomen. These openings connect to the tubular network, allowing oxygen to pass into the body (Figure 39.6) and regulating the diffusion of CO2 and water vapor. Air enters and leaves the tracheal system through the spiracles. Some insects can ventilate the tracheal system with body movements. Figure 39.6 Insects perform respiration via a tracheal system. Mammalian Systems In mammals, pulmonary ventilation occurs via inhalation (breathing). During inhalation, air enters the body through the nasal cavity located just inside the nose (Figure 39.7). As air passes through the nasal cavity, the air is warmed to body temperature and humidified. The respiratory tract is coated with mucus to seal the tissues from direct contact with air. Mucus is high in water. As air crosses these surfaces of the mucous membranes, it picks up water. These processes help equilibrate the air to the body conditions, reducing any damage that cold, dry air can cause. Particulate matter that is floating in the air is removed in the nasal passages via mucus and cilia. The processes of warming, humidifying, and removing particles are important protective mechanisms that prevent damage to the trachea and lungs. Thus, inhalation serves several purposes in addition to bringing oxygen into the respiratory system. 1126 Chapter 39 • The Respiratory System Access for free at openstax.org. VISUAL CONNECTION Figure 39.7 Air enters the respiratory system through the nasal cavity and pharynx, and then passes through the trachea and into the bronchi, which bring air into the lungs. (credit: modification of work by NCI) Which of the following statements about the mammalian respiratory system is false? a. When we breathe in, air travels from the pharynx to the trachea. b. The bronchioles branch into bronchi. c. Alveolar ducts connect to alveolar sacs. d. Gas exchange between the lung and blood takes place in the alveolus. From the nasal cavity, air passes through the pharynx (throat) and the larynx (voice box), as it makes its way to the trachea (Figure 39.7). The main function of the trachea is to funnel the inhaled air to the lungs and the exhaled air back out of the body. The human trachea is a cylinder about 10 to 12 cm long and 2 cm in diameter that sits in front of the esophagus and extends from the larynx into the chest cavity where it divides into the two primary bronchi at the midthorax. It is made of incomplete rings of hyaline cartilage and smooth muscle (Figure 39.8). The trachea is lined with mucus-producing goblet cells and ciliated epithelia. The cilia propel foreign particles trapped in the mucus toward the pharynx. The cartilage provides strength and support to the trachea to keep the passage open. The smooth muscle can contract, decreasing the trachea’s diameter, which causes expired air to rush upwards from the lungs at a great force. The forced exhalation helps expel mucus when we cough. Smooth muscle can contract or relax, depending on stimuli from the external environment or the body’s nervous system. 39.1 • Systems of Gas Exchange 1127 Figure 39.8 The trachea and bronchi are made of incomplete rings of cartilage. (credit: modification of work by Gray's Anatomy) Lungs: Bronchi and Alveoli The end of the trachea bifurcates (divides) to the right and left lungs. The lungs are not identical. The right lung is larger and contains three lobes, whereas the smaller left lung contains two lobes (Figure 39.9). The muscular diaphragm, which facilitates breathing, is inferior to (below) the lungs and marks the end of the thoracic cavity. Figure 39.9 The trachea bifurcates into the right and left bronchi in the lungs. The right lung is made of three lobes and is larger. To accommodate the heart, the left lung is smaller and has only two lobes. In the lungs, air is diverted into smaller and smaller passages, or bronchi. Air enters the lungs through the two primary (main) bronchi (singular: bronchus). Each bronchus divides into secondary bronchi, then into tertiary bronchi, which in turn divide, creating smaller and smaller diameter bronchioles as they split and spread through the lung. Like the trachea, the bronchi are made of cartilage and smooth muscle. At the bronchioles, the cartilage is replaced with elastic fibers. Bronchi are innervated by nerves of both the parasympathetic and sympathetic nervous systems that control muscle contraction (parasympathetic) or relaxation (sympathetic) in the bronchi and bronchioles, depending on the nervous system’s cues. In humans, bronchioles with a diameter smaller than 0.5 mm are the respiratory bronchioles. They lack cartilage and therefore rely on inhaled air to support their shape. As the passageways decrease in diameter, the relative amount of smooth muscle increases. The terminal bronchioles subdivide into microscopic branches called respiratory bronchioles. The respiratory bronchioles subdivide into several alveolar ducts. Numerous alveoli and alveolar sacs surround the alveolar ducts. The alveolar sacs resemble bunches of grapes tethered to the end of the bronchioles (Figure 39.10). In the acinar region, the alveolar ducts are attached to the end of each bronchiole. At the end of each duct are approximately 100 alveolar sacs, each containing 20 to 30 alveoli that are 200 to 300 microns in diameter. Gas exchange occurs only in alveoli. Alveoli are made of thin-walled parenchymal cells, typically one-cell thick, that look like tiny bubbles within the sacs. Alveoli are in direct contact with capillaries (one-cell thick) of the 1128 Chapter 39 • The Respiratory System Access for free at openstax.org. circulatory system. Such intimate contact ensures that oxygen will diffuse from alveoli into the blood and be distributed to the cells of the body. In addition, the carbon dioxide that was produced by cells as a waste product will diffuse from the blood into alveoli to be exhaled. The anatomical arrangement of capillaries and alveoli emphasizes the structural and functional relationship of the respiratory and circulatory systems. Because there are so many alveoli (~300 million per lung) within each alveolar sac and so many sacs at the end of each alveolar duct, the lungs have a sponge-like consistency. This organization produces a very large surface area that is available for gas exchange. The surface area of alveoli in the lungs is approximately 75 m2 . This large surface area, combined with the thin-walled nature of the alveolar parenchymal cells, allows gases to easily diffuse across the cells. Figure 39.10 Terminal bronchioles are connected by respiratory bronchioles to alveolar ducts and alveolar sacs. Each alveolar sac contains 20 to 30 spherical alveoli and has the appearance of a bunch of grapes. Air flows into the atrium of the alveolar sac, then circulates into alveoli where gas exchange occurs with the capillaries. Mucous glands secrete mucous into the airways, keeping them moist and flexible. (credit: modification of work by Mariana Ruiz Villareal) LINK TO LEARNING Watch the following video to review the respiratory system. Click to view content (https://www.openstax.org/l/lungs_pulmonary) Protective Mechanisms The air that organisms breathe contains particulate matter such as dust, dirt, viral particles, and bacteria that can damage the lungs or trigger allergic immune responses. The respiratory system contains several protective mechanisms to avoid problems or tissue damage. In the nasal cavity, hairs and mucus trap small particles, viruses, bacteria, dust, and dirt to prevent their entry. If particulates do make it beyond the nose, or enter through the mouth, the bronchi and bronchioles of the lungs also contain several protective devices. The lungs produce mucus—a sticky substance made of mucin, a complex glycoprotein, as well as salts and water—that traps particulates. The bronchi and bronchioles contain cilia, small hair-like projections that line the walls of the bronchi and bronchioles (Figure 39.11). These cilia beat in unison and move mucus and particles out of the bronchi and bronchioles back up to the throat where it is swallowed and eliminated via the esophagus. 39.1 • Systems of Gas Exchange 1129 In humans, for example, tar and other substances in cigarette smoke destroy or paralyze the cilia, making the removal of particles more difficult. In addition, smoking causes the lungs to produce more mucus, which the damaged cilia are not able to move. This causes a persistent cough, as the lungs try to rid themselves of particulate matter, and makes smokers more susceptible to respiratory ailments. Figure 39.11 The bronchi and bronchioles contain cilia that help mov39.4 Transport of Gases in Human Bodily Fluids By the end of this section, you will be able to do the following: • Describe how oxygen is bound to hemoglobin and transported to body tissues • Explain how carbon dioxide is transported from body tissues to the lungs Once the oxygen diffuses across the alveoli, it enters the bloodstream and is transported to the tissues where it is unloaded, and carbon dioxide diffuses out of the blood and into the alveoli to be expelled from the body. Although gas exchange is a continuous process, the oxygen and carbon dioxide are transported by different mechanisms. Transport of Oxygen in the Blood Although oxygen dissolves in blood, only a small amount of oxygen is transported this way. Only 1.5 percent of oxygen in the blood is dissolved directly into the blood itself. Most oxygen—98.5 percent—is bound to a protein called hemoglobin and carried to the tissues. Hemoglobin Hemoglobin, or Hb, is a protein molecule found in red blood cells (erythrocytes) made of four subunits: two alpha subunits and two beta subunits (Figure 39.19). Each subunit surrounds a central heme group that contains iron and binds one oxygen molecule, allowing each hemoglobin molecule to bind four oxygen molecules. Molecules with more oxygen bound to the heme groups are brighter red. As a result, oxygenated arterial blood where the Hb is carrying four oxygen molecules is bright red, while venous blood that is deoxygenated is darker red. 1140 Chapter 39 • The Respiratory System Access for free at openstax.org. Figure 39.19 The protein inside (a) red blood cells that carries oxygen to cells and carbon dioxide to the lungs is (b) hemoglobin. Hemoglobin is made up of four symmetrical subunits and four heme groups. Iron associated with the heme binds oxygen. It is the iron in hemoglobin that gives blood its red color. It is easier to bind a second and third oxygen molecule to Hb than the first molecule. This is because the hemoglobin molecule changes its shape, or conformation, as oxygen binds. The fourth oxygen is then more difficult to bind. The binding of oxygen to hemoglobin can be plotted as a function of the partial pressure of oxygen in the blood (x-axis) versus the relative Hb-oxygen saturation (y-axis). The resulting graph—an oxygen dissociation curve—is sigmoidal, or S-shaped (Figure 39.20). As the partial pressure of oxygen increases, the hemoglobin becomes increasingly saturated with oxygen. VISUAL CONNECTION Figure 39.20 The oxygen dissociation curve demonstrates that, as the partial pressure of oxygen increases, more oxygen binds hemoglobin. However, the affinity of hemoglobin for oxygen may shift to the left or the right depending on environmental conditions. The kidneys are responsible for removing excess H+ ions from the blood. If the kidneys fail, what would happen to blood pH and to hemoglobin affinity for oxygen? Factors That Affect Oxygen Binding The oxygen-carrying capacity of hemoglobin determines how much oxygen is carried in the blood. In addition to , other environmental factors and diseases can affect oxygen carrying capacity and delivery. Carbon dioxide levels, blood pH, and body temperature affect oxygen-carrying capacity (Figure 39.20). When carbon dioxide is 39.4 • Transport of Gases in Human Bodily Fluids 1141 in the blood, it reacts with water to form bicarbonate and hydrogen ions (H+ ). As the level of carbon dioxide in the blood increases, more H+ is produced and the pH decreases. This increase in carbon dioxide and subsequent decrease in pH reduce the affinity of hemoglobin for oxygen. The oxygen dissociates from the Hb molecule, shifting the oxygen dissociation curve to the right. Therefore, more oxygen is needed to reach the same hemoglobin saturation level as when the pH was higher. A similar shift in the curve also results from an increase in body temperature. Increased temperature, such as from increased activity of skeletal muscle, causes the affinity of hemoglobin for oxygen to be reduced. Diseases like sickle cell anemia and thalassemia decrease the blood’s ability to deliver oxygen to tissues and its oxygen-carrying capacity. In sickle cell anemia, the shape of the red blood cell is crescent-shaped, elongated, and stiffened, reducing its ability to deliver oxygen (Figure 39.21). In this form, red blood cells cannot pass through the capillaries. This is painful when it occurs. Thalassemia is a rare genetic disease caused by a defect in either the alpha or the beta subunit of Hb. Patients with thalassemia produce a high number of red blood cells, but these cells have lower-than-normal levels of hemoglobin. Therefore, the oxygencarrying capacity is diminished. Figure 39.21 Individuals with sickle cell anemia have crescent-shaped red blood cells. (credit: modification of work by Ed Uthman; scalebar data from Matt Russell) Transport of Carbon Dioxide in the Blood Carbon dioxide molecules are transported in the blood from body tissues to the lungs by one of three methods: dissolution directly into the blood, binding to hemoglobin, or carried as a bicarbonate ion. Several properties of carbon dioxide in the blood affect its transport. First, carbon dioxide is more soluble in blood than oxygen. About 5 to 7 percent of all carbon dioxide is dissolved in the plasma. Second, carbon dioxide can bind to plasma proteins or can enter red blood cells and bind to hemoglobin. This form transports about 10 percent of the carbon dioxide. When carbon dioxide binds to hemoglobin, a molecule called carbaminohemoglobin is formed. Binding of carbon dioxide to hemoglobin is reversible. Therefore, when it reaches the lungs, the carbon dioxide can freely dissociate from the hemoglobin and be expelled from the body. Third, the majority of carbon dioxide molecules (85 percent) are carried as part of the bicarbonate buffer system. In this system, carbon dioxide diffuses into the red blood cells. Carbonic anhydrase (CA) within the red blood cells quickly converts the carbon dioxide into carbonic acid (H2CO3 ). Carbonic acid is an unstable intermediate molecule that immediately dissociates into bicarbonate ions and hydrogen (H+ ) ions. Since carbon dioxide is quickly converted into bicarbonate ions, this reaction allows for the continued uptake of carbon dioxide into the blood down its concentration gradient. It also results in the production of H+ ions. If too much H+ is produced, it can alter blood pH. However, hemoglobin binds to the free H+ ions and thus limits shifts in pH. The newly synthesized bicarbonate ion is transported out of the red blood cell into the liquid component of the blood in exchange for a chloride ion (Cl- ); this is called the chloride shift. When the blood reaches the lungs, the bicarbonate ion is transported back into the red blood cell in exchange for the chloride ion. The H+ ion dissociates from the hemoglobin and binds to the bicarbonate ion. This produces the carbonic acid intermediate, which is converted back into carbon dioxide through the enzymatic action of CA. The carbon dioxide produced is expelled through the lungs during exhalation. 1142 Chapter 39 • The Respiratory System Access for free at openstax.org. The benefit of the bicarbonate buffer system is that carbon dioxide is “soaked up” into the blood with little change to the pH of the system. This is important because it takes only a small change in the overall pH of the body for severe injury or death to result. The presence of this bicarbonate buffer system also allows for people to travel and live at high altitudes: When the partial pressure of oxygen and carbon dioxide change at high altitudes, the bicarbonate buffer system adjusts to regulate carbon dioxide while maintaining the correct pH in the body. Carbon Monoxide Poisoning While carbon dioxide can readily associate and dissociate from hemoglobin, other molecules such as carbon monoxide (CO) cannot. Carbon monoxide has a greater affinity for hemoglobin than oxygen. Therefore, when carbon monoxide is present, it binds to hemoglobin preferentially over oxygen. As a result, oxygen cannot bind to hemoglobin, so very little oxygen is transported through the body (Figure 39.22). Carbon monoxide is a colorless, odorless gas and is therefore difficult to detect. It is produced by gas-powered vehicles and tools. Carbon monoxide can cause headaches, confusion, and nausea; long-term exposure can cause brain damage or death. Administering 100 percent (pure) oxygen is the usual treatment for carbon monoxide poisoning. Administration of pure oxygen speeds up the separation of carbon monoxide from hemoglobin.

40.3 Mammalian Heart and Blood Vessels By the end of this section, you will be able to do the following: • Describe the structure of the heart and explain how cardiac muscle is different from other muscles • Describe the cardiac cycle • Explain the structure of arteries, veins, and capillaries, and how blood flows through the body The heart is a complex muscle that pumps blood through the three divisions of the circulatory system: the coronary (vessels that serve the heart), pulmonary (heart and lungs), and systemic (systems of the body), as shown in Figure 40.10. Coronary circulation intrinsic to the heart takes blood directly from the main artery (aorta) coming from the heart. For pulmonary and systemic circulation, the heart has to pump blood to the lungs or the rest of the body, respectively. In vertebrates, the lungs are relatively close to the heart in the thoracic cavity. The shorter distance to pump means that the muscle wall on the right side of the heart is not as thick as the left side which must have enough pressure to pump blood all the way to your big toe. 1158 Chapter 40 • The Circulatory System Access for free at openstax.org. VISUAL CONNECTION Figure 40.10 The mammalian circulatory system is divided into three circuits: the systemic circuit, the pulmonary circuit, and the coronary circuit. Blood is pumped from veins of the systemic circuit into the right atrium of the heart, then into the right ventricle. Blood then enters the pulmonary circuit, and is oxygenated by the lungs. From the pulmonary circuit, blood reenters the heart through the left atrium. From the left ventricle, blood reenters the systemic circuit through the aorta and is distributed to the rest of the body. The coronary circuit, which provides blood to the heart, is not shown. Which of the following statements about the circulatory system is false? a. Blood in the pulmonary vein is deoxygenated. b. Blood in the inferior vena cava is deoxygenated. c. Blood in the pulmonary artery is deoxygenated. d. Blood in the aorta is oxygenated. Structure of the Heart The heart muscle is asymmetrical as a result of the distance blood must travel in the pulmonary and systemic circuits. Since the right side of the heart sends blood to the pulmonary circuit it is smaller than the left side which must send blood out to the whole body in the systemic circuit, as shown in Figure 40.11. In humans, the heart is about the size of a clenched fist; it is divided into four chambers: two atria and two ventricles. There is one atrium and one ventricle on the right side and one atrium and one ventricle on the left side. The atria are the chambers that receive blood, and the ventricles are the chambers that pump blood. The right atrium receives deoxygenated blood from the superior vena cava, which drains blood from the jugular vein that comes from the brain and from the veins that come from the arms, as well as from the inferior vena cava which drains blood from the veins that come from the lower organs and the legs. In addition, the right atrium receives blood from the coronary sinus which drains deoxygenated blood from the heart itself. This deoxygenated blood then passes to the right ventricle through the atrioventricular valve or the tricuspid valve, a flap of connective tissue that opens in only one direction to prevent the backflow of blood. The valve separating the chambers on the left side of the heart valve is called the biscuspid or mitral valve. After it is filled, the right ventricle pumps the blood through the pulmonary arteries, bypassing the semilunar valve (or pulmonic valve) to the lungs for re-oxygenation. After blood passes through the pulmonary arteries, the right semilunar valves close preventing the blood from flowing backwards into the right ventricle. The left atrium then receives the oxygen-rich blood from the lungs via the pulmonary veins. This blood passes through the bicuspid valve or mitral valve (the atrioventricular valve on the left side of the heart) to the left ventricle where the blood is pumped out through the aorta, the major artery of the body, taking oxygenated blood to the organs and muscles of the body. Once blood is pumped out of the left ventricle and into the aorta, the aortic semilunar valve (or aortic valve) closes preventing blood from flowing backward into the left ventricle. This pattern of pumping is referred to as double circulation and is found in all mammals. 40.3 • Mammalian Heart and Blood Vessels 1159 VISUAL CONNECTION Figure 40.11 (a) The heart is primarily made of a thick muscle layer, called the myocardium, surrounded by membranes. One-way valves separate the four chambers. (b) Blood vessels of the coronary system, including the coronary arteries and veins, keep the heart musculature oxygenated. Which of the following statements about the heart is false? a. The mitral valve separates the left ventricle from the left atrium. b. Blood travels through the bicuspid valve to the left atrium. c. Both the aortic and the pulmonary valves are semilunar valves. d. The mitral valve is an atrioventricular valve. The heart is composed of three layers; the epicardium, the myocardium, and the endocardium, illustrated in Figure 40.11. The 1160 Chapter 40 • The Circulatory System Access for free at openstax.org. inner wall of the heart has a lining called the endocardium. The myocardium consists of the heart muscle cells that make up the middle layer and the bulk of the heart wall. The outer layer of cells is called the epicardium, of which the second layer is a membranous layered structure called the pericardium that surrounds and protects the heart; it allows enough room for vigorous pumping but also keeps the heart in place to reduce friction between the heart and other structures. The heart has its own blood vessels that supply the heart muscle with blood. The coronary arteries branch from the aorta and surround the outer surface of the heart like a crown. They diverge into capillaries where the heart muscle is supplied with oxygen before converging again into the coronary veins to take the deoxygenated blood back to the right atrium where the blood will be re-oxygenated through the pulmonary circuit. The heart muscle will die without a steady supply of blood. Atherosclerosis is the blockage of an artery by the buildup of fatty plaques. Because of the size (narrow) of the coronary arteries and their function in serving the heart itself, atherosclerosis can be deadly in these arteries. The slowdown of blood flow and subsequent oxygen deprivation that results from atherosclerosis causes severe pain, known as angina, and complete blockage of the arteries will cause myocardial infarction: the death of cardiac muscle tissue, commonly known as a heart attack. The Cardiac Cycle The main purpose of the heart is to pump blood through the body; it does so in a repeating sequence called the cardiac cycle. The cardiac cycle is the coordination of the filling and emptying of the heart of blood by electrical signals that cause the heart muscles to contract and relax. The human heart beats over 100,000 times per day. In each cardiac cycle, the heart contracts (systole), pushing out the blood and pumping it through the body; this is followed by a relaxation phase (diastole), where the heart fills with blood, as illustrated in Figure 40.12. The atria contract at the same time, forcing blood through the atrioventricular valves into the ventricles. Closing of the atrioventricular valves produces a monosyllabic “lup” sound. Following a brief delay, the ventricles contract at the same time forcing blood through the semilunar valves into the aorta and the artery transporting blood to the lungs (via the pulmonary artery). Closing of the semilunar valves produces a monosyllabic “dup” sound. Figure 40.12 During (a) cardiac diastole, the heart muscle is relaxed and blood flows into the heart. During (b) atrial systole, the atria contract, pushing blood into the ventricles. During (c) atrial diastole, the ventricles contract, forcing blood out of the heart. The pumping of the heart is a function of the cardiac muscle cells, or cardiomyocytes, that make up the heart muscle. Cardiomyocytes, shown in Figure 40.13, are distinctive muscle cells that are striated like skeletal muscle but pump rhythmically and involuntarily like smooth muscle; they are connected by intercalated disks exclusive to cardiac muscle. They are selfstimulated for a period of time and isolated cardiomyocytes will beat if given the correct balance of nutrients and electrolytes. 40.3 • Mammalian Heart and Blood Vessels 1161 Figure 40.13 Cardiomyocytes are striated muscle cells found in cardiac tissue. (credit: modification of work by Dr. S. Girod, Anton Becker; scale-bar data from Matt Russell) The autonomous beating of cardiac muscle cells is regulated by the heart’s internal pacemaker that uses electrical signals to time the beating of the heart. The electrical signals and mechanical actions, illustrated in Figure 40.14, are intimately intertwined. The internal pacemaker starts at the sinoatrial (SA) node, which is located near the wall of the right atrium. Electrical charges spontaneously pulse from the SA node causing the two atria to contract in unison. The pulse reaches a second node, called the atrioventricular (AV) node, between the right atrium and right ventricle where it pauses for approximately 0.1 second before spreading to the walls of the ventricles. From the AV node, the electrical impulse enters the bundle of His, then to the left and right bundle branches extending through the interventricular septum. Finally, the Purkinje fibers conduct the impulse from the apex of the heart up the ventricular myocardium, and then the ventricles contract. This pause allows the atria to empty completely into the ventricles before the ventricles pump out the blood. The electrical impulses in the heart produce electrical currents that flow through the body and can be measured on the skin using electrodes. This information can be observed as an electrocardiogram (ECG)—a recording of the electrical impulses of the cardiac muscle. 1162 Chapter 40 • The Circulatory System Access for free at openstax.org. Figure 40.14 The beating of the heart is regulated by an electrical impulse that causes the characteristic reading of an ECG. The signal is initiated at the sinoatrial valve. The signal then (a) spreads to the atria, causing them to contract. The signal is (b) delayed at the atrioventricular node before it is passed on to the (c) heart apex. The delay allows the atria to relax before the (d) ventricles contract. The final part of the ECG cycle prepares the heart for the next beat. LINK TO LEARNING Visit this site (http://openstax.org/l/electric_heart) to see the heart’s “pacemaker” in action. Arteries, Veins, and Capillaries The blood from the heart is carried through the body by a complex network of blood vessels (Figure 40.15). Arteries take blood away from the heart. The main artery is the aorta that branches into major arteries that take blood to different limbs and organs. These major arteries include the carotid artery that takes blood to the brain, the brachial arteries that take blood to the arms, and the thoracic artery that takes blood to the thorax and then into the hepatic, renal, and gastric arteries for the liver, kidney, and stomach, respectively. The iliac artery takes blood to the lower limbs. The major arteries diverge into minor arteries, and then smaller vessels called arterioles, to reach more deeply into the muscles and organs of the body. 40.3 • Mammalian Heart and Blood Vessels 1163 Figure 40.15 The major human arteries and veins are shown. (credit: modification of work by Mariana Ruiz Villareal) Arterioles diverge into capillary beds. Capillary beds contain a large number (10 to 100) of capillaries that branch among the cells and tissues of the body. Capillaries are narrow-diameter tubes that can fit red blood cells through in single file and are the sites for the exchange of nutrients, waste, and oxygen with tissues at the cellular level. Fluid also crosses into the interstitial space from the capillaries. The capillaries converge again into venules that connect to minor veins that finally connect to major veins that take blood high in carbon dioxide back to the heart. Veins are blood vessels that bring blood back to the heart. The major veins drain blood from the same organs and limbs that the major arteries supply. Fluid is also brought back to the heart via the lymphatic system. The structure of the different types of blood vessels reflects their function or layers. There are three distinct layers, or tunics, that form the walls of blood vessels (Figure 40.16). The first tunic is a smooth, inner lining of endothelial cells that are in contact with the red blood cells. The endothelial tunic is continuous with the endocardium of the heart. In capillaries, this single layer of cells is the location of diffusion of oxygen and carbon dioxide between the endothelial cells and red blood cells, as well as the exchange site via endocytosis and exocytosis. The movement of materials at the site of capillaries is regulated by vasoconstriction, narrowing of the blood vessels, and vasodilation, widening of the blood vessels; this is important in the overall regulation of blood pressure. Veins and arteries both have two further tunics that surround the endothelium: the middle tunic is composed of smooth muscle and the outermost layer is connective tissue (collagen and elastic fibers). The elastic connective tissue stretches and supports the blood vessels, and the smooth muscle layer helps regulate blood flow by altering vascular resistance through vasoconstriction and vasodilation. The arteries have thicker smooth muscle and connective tissue than the veins to accommodate the higher pressure and speed of freshly pumped blood. The veins are thinner walled as the pressure and rate of flow are much lower. In addition, veins are structurally different than arteries in that veins have valves to prevent the backflow of blood. Because veins have to work against gravity to get blood back to the heart, contraction of skeletal muscle assists with the flow of blood back to the heart. 1164 Chapter 40 • The Circulatory System Access for free at openstax.org. Figure 40.16 Arteries and veins consist of three layers: an outer tunica externa, a middle tunica media, and an inner tunica intima. Capillaries consist of a single layer of epithelial cells, the tunica intima. (credit: modification of work by NCI, NIH)

18.1 Understanding Evolution

• Goals of the section: describe how the present-day theory of evolution was developed; define adaptation; explain convergent and divergent evolution; describe homologous and vestigial structures; discuss common misconceptions about evolution.

• Evolution by natural selection as a mechanism for evolution:

  • Pre-Darwin ideas laid groundwork for thinking about species change (Plato’s static view vs. evolving ideas; Buffon’s observations of regional differences and extinct species; Hutton and Lyell arguing gradual geological change over long time—giving more time for evolution; Lamarck’s mechanism of inheritance of acquired characteristics).

  • Darwin and Wallace (mid-1800s) independently described natural selection after expeditions; Darwin’s voyage on HMS Beagle (1831–1836) showed island species that were similar yet different; Galápagos finches as a key example with beak variation tied to food type (seed-eaters vs. insect-eaters).

  • Natural selection defined as generation-to-generation change driven by differential survival and reproduction of individuals with favorable traits in a changing environment; Darwin’s three key principles (as summarized in the text):

  • Inheritance of most characteristics (traits passed from parent to offspring).

  • More offspring produced than can survive; competition for resources.

  • Offspring vary, with some inherited traits allowing better competition and survival, leading to changes in populations over generations (descent with modification).

  • Result: evolution leads to greater adaptation to local environments; natural selection described as the primary mechanism for adaptive evolution; described as the only known mechanism for adaptive evolution in the text.

• Darwin and Wallace’s Linnean Society presentation (1858) and Darwin’s On the Origin of Species (1859) solidified the theory; the Galápagos finches, studied by Peter and Rosemary Grant since 1976, provide ongoing evidence of natural selection in action.

• Example: Galápagos giant tortoises on dry lowland islands tend to have shorter necks; populations on other islands with drought and leaf scarcity show selection for longer necks to reach leaves. Grants documented shifts in beak size distribution in the Daphne Major medium ground finch during drought (El Niño years):

  • Small hard seeds scarce; small-billed birds thrive on abundant small soft seeds; selection shifts toward smaller average beak size after drought; as conditions improve, the trend reverses.

  • Important conclusion: bill size is heritable; environmental changes shift which phenotypes are favored; the mean trait value in the population can change even though individual bills do not physically change.

• Important misconceptions (addressed later in the section):

  • Evolution is not “just a theory” in the everyday sense; in science, a theory is a well-supported explanation of observations.

  • Evolution does not imply that individuals evolve within their lifetimes; populations evolve across generations via the differential survival and reproduction of individuals with heritable traits.

  • Evolution does not explain the origin of life (it explains descent with modification and diversification once life exists).

• Evidence for evolution (overview):

  • Fossils show progressive changes and common ancestry; fossil records for humans, horses, etc., and homologies across taxa.

  • Anatomy and embryology reveal homologous structures across diverse taxa that imply common ancestry; vestigial structures indicate historical use of traits now reduced or lost.

  • Biogeography explains distributions consistent with shared ancestry and plate tectonics.

  • Molecular biology (DNA) shows shared genetic codes and molecular similarity reflecting descent with modification; gene duplication can lead to new protein functions.

• Key concepts introduced:

  • Adaptation: a heritable trait that increases an organism’s fitness in its present environment (e.g., platypus webbed feet for swimming; snow leopard thick fur for cold climates; cheetah speed for catching prey).

  • Variation must have a genetic basis for natural selection to act.

  • Environmental change shifts which traits are favored; a trait that is favorable in one environment may be neutral or deleterious in another.

  • Convergent evolution: unrelated species evolve similar traits due to similar selection pressures (e.g., wings for flight in bats and insects; wings arising from different embryonic origins).

• Important terminology:

  • Descent with modification: Darwin’s term for how new species arise from ancestral ones via heritable variation and differential survival and reproduction.

  • Adaptation, natural selection, and fitness (as a concept of reproductive success).

• Connections to broader biology: ties to genetics, ecology, paleontology, embryology, and molecular biology; forms a foundation for understanding speciation and diversity.

18.1 Evidence, Misconceptions, and Key Concepts (continued)

• Evidence types and examples:

  • Fossils show progression of form over millions of years; fossil records for humans and horses illustrate gradual changes; whale flippers share morphology with bird and mammal appendages, implying a common ancestor.

  • Anatomy and embryology: homologous structures (e.g., bones in human, dog, bird, and whale limbs) arise from common ancestral origins; vestigial structures (wings of flightless birds, nonfunctional leaves on some cacti, hind limb bones in whales) show historical function.

  • Analogy (homoplasy) arises when similar traits evolve independently in distantly related organisms due to similar environmental constraints (e.g., insect wings vs. bat wings).

  • Biogeography: patterns of distribution (Proteaceae in Australia, southern Africa, and South America; marsupial diversification in Australia) reflect historical isolation and continental movements.

  • Molecular biology: DNA sequence similarity tracks descent; gene duplication events allow protein functional innovation.

• Summary of a central theme: evolution is a historical process that leaves multiple lines of evidence across different levels of biological organization (molecular to ecological).

18.2 Formation of New Species

• Defining species and reproductive isolation:

  • Biological species concept: a species is a group of interbreeding populations that produce fertile offspring; species are distinct when mating between species does not yield fertile offspring.

  • Hybrids can occur between somewhat related species, but hybrids are often sterile or nonviable, preserving species boundaries.

  • Gene pool concept: populations share a gene pool; heritable variation is key to evolutionary potential.

  • Reproduction requires heritable variation; only traits with a genetic basis can be passed to offspring.

• Speciation as a branching process:

  • Darwin’s diagram in On the Origin of Species conceptualized speciation as branching events creating diversity.

  • Two main pathways to speciation:

  • Allopatric speciation (allo- = other, -patric = homeland): geographic separation of populations leading to divergence and reproductive isolation.

  • Sympatric speciation (sym- = same, -patric = homeland): speciation within a single, shared geographic area.

• Allopatric speciation details:

  • Gene flow is disrupted by geographic barriers (rivers, mountains, dispersal to new areas); populations diverge genetically due to mutation, genetic drift, and local adaptation by natural selection.

  • Dispersal vs vicariance: dispersal is tiny group movement to establish a new population; vicariance is a geographic barrier that physically splits a population.

  • Examples: northern spotted owl vs Mexican spotted owl (geographically separate populations with distinct adaptations) illustrate allopatric speciation.

  • Greater geographic distance often increases the likelihood of speciation due to differing environments and limited gene flow.

• Adaptive radiation and island examples:

  • Adaptive radiation: single ancestral species gives rise to many descendant species each adapted to a different niche; island systems (e.g., Hawaiian honeycreepers) are classic examples.

  • Darwin’s finches are another example of adaptive radiation in archipelagos, with beak shapes tailored to food sources.

  • Island archipelagoes like Hawaii promote isolation and diversification, leading to many closely related species with distinct ecologies.

• Sympatric speciation mechanisms (non-geographic barriers):

  • Polyploidy (common in plants):

  • Autopolyploidy: same species yields individuals with extra chromosome sets (e.g., 2n → 4n), leading to immediate reproductive isolation.

  • Allopolyploidy: hybridization between two species followed by chromosome doubling, producing a new, fertile lineage.

  • Examples include cultivated wheat, cotton, and tobacco; polyploidy accounts for a substantial fraction of plant diversity.

  • Other sympatric mechanisms: disruptive selection and niche partitioning can drive divergence even in the same habitat (e.g., fish diverging by depth, feeding niches).

• Prezygotic and postzygotic barriers:

  • Prezygotic barriers prevent fertilization: temporal isolation (breeding seasons differ), habitat isolation (different habitats), behavioral isolation (courtship differences), gametic isolation (sperm/egg incompatibility), and mechanical isolation (reproductive structures incompatible).

  • Postzygotic barriers occur after fertilization: hybrid inviability (zygote fails to develop), hybrid sterility (offspring infertile), and hybrid breakdown in subsequent generations.

• Examples and case studies:

  • Lake Apoyeque cichlid population in Nicaragua shows sympatric divergence into two morphs with different diets within the same lake.

  • Lake Victoria cichlids illustrate rapid sympatric speciation with hundreds of lineages arising in a relatively short time.

19.2 Population Genetics

• Population variation and genetic basis:

  • Polymorphisms: populations can be polymorphic with two or more variants of a trait.

  • Heritable variation: only heritable traits can respond to natural selection; acquired traits are not generally passed on.

  • Genetic variance vs phenotypic variance: genetic variance underpins heritable variation; phenotype results from the interaction of genotype with the environment.

• Key evolutionary forces that alter allele frequencies:

  • Mutation: ultimate source of new alleles; creates genetic variation.

  • Sexual reproduction: recombination of alleles creates novel genotypes.

  • Gene flow: migration of individuals or gametes introduces new alleles to populations.

  • Genetic drift: random changes in allele frequencies, especially strong in small populations; includes bottlenecks (random loss of diversity after a disaster) and founder effects (new populations founded by a small subset of individuals).

  • Nonrandom mating: assortative mating and other mating biases alter genotype frequencies without changing allele frequencies directly.

  • Natural selection: acts only on heritable variation; increases frequency of beneficial alleles and decreases deleterious ones.

• Heritability and genetic variance:

  • Heritability is the fraction of phenotypic variation due to genetic differences among individuals in a population.

  • High heritability implies that evolutionary forces (especially natural selection) can more effectively act on the variation.

  • Inbreeding depression: mating among relatives increases homozygosity, elevating deleterious recessive traits.

• Population genetics in action (conceptual examples from the text):

  • Small population size makes genetic drift more powerful; a single death in a small population can dramatically shift allele frequencies.

  • Bottleneck effect: a drastic, random reduction in population size that changes genetic structure.

  • Founder effect: a new population founded by a small number of individuals whose gene pool differs from the original population (example: certain mutations common in Afrikaner populations).

• Linkage and constraints:

  • Not all evolution is adaptive; drift and gene flow can introduce deleterious alleles.

  • Alleles can be linked (linkage disequilibrium), so selection on one allele affects others nearby on the chromosome.

• Population genetics equations (key ideas):

  • Allele frequencies satisfy p + q = 1, where p is the frequency of one allele and q of the other.

  • Hardy–Weinberg expectations for genotype frequencies in a non-evolving population: p^2 + 2pq + q^2 = 1 and p + q = 1.

  • These equations describe expected genotype frequencies if there is no evolutionary force acting.

19.3 Adaptive Evolution

• Natural selection shapes populations by increasing alleles that enhance survival and reproduction and decreasing deleterious alleles.

• Fitness concepts:

  • Darwinian fitness refers to an organism’s contribution to the gene pool of the next generation.

  • Relative fitness compares the reproductive success of different genotypes within a population.

• Modes of natural selection:

  • Stabilizing selection: favors intermediate phenotypes and reduces variation; example: a woodland mouse population with coat color blending into the background.

  • Directional selection: shifts the population toward one extreme; example: peppered moths shifting toward darker morphs when pollutants darken trees; modern debates question specific historical details but illustrate the directional concept.

  • Diversifying (disruptive) selection: favors multiple extreme phenotypes over intermediates; example: male morphs with different mating strategies; can increase overall genetic variance.

  • Frequency-dependent selection: fitness of phenotypes depends on their frequency; positive frequency-dependent selection reduces variation by favoring common phenotypes; negative frequency-dependent selection increases variation by favoring rare phenotypes (example: side-blotched lizards with color morphs).

  • Sexual selection: differences between males and females (sexual dimorphism) in traits that improve reproductive success; can lead to traits that reduce survival (e.g., peacock tail) but increase mating success; theories include handicap principle and good genes hypothesis; Fisher’s runaway model discusses selection driven by female preference.

• Non-adaptive evolution:

  • Not all evolution is adaptive; drift, gene flow, and other forces can introduce deleterious alleles or reduce variation.

  • Environment and genetic architecture constrain what can evolve; some trait combinations are effectively blocked by intermediate less-fit phenotypes.

• Limits of natural selection:

  • It cannot create perfect organisms; it works with existing variation and is constrained by linkage and genetic correlations among traits.

  • Trade-offs exist; selection acts on the net effect of multiple loci and alleles, not on isolated traits.

• Practical takeaways:

  • Evolution is not goal-directed; environments, populations, and genetic variation determine which traits persist.

  • Evolutionary changes reflect current environmental conditions and genetic variation, which may change over time.

20.1 Organizing Life on Earth

• Taxonomy and systematics:

  • Phylogeny: evolutionary history and relationships among organisms.

  • Systematics combines data from fossils, morphology, and molecular biology (DNA) to infer phylogenies.

  • Phylogenetic trees are hypotheses; they may change with new data.

  • Rooted trees show a common ancestor at the base; unrooted trees show relationships but not a designated common ancestor.

• Phylogenetic concepts:

  • Root, branch points (nodes), basal taxon, sister taxa, polytomy.

  • Clades: groups derived from a single ancestor; a monophyletic group includes all descendants from a common ancestor.

  • Cladistics organizes traits using homologous characters and derives evolutionary relationships.

  • Shared characters: ancestral vs. derived; shared ancestral characters are traits common to a broader clade; shared derived characters arose in a particular lineage and define clades.

• Taxonomic levels and binomial nomenclature:

  • Hierarchical levels (from broad to specific): domains, kingdoms, phyla, classes, orders, families, genera, species.

  • The common dog example: Canis lupus familiaris; binomial nomenclature uses genus and species, italicized; subspecies denotes geographic or other isolation.

  • The eight-term taxonomic path for Canis lupus familiaris (domain to species) illustrates hierarchical organization.

• Contemporary phylogeny and taxonomy:

  • DNA data have refined classifications; some earlier groupings were revised as molecular data became available.

  • Phylogenies guide the understanding of evolutionary relationships and the tree of life.

• Practical connections:

  • Phylogeny informs fields from botany to medicine; DNA markers can identify relationships and potential medicinal properties in plants.

  • Maximum parsimony: the principle of choosing the simplest explanation with the fewest major evolutionary events consistent with the data.

20.2 Determining Evolutionary Relationships

• Evidence sources for phylogeny:

  • Morphologic (form and function) and genetic data; homologous traits indicate common ancestry; analogous traits indicate convergent evolution.

  • Molecular data (DNA sequences) complement and refine morphology-based classifications.

• Homology vs analogy (homoplasy):

  • Homologous structures share embryonic origin and developmental pathways, suggesting common ancestry (e.g., bat and bird wings are homologous as forelimbs, yet their wings are not structurally the same in origin).

  • Analogous structures perform similar functions but do not share a recent common ancestor (e.g., insect wings vs. bat wings).

• Molecular systematics:

  • DNA data reveal correlations and sometimes surprising relationships; high similarity does not always imply close relatedness due to convergent sequences or molecular changes.

  • In some cases, very close relatives may appear unrelated due to frame shifts or indels changing reading frames; molecular analyses require careful interpretation.

• Building phylogenies with cladistics:

  • Groups organisms into clades (monophyletic groups) based on shared ancestry.

  • Maximum parsimony guides the selection of the simplest, most plausible evolutionary pathway consistent with the data.

• Practical implications:

  • Phylogeny informs taxonomy, comparative biology, and biomedical research by highlighting evolutionary relationships and shared traits.

33.1 Animal Form and Function

• Body plans and symmetry:

  • Animals can be asymmetrical (e.g., sponges), radially symmetric (e.g., sea anemone), or bilaterally symmetric (e.g., goat).

  • Terms: anterior (front), posterior (rear), dorsal (toward back), ventral (toward the belly).

• Size and shape limits; fusiform shape in aquatic animals reduces drag and enhances swimming speed.

• Speeds (examples from Table 33.1):

  • Cheetah: 113 ext{ km/h}; Quarter horse: 77 ext{ km/h}; Fox: 68 ext{ km/h}; Shortfin mako shark: 50 ext{ km/h}; Human: 45 ext{ km/h}; Dolphin: 32 ext{–}40 ext{ km/h}; Mouse: 13 ext{ km/h}; Snail: 0.05 ext{ km/h}.

• Exoskeletons and endoskeletons:

  • Exoskeletons (arthropods, e.g., insects, spiders, crustaceans) protect against predators and water loss and serve as muscle attachment sites (apodemes).

  • Growth requires molting; thickness must increase with size; endoskeletons are more efficient for larger sizes because muscles attach externally.

• Diffusion limits and multicellularity:

  • Diffusion limits on size; surface-to-volume ratio decreases as size increases; multicellularity and organ systems overcome diffusion limits by specialization (circulatory, respiratory systems).

• Body mass, metabolism, and heat: energy flow and metabolic rate (BMR/SMR) relate to body size and activity levels; smaller animals have higher mass-specific metabolic rates; energy content of foods: carbohydrates/proteins ~ 4.5 ext{–}5 ext{ kcal/g}; fat ~ 9 ext{ kcal/g}.

• Temperature regulation and torpor:

  • Torpor as a response to temperature or resource scarcity; hibernation (long-term torpor) and estivation (summer torpor); daily torpor in some species.

• Planes and cavities in vertebrates:

  • Planes (sagittal, midsagittal, frontal/coronal, transverse/axial) for describing body sections.

  • Major body cavities: dorsal (cranial, vertebral) and ventral (thoracic, abdominopelvic); diaphragm separates thoracic and abdominopelvic cavities.

33.2 Animal Primary Tissues

• Four primary tissue types:

  • Epithelial: covers body surfaces and lines lumens; classified by cell shape (squamous, cuboidal, columnar) and layers (simple, stratified, pseudostratified, transitional).

  • Connective: matrix-based tissues with cells (fibroblasts, chondrocytes, osteocytes, etc.) and fibers (collagen, elastic, reticular); includes loose areolar, dense fibrous, cartilage, bone, adipose, and blood.

  • Muscle: smooth, skeletal, and cardiac; distinctions include striations, nuclei number/location, involuntary vs. voluntary control, and location.

  • Nervous: neurons and glial cells; neurons consist of soma, dendrites (input), and axons (output); glial cells provide support and insulation (astrocytes, oligodendrocytes).

• Epithelial subtypes and functions (selected examples):

  • Squamous epithelia: diffusion (simple) and protection (stratified) in locations like lungs, capillaries, skin, mouth.

  • Cuboidal epithelia: glandular secretion and kidney tubule function (simple cuboidal).

  • Columnar epithelia: absorption in digestive tract; goblet cells secrete mucus; pseudostratified columnar with cilia in respiratory tract.

  • Transitional epithelia: can stretch (urinary bladder) and change thickness with filling.

• Connective tissues:

  • Loose/areolar: surrounds vessels, anchors epithelia; fibers include collagen and elastin.

  • Dense fibrous: tendons and ligaments (regular) or dermis (irregular).

  • Cartilage: hyaline (glassy matrix, lacunae, chondrocytes), fibrocartilage (high collagen content), elastic cartilage (elastic fibers in matrix).

  • Bone: organic matrix (collagen, elastin) and inorganic matrix (calcium salts) giving hardness; cells: osteoblasts, osteocytes, osteoclasts; two types: compact (osteons) and spongy (trabeculae).

  • Adipose tissue: adipocytes store triglycerides; acts as insulation and cushioning.

  • Blood: liquid matrix (plasma) with erythrocytes and leukocytes; platelets for coagulation.

• Muscular tissues:

  • Smooth muscle: non-striated; single central nucleus; involuntary control; walls of vessels and organs.

  • Skeletal muscle: striated; multiple peripheral nuclei; voluntary control; attached to bones.

  • Cardiac muscle: striated with single central nucleus; intercalated discs; involuntary but modulated by autonomic nervous system; specialized conduction system.

• Nervous tissue:

  • Neurons for signaling; glial cells for support; astrocytes regulate extracellular environment; oligodendrocytes insulate axons (myelin).

33.3 Homeostasis

• Homeostasis defined as dynamic equilibrium (steady state) in the body, maintaining values around set points.

• Basic control loop components:

  • Receptor detects a change (stimulus).

  • Control center (often brain) processes signal.

  • Effector (muscle or gland) executes response to restore set point.

• Negative feedback vs positive feedback:

  • Negative feedback: response reduces deviation from set point (e.g., blood glucose regulation by insulin and glucagon; calcium regulation by PTH).

  • Positive feedback: response reinforces the stimulus; examples include blood coagulation cascade and uterine contractions during childbirth.

• Set point and acclimatization:

  • Set point can be adjusted (e.g., elevated blood pressure leads to a higher maintained level).

  • Acclimatization: physiological adjustments to environmental changes (e.g., higher altitude increasing red blood cell counts).

• Thermoregulation and the nervous system:

  • Hypothalamus acts as the central thermostat; reflexes regulate vasodilation/constriction and sweating/shivering.

  • Pyrogens from pathogens can raise body temperature (fever) to enhance immune response.

• Endothermy vs ectothermy:

  • Endotherms (homeotherms) maintain relatively constant body temperature via internal heat production (metabolic processes) and insulation; higher metabolic rate with heat production.

  • Ectotherms rely on environmental heat sources; lower energy expenditure but more temperature-sensitive.

• Heat exchange mechanisms:

  • Radiation, evaporation, convection, conduction.

• Heat conservation strategies:

  • Insulation (fur, fat, feathers); countercurrent heat exchange; vasomotor responses; behavioral adaptations (seeking shade, burrowing).

• Note on energy and activity:

  • Metabolic rate (BMR for endotherms; SMR for ectotherms) describes energy use at rest (or standardized conditions); higher activity increases energy demand.

39.1 Systems of Gas Exchange

• Primary function: deliver O2 to tissues; remove CO2 waste.

• Gas exchange strategies across life forms reflect size and ecology:

  • Diffusion suffices for very small or flattened organisms; diffusion distance limits growth in single cells or thin bodies.

  • Larger organisms require specialized respiratory systems and circulatory systems.

• Direct diffusion:

  • Effective for organisms <1 mm thick; cells near external environment; example: flatworms.

• Skin and gills:

  • Some animals (e.g., earthworms, amphibians) respire via skin; gills in aquatic organisms provide large surface area for gas exchange with water.

  • Water contains lower O2 than air; gill surfaces designed to maximize diffusion; countercurrent exchange enhances O2 uptake.

• Insect tracheal system:

  • Insects respire via a network of tracheae and tracheoles; spiracles allow air entry; gas exchange largely independent of circulatory system.

• Mammalian respiratory system:

  • Pulmonary ventilation: inhalation and exhalation; air is warmed and humidified in nasal cavity; mucus traps particulates; cilia move mucus to the pharynx.

  • Trachea and bronchi: reinforced by incomplete cartilaginous rings; lined with mucus and ciliated epithelia; bronchial smooth muscle regulates airflow via constriction/dilation.

  • Lungs: trachea divides into bronchi, bronchioles, and alveolar sacs; alveoli are the gas-exchange units; approximately 300 million alveoli per lung; total surface area ~ 75 ext{ m}^2; alveolar walls are thin to allow diffusion to capillaries.

• Gas exchange physiology:

  • Oxygen diffuses from alveoli into blood; CO2 diffuses from blood into alveoli for exhalation; intimate relationship between respiratory and circulatory systems.

• Protective mechanisms:

  • Nasal hairs and mucus trap particulates; mucus and cilia in bronchial passages move debris to the pharynx; smoking damages cilia and increases mucus production.

39.4 Transport of Gases in Human Bodily Fluids

• Oxygen transport in blood:

  • Only about 1.5 ext{%} of oxygen is dissolved in plasma; ~ 98.5 ext{%} is bound to hemoglobin (Hb) in red blood cells.

  • Hemoglobin structure: tetramer with four heme groups (each binds one O2); 2 α and 2 β subunits.

  • Oxygen binding is cooperative; binding of the first O2 facilitates subsequent bindings; saturation is sigmoidal with respect to PO2.

  • Factors affecting Hb-O2 affinity: pH (Bohr effect), CO2 levels, temperature, and certain diseases (e.g., sickle cell anemia, thalassemia).

  • Left shift: higher affinity (more saturated at a given PO2); Right shift: lower affinity (less saturated at same PO2).

• CO2 transport and bicarbonate buffering:

  • CO2 transport mechanisms: dissolved CO2 in plasma (~5–7%), carbaminohemoglobin bound to Hb (~10%), and bicarbonate in plasma (approx. 85%).

  • In RBCs, CO2 + H2O ⇌ H2CO3 via carbonic anhydrase; H2CO3 ⇌ HCO3− + H+. The bicarbonate is exchanged with Cl− (chloride shift) and moves into plasma; at the lungs, the reaction reverses and CO2 is expelled.

  • Hemoglobin buffers H+ to moderate pH changes.

40.3 Mammalian Heart and Blood Vessels

• Overview of circulatory circuits:

  • Three circuits: systemic, pulmonary, and coronary; the systemic circuit supplies the body, the pulmonary circuit handles gas exchange in the lungs, and the coronary circuit nourishes the heart itself.

  • The heart pumps blood through all circuits; right heart handles deoxygenated blood to lungs; left heart handles oxygenated blood to the body.

• Heart structure and function:

  • Four chambers: right atrium, right ventricle, left atrium, left ventricle.

  • Valves ensure unidirectional flow: tricuspid (right AV), mitral/bicuspid (left AV), pulmonary semilunar, aortic semilunar.

  • The heart walls are composed of three layers: epicardium, myocardium (cardiac muscle), and endocardium.

  • Pericardium surrounds the heart and reduces friction; coronary arteries supply the heart muscle with oxygenated blood; coronary veins drain deoxygenated blood.

• The cardiac cycle and conduction system:

  • Cardiac cycle consists of systole (contraction) and diastole (relaxation); atrial contraction precedes ventricular contraction.

  • Electrical conduction pathway: SA node → AV node (pause ~0.1 s) → bundle of His → bundle branches → Purkinje fibers; coordinated contraction generates the heartbeat.

  • ECG records electrical activity of the heart; the SA node establishes the pace; the AV node coordinates timing; abnormalities can indicate conduction problems.

• Blood vessels and blood flow:

  • Arteries carry blood away from the heart; veins return blood to the heart; capillaries are the sites of nutrient, gas, and waste exchange.

  • Major arteries include the aorta and its branches (e.g., carotid, brachial, iliac); capillaries form networks that connect arterioles to venules.

  • Veins have valves to prevent backflow and rely on skeletal muscle contraction and respiratory movements to help return blood to the heart.

• Vessel structure and function:

  • Three tunics (layers) form the walls of arteries and veins: tunica intima (inner), tunica media (middle smooth muscle), tunica externa (outer connective tissue).

  • Capillaries consist of a single endothelial cell layer (tunica intima) to facilitate diffusion.

  • Arteries have thicker walls and more smooth muscle to withstand higher pressures; veins have thinner walls and lower pressure.

• Pathophysiology and clinical notes:

  • Atherosclerosis: buildup of fatty plaques in coronary arteries, reducing blood flow to the heart muscle and potentially causing a heart attack.

  • Carbon monoxide poisoning: CO binds hemoglobin with higher affinity than O2, reducing O2 transport; treatment often involves 100% oxygen therapy to displace CO.

  • Angina and myocardial infarction: insufficient blood flow to the heart leads to chest pain and potential heart muscle death.

39.1 Systems of Gas Exchange (revisited – key recap and links)

• Recap of diffusion principles, respiratory surface areas, and protective mechanisms against particulates; links to learning resources and animations are provided in the text for deeper understanding.

40.3 (Summary) Key Circulatory System Concepts

• The heart supports three circuits and relies on a coordinated conduction system to maintain rhythmic pumping.

• Blood vessels show specialization (arteries vs. veins vs. capillaries) to optimize pressure, flow, and exchange.

• Gas exchange and transport are tightly linked through the respiratory and circulatory systems; diseases like atherosclerosis or CO poisoning illustrate the clinical relevance of these systems.

• Mathematical and terminological recap:

  • Allele frequencies and Hardy–Weinberg basics: p+q=1,\ ext{} p^2+2pq+q^2=1. These form the baseline expectations when evolution is not acting.

  • Fitness and population genetics concepts are often quantified with relative measures (e.g., relative fitness values, selection coefficients). A complete treatment would include population-genetic model equations when needed for specific problems.

  • Oxygen binding to Hb is a cooperative process yielding a sigmoid dissociation curve; shifts in the curve reflect changes in pH, CO2, or temperature, altering oxygen delivery to tissues.

• Connections across topics:

  • Evolutionary concepts underpin the diversity of life and the mechanisms by which populations adapt to their environments.

  • The study of tissues, homeostasis, and organ systems provides the mechanistic basis for understanding how organisms maintain stability while adapting to varying ecological niches.

  • Phylogeny and classification contextualize how organisms are related and how shared traits reflect common ancestry, an essential backdrop for comparative biology.

• Key terms to review (quick list):

  • adaptation, divergent/convergent evolution, homologous/vestigial structures, misperceptions about evolution, speciation (allopatric/sympatric), adaptive radiation, polymorphism, genetic drift, bottleneck, founder effect, gene flow, mutation, nonrandom mating, environmental variance, stabilizing/directive/diversifying/frequency-dependent/sexual selection, fitness, Hardy–Weinberg, monophyly/clades, maximum parsimony, homologous vs analogous traits, amniotic egg, clades, synapomorphy, endothermy/ectothermy, diffusion, tracheal system, alveoli, hemoglobin, Bohr effect, bicarbonate buffering, chloride shift, conduction system of the heart, ECG, etc.