CHAPTER 22 EVOLUTION

Evolution is a heritable change in one or more characteristics of a population from one generation to the next. This process not only alters the characteristics of populations, it also leads to the formation of new species.

We will begin this unit by considering the fundamental concepts of evolution, with an emphasis on natural selection (Chapter 22). We will examine observations of evolutionary change, which include (1) the fossil record, (2) a comparison of the characteristics of modern species, and (3) an analysis of molecular data. Chapter 23 continues our discussion of evolution at the molecular level and focuses on how changes in allele and genotype frequencies from one generation to the next are driven by a variety of different factors. By comparison, Chapter 24 shifts the emphasis of evolution to the level of species. We will examine how species are identified and discuss the mechanisms by which new species arise via evolution. In Chapter 25, we will examine how biologists determine the evolutionary relationships among different species and produce diagrams called evolutionary trees to describe those relationships. Finally, in Chapter 26, we will examine a timeline for the evolution of species from 4 billion years ago to the present and consider the topic of human evolution.

The following Core Concepts and Core Skills will be emphasized in this unit:

• Evolution: This concept will be emphasized throughout the entire unit.

• Information: As discussed in Chapters 22 and 23, evolution involves changes in genes.

• Systems: Living organisms interact with their environment. As discussed in Chapters 22 through 24, natural selection is a process in which certain individuals have greater reproductive success. This success is often due to their ability to survive in a given environment.

• Structure and Function: Chapters 22 and 24 will also consider how structural features change during the evolution of new species. Such changes are related to changes in function.

• Process of Science: Every chapter has a Feature Investigation describing a pivotal experiment that provided insights into our understanding of evolution.

• Quantitative Reasoning: Chapter 23 focuses on changes in allele and genotype frequencies in populations.

• Modeling: Every chapter has a Modeling Challenging to help you refine this important skill.

• Organic life beneath the shoreless waves

• Was born and nurs’d in Ocean’s pearly caves

• First forms minute, unseen by spheric glass,

• Move on the mud, or pierce the watery mass;

• These, as successive generations bloom,

• New powers acquire, and larger limbs assume;

• Whence countless groups of vegetation spring,

• And breathing realms of fin, and feet, and wing.

By Erasmus Darwin, grandfather of Charles Darwin.

Published posthumously in 1803.

The term biological evolution, or simply evolution, is used to describe a heritable change in one or more characteristics of a population from one generation to the next. Evolution can be viewed on different scales. Microevolution involves changes in a single gene or in allele frequencies in a population over time. Macroevolution refers to the formation of a new species or groups of related species.

In the first section of this chapter, we will examine the development of evolutionary thought and some of the basic tenets of evolution, particularly those proposed by the British naturalists Charles Darwin and Alfred Wallace in the mid-1800s. The theory of evolution has been refined over the past 170 years or so, but the fundamental principle of evolution remains unchanged and has provided a cornerstone for our understanding of biology. 

Ukrainian-born American geneticist Theodosius Dobzhansky, an influential evolutionary scientist of the 1900s, once said, “Nothing in biology makes sense except in the light of evolution.” The extraordinarily diverse and often seemingly bizarre array of species on our planet can be explained within the context of evolution. As is the case with all scientific theories, evolution is called a theory because it is supported by a substantial body of evidence and because it explains a wide range of observations. The theory of evolution provides answers to many questions related to the diversity of life. In biology, theories such as this are viewed as scientific knowledge.

In the second section of this chapter, we will survey the extensive data that illustrate the processes by which evolution occurs. These data not only support the theory of evolution but also allow us to understand the interrelatedness of different species, whose similarities are often due to descent from a common ancestor. Much of the early evidence supporting evolution came from direct observations and comparisons of living and extinct species. More recently, advances in molecular genetics, particularly those related to DNA sequencing and genomics, have revolutionized the study of evolution. Scientists now have information that allows us to understand how evolution involves changes in the DNA sequences of a given species. These changes affect both a species’ genes and the proteins they encode. 

Molecular evolution refers to the process of evolution at the level of genes and proteins. Comparisons of gene or protein sequences in different organisms can reveal evolutionary relationships that cannot be seen in morphology. A major focus of this textbook is to provide an understanding of these changes. In the last section of this chapter, we consider some of the exciting new ways of exploring evolutionary change at the molecular level. In the following chapters of this unit, we will examine how such changes are acted upon by evolutionary factors in ways that alter the traits of a given species and may eventually lead to the formation of new species.

Undoubtedly, the question “Where did we come from?” has been asked and debated by people for thousands of years. Many of the early ideas regarding the existence of living organisms were strongly influenced by religion and philosophy. Some of these ideas suggested that all forms of life have remained the same since their creation. In the 1600s, however, scholars in Europe began a revolution that created the basis of empirical and scientific thought. Empirical thought relies on observation to form an idea or hypothesis rather than trying to understand life from a nonphysical or spiritual point of view. As described in this section, the shift toward empirical thought encouraged scholars to look for the basic rationale behind a given process or phenomenon. This perspective played a key role in the development of the theory of evolution.

For our discussion of evolution, let’s begin with a working definition of a species. Biologists often define a species as a group of related organisms that share a distinctive form. Among species that reproduce sexually, such as plants and animals, members of the same species are capable of interbreeding in nature to produce viable and fertile offspring. The term population refers to all members of a species that live in the same area at the same time and have the opportunity to interbreed. As we will see in Chapter 24, some of the emphasis in the study of evolution is on understanding how populations change over the course of many generations to produce new species.

The Work of Several Scientists Set the Stage for the Ideas of Darwin and Wallace

In the mid- to late-1600s, the first scientist to carry out a thorough study of the living world was an English naturalist named John Ray, who developed an early classification system for plants and animals based on anatomy and physiology. He established the modern concept of a species, noting that organisms of one species do not interbreed with members of another, and used it as the basic unit of his classification system. Ray’s ideas on classification were later expanded by the Swedish naturalist Carolus Linnaeus. How did their work contribute to the development of evolutionary theory? Neither Ray nor Linnaeus proposed that evolutionary change promotes the formation of new species. However, their systematic classification of plants and animals helped scholars of this period perceive the similarities and differences among living organisms.

Late in the 1700s, a small number of European scientists began to quietly suggest that life-forms are not fixed and unchanging. A French zoologist, Georges Buffon, actually proposed that populations of living things change through time. However, Buffon was careful to hide his views in a 44-volume series of books on natural history. Around the same time, a French naturalist named Jean-Baptiste Lamarck suggested an intimate relationship between variation and evolution. By examining fossils, he realized that some species had remained the same over the millennia but others had changed. Lamarck popularized the view that species change over the course of many generations by adapting to new environments. He advocated the idea that living things evolved in a continuously upward direction, from dead matter, through simple to more complex forms, toward “human perfection.” 

According to Lamarck, organisms altered their behavior in response to environmental changes. He thought that behavioral changes could modify traits and hypothesized that these modified traits were inherited by offspring. He called this idea the inheritance of acquired characteristics. For example, according to Lamarck’s hypothesis, giraffes developed their elongated necks and front legs by feeding on the leaves at the top of trees. The exercise of stretching up to the leaves altered the neck and legs, and Lamarck presumed that these acquired characteristics were transmitted to offspring. However, further research has rejected Lamarck’s idea that most acquired traits can be inherited. (Note: An acquired trait can sometimes be transmitted from parent to offspring via epigenetic changes, which are described in Chapter 18.) Lamarck’s work was important in promoting the idea of evolutionary change.

Interestingly, Erasmus Darwin, the grandfather of Charles Darwin, was a contemporary of Buffon and Lamarck and an early advocate of evolutionary change. He was a physician, a plant biologist, and also a poet (see the poem at the beginning of the chapter). He was aware that modern species were different from similar types of fossilized organisms, and he noted how plant and animal breeders used breeding practices to change the traits of domesticated species (see the chapter opening photo). He knew that offspring inherited features from their parents and went so far as to say that life on Earth could have descended from a common ancestor.

Darwin and Wallace Proposed That Existing Species Are Derived from Pre-existing Species

Darwin and Wallace played a central role in developing the foundation of evolutionary biology: that existing species have evolved from pre-existing ones. Their unique perspectives and ability to formulate evolutionary theory were shaped by several different fields of study, including ideas of his time about geology and population growth, as well as his own observations.

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Hypotheses about Geology

Two main hypotheses about geological processes predominated in the early 19th century: 

1. Catastrophism was first proposed by French zoologist and paleontologist Georges Cuvier. He suggested that the Earth has largely been shaped by sudden, violent events, which may have been worldwide in scope. Many of his contemporaries hypothesized that Earth was about 6,000 years ago. This idea fit well with certain religious teachings. However, catastrophism fit better with a much older age for Earth. 

2. Uniformitarianism, proposed by Scottish geologist James Hutton and popularized by another Scottish geologist, Charles Lyell, suggested that changes in the Earth are directly caused by recurring events. For example, they suggested that geological processes such as erosion existed in the past and happened at the same gradual rate as they do now. For such slow geological processes to lead eventually to substantial changes in the Earth’s characteristics, a great deal of time is required. The ideas of Hutton and Lyell helped to shape Darwin’s view of the world.

Population Growth

An Essay on the Principle of Population by Thomas Malthus, an English economist, asserted that the population size of humans can, at best, increase linearly due to increased land usage and improvements in agriculture, whereas our reproductive potential is exponential (for example, doubling with each generation). He argued that famine, war, and disease, especially among the poor, keep population growth within existing resources. The relevant message from Malthus’s work was that not all members of any population will survive and reproduce. After becoming experienced naturalists, Darwin and Wallace became aware of Malthus’s work, and it influenced their thinking.

Voyage of the Beagle

Darwin’s and Wallace’s ideas about evolution were largely initiated by their experiences and observations as naturalists. Darwin's work as a young man aboard the HMS Beagle, a survey ship, lasted from 1831 to 1836 and involved a careful examination of many different species (Figure 22.1). The main mission of the Beagle was to map the coastline of southern South America and take oceanographic measurements. As the ship’s naturalist, Darwin’s job was to record information about the weather, geological features, plants, animals, fossils, rocks, minerals, and indigenous people.

Though Darwin made many interesting observations on his journey, he was particularly struck by the distinctive traits of island species compared to mainland species. For example, Darwin observed several species of finches found on the Galápagos Islands, a group of volcanic islands 600 miles from the coast of Ecuador. Though it is often assumed that Darwin’s personal observations of these finches directly inspired his theory of evolution, this is not the case. Initially, Darwin thought the birds were various species of blackbirds, grosbeaks, and finches. Later, however, the bird specimens from the islands were given to British ornithologist John Gould, who identified them as several new finch species. Gould’s observations helped Darwin in the later formulation of his theory.

As seen in Table 22.1, the finches differ widely in the size and shape of their beaks and in their feeding habits. Darwin clearly saw the similarities among these species, yet he noted the differences that provided them with specialized feeding strategies. We now know that all of these finches evolved from a single species similar to the dull-colored grassquit finch (Tiaris obscura), commonly found along the Pacific coast of South America. Once they arrived on the Galápagos Islands, the finches’ ability to obtain particular types of food in their new habitat depended, in part, on the relative sizes and shapes of their beaks, which, in turn, influenced their abilities to survive and reproduce.

Similarly, Wallace also was involved with specimen collection activities, but in the Malay archipelago, which now comprises Indonesia, Malaysia, and Singapore. In 1855, he published On the Law Which Has Regulated the Introduction of New Species, which was based on his observations of the variation of characteristics of species on different islands.

Darwin and Wallace Arrived at the Same Conclusions

With an understanding of geology and population growth, and his observations from his voyage on the Beagle, Darwin had formulated his theory of evolution by the mid-1840s. He had also catalogued and described all of the species he had collected on his Beagle voyage except for one type of barnacle. Some have speculated that Darwin may have felt that he should establish himself as an expert on one species before making generalizations about all of them. Therefore, he spent several additional years studying barnacles. During this time, the geologist Charles Lyell, who had greatly influenced Darwin’s thinking, strongly encouraged Darwin to publish his work on evolution. 

In 1856, Darwin began to write a long book to explain his ideas. In 1858, however, Wallace sent Darwin an unpublished manuscript to read prior to its publication. In it, Wallace proposed the same ideas concerning evolution. In response to this, Darwin decided to use some of his own writings on this subject, and two papers, one by Darwin and one by Wallace, were published in the Proceedings of the Linnean Society of London. These papers were not widely recognized. A year later, however, Darwin finished his book On the Origin of Species (1859), which described his ideas in greater detail and included observational support. Some of his ideas in this book were incomplete because the genetic basis of traits was not understood at that time. Even so, the work of Darwin and Wallace remains a foundation of our understanding of biology.

Natural Selection Changes Populations from Generation to Generation

Darwin and Wallace hypothesized that existing life-forms on our planet have resulted from the modification of pre-existing life-forms. Darwin expressed this concept of evolution as “the theory of descent with modification through variation and natural selection.” The term evolution refers to change. What factors bring about evolutionary change? According to Darwin’s and Wallace’s ideas, evolution occurs from generation to generation due to two interacting factors, genetic variation and natural selection:

1. Variation in traits may occur among individuals of a given species. Variable aspects of those traits are heritable: They are passed from parent to offspring. For example, finches may vary with regard to beak size, and offspring may inherit larger or smaller beaks. The genetic basis for variation within a species was not understood in the mid-1800s. We now know that such variation is due to different types of genetic changes such as random mutations in genes. Even though Darwin and Wallace did not fully appreciate the genetic basis of variation, they and many other people before them observed that offspring resemble their parents more than they do unrelated individuals. Therefore, Darwin and Wallace assumed that some traits are passed from parent to offspring.

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2. In each generation, many more offspring are usually produced than will survive and reproduce. Often, resources in the environment are limiting for an organism’s survival. During the process of natural selection, individuals with heritable traits that make them better suited to their native environment tend to flourish and reproduce, whereas other individuals are less likely to survive and reproduce. As a result of natural selection, certain traits that favor reproductive success become more prevalent in a population over time.

As an example, we can consider a population of finches that migrates from the South American mainland to a distant island (Figure 22.2). Variation exists in the beak sizes among the migrating birds. Furthermore, variation in beak size is heritable—it can be passed from parents to offspring. Parents with larger beak sizes tend to have offspring with larger beaks, and parents with smaller beak sizes tend to have offspring with smaller beaks.

Let’s suppose the seeds produced on the distant island are larger than those produced on the mainland. Those birds with larger beaks would be better able to feed on these larger seeds and therefore would be more likely to survive and pass that trait to their offspring. What are the consequences of this selection process? In succeeding generations, the population tends to have a greater proportion of finches with larger beaks. 

Alternatively, if a trait happens to be detrimental to an individual’s ability to survive and reproduce, natural selection is likely to eliminate this type of variation. For example, if a finch on the island where the seeds are larger had a small beak, this bird would be less likely to acquire enough food, which would decrease its ability to survive and pass this trait to its offspring. Natural selection may ultimately result in a new species with a combination of multiple traits that are quite different from those of the original species, such as finches with larger beaks and changes in coloration. In other words, the newer species has evolved from a pre-existing one.

Feature Investigation | The Grants Observed Natural Selection in Galápagos Finches

Since 1973, British evolutionary biologists Peter Grant, Rosemary Grant, and their colleagues have studied natural selection in finches found on the Galápagos Islands. For nearly 50 years, the Grants focused much of their work on one of the Galápagos Islands known as Daphne Major (Figure 22.3a). This small island (with an area of 0.34 km²) has a moderate degree of isolation (it is 8 km from the nearest island), an undisturbed habitat, and a resident population of Geospiza fortis, the medium ground finch (Figure 22.3b).

To study natural selection, the Grants observed various traits in finches over the course of many years. One trait they observed is beak size. The medium ground finch has a relatively small, crushing beak, allowing it to feed more easily on small, tender seeds (see Table 22.1). The Grants quantified beak size among the medium ground finches of Daphne Major by carefully measuring beak depth—a measurement of the beak from top to bottom (Figure 22.4). The small size of the island made it possible for them to measure a large percentage of the finches living there and their offspring. During the course of their studies, the Grants compared the beak depths of parents and offspring by examining many broods over several years and found that the depth of the beak was transmitted from parents to offspring, regardless of environmental conditions, indicating that differences in beak depths are due to genetic differences in the population. In other words, they found that beak depth was a heritable trait.

By measuring the beaks of many birds every year, the Grants were able to assemble a detailed portrait of natural selection in action. In the study shown in Figure 22.4, they measured beak depth from 1976 to 1978. In the wet year of 1976, the plants of Daphne Major produced an abundance of the small, tender seeds that these finches could easily eat. However, a severe drought occurred in 1977. During this year, the plants on Daphne Major tended to produce few of the smaller seeds, which the finches rapidly consumed. Therefore, the finches resorted to eating larger, drier seeds, which are harder to crush. As a result, birds with larger beaks were more likely to survive and reproduce because they were better at breaking open the large seeds. As shown in the data, the average beak depth of birds in the population increased substantially, from 8.8 mm in predrought offspring to 9.8 mm in postdrought offspring.

How do we explain these results? According to evolutionary theory, birds with larger beaks were more likely to survive and pass this trait to their offspring. Overall, these results illustrate the power of natural selection to alter the features of a trait—in this case, beak depth—in a given population from one generation to the next.

Learning Outcomes:

1. Summarize the different types of evidence for evolutionary change, including studies of natural selection, selective breeding, biogeography, convergent traits, the fossil record, and homologies.

2. Provide examples of three types of homologies.

Evidence that supports the theory of evolution has been gleaned from many sources (Table 22.2). As we have already seen, the Grants were able to observe changes in a finch population that resulted after a drought. Historically, the first evidence of biological evolution came from studies of the fossil record, the distribution of related species on our planet, selective breeding experiments, and the comparison of similar anatomical features in different species. More recently, additional evidence that illustrates the process of evolution has been found at the molecular level. By comparing DNA sequences from many different species, evolutionary biologists have gained great insight into the relationship between the evolution of species and the associated changes in the genetic material. In this section, we will survey the various types of evidence that show the process of evolutionary change.

Selective Breeding Is a Human-Driven Form of Selection

In the preceding section, we considered how the traits of populations can evolve from one generation to the next by the mechanism of natural selection (see Figure 22.4). The term selective breeding refers to programs and procedures designed to modify traits in domesticated species. This practice, also called artificial selection, is related to natural selection. In forming his theory of evolution, Darwin was influenced by his observations of selective breeding by pigeon breeders. 

The primary difference between natural and artificial selection is how the parents are chosen. Natural selection occurs because of genetic variation in reproductive success. Organisms that are able to survive and reproduce are more likely to pass their genes to future generations. Environmental factors often determine which individuals will be successful parents. In artificial selection, the breeder chooses as parents those individuals with traits that are desirable from a human perspective.

The underlying phenomenon that makes selective breeding possible is genetic variation. Within a population, variation may exist in a trait of interest. For selective breeding to be successful, the underlying cause of the phenotypic variation is typically due to differences in alleles, variant forms of a particular gene, that determine the trait. The breeder chooses parents with alleles that confer desirable phenotypic characteristics. For centuries, humans have employed selective breeding to obtain domesticated species with interesting or agriculturally useful characteristics.

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Artificial Selection in Dogs

The various breeds of dog are the result of selective breeding strategies (Figure 22.5). All dogs are members of the same species, Canis lupus, subspecies familiaris, so they can interbreed to produce offspring. Selective breeding can dramatically modify the traits in a species. When you compare certain breeds of dogs (for example, a greyhound and a dachshund), they hardly look like members of the same species! A study in 2007 by American geneticist Nathan Sutter and colleagues indicated that the size of dogs may be determined by alleles of the Igf1 gene, which encodes a growth hormone called insulin-like growth factor 1. A particular allele of this gene was found to be common to all small breeds of dogs but nearly absent from very large breeds, suggesting that this allele is a major contributor to body size in small breeds of dogs.

Figure 22.5Common breeds of dogs that have been obtained by selective breeding. By selecting individuals carrying the alleles that influence traits desirable to humans, dog breeders have produced breeds with distinctive features. All the dogs in this figure carry the same kinds of genes (for example, genes that affect their size, shape, and fur color). However, the alleles for many of these genes are different among these dogs, thereby allowing dog breeders to select for or against them and produce breeds with strikingly different phenotypes.a: WilleeCole/iStockphoto/Getty Images; b: Martin Rugner/age fotostock; c: Juniors Bildarchiv/age fotostock

Artificial Selection in Agricultural Species

Most of the food we eat—including products such as grains, fruits, vegetables, meat, milk, and juices—is also obtained from species that have been profoundly modified by selective breeding strategies. For example, certain characteristics in the wild mustard plant (Brassica oleracea) have been modified by selective breeding to produce several varieties of domesticated crops, including broccoli, Brussels sprouts, cabbage, and cauliflower (Figure 22.6). The wild mustard plant is native to Europe and Asia, and plant breeders began to modify its traits approximately 4,000 years ago. As seen here, certain traits in the domestic strains differ dramatically from those of the original wild species. These varieties are all members of the same species. They can interbreed to produce viable offspring. For example, in the grocery store you may have seen broccoflower, a vegetable produced from a cross between broccoli and cauliflower.

Figure 22.6Crop plants developed by selective breeding of the wild mustard plant. Although these six agricultural plants look quite different from each other, they carry many of the same alleles as the wild mustard plant. However, they differ from each other in alleles that affect the formation of stems, leaves, and flowers.

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As another example, Figure 22.7 shows the results of a selective breeding experiment on corn begun at the University of Illinois Agricultural Experiment Station in 1896, several years before the rediscovery of Mendel’s laws. This study began with 163 ears of corn with an oil content ranging from 4% to 6%. In each of 80 succeeding generations, corn plants were divided into two separate groups. In one group, members with the highest oil content in the kernels were chosen as parents of the next generation. In the other group, members with the lowest oil content were chosen. After many generations, the oil content in the first group rose to over 18%. In the other group, it dropped to less than 1%. These results show that selective breeding can modify a trait in a very directed manner.

Figure 22.7Results of selective breeding for oil content in corn plants. In this example, corn plants were selected for breeding based on high or low oil content of the kernels. Over the course of many generations, this artificial selection had a major influence on the amount of corn oil (an agriculturally important product) made by the two groups of plants.

Concept Check: When comparing Figures 22.5, 22.6, and 22.7, what general effects of artificial selection do you observe? Answer

Biogeography Indicates That Species in a Given Area Have Evolved from Pre-existing Species

Biogeography is the study of the geographic distribution of extinct and living species. Patterns of past evolution are often found in the natural geographic distribution of related species. From such studies, scientists have discovered that isolated continents and island groups have evolved their own distinct plant and animal communities.

Islands

Islands, which are isolated from other landmasses, provide numerous examples in which geography has played a key role in the evolution of new species. Islands often have many species of plants and animals that are endemic, which means they are naturally found only in a particular location. Most endemic island species have closely related relatives on nearby islands or the mainland.

As an example, let’s consider the island fox (Urocyon littoralis), which lives on the Channel Islands located off the coast near Santa Barbara in southern California (Figure 22.8). This type of fox is found nowhere else in the world. It weighs about 3–6 pounds and feeds largely on insects, mice, and fruits. The island fox evolved from the mainland gray fox (Urocyon cinereoargenteus), which is much larger, usually 7–11 pounds. During the last Ice Age, about 16,000–18,000 years ago, the Santa Barbara channel was frozen and narrow enough for ancestors of the mainland gray fox to cross over to the Channel Islands. When the Ice Age ended, the ice melted and sea levels rose, causing the foxes to be cut off from the mainland. Over the last 16,000–18,000 years, the population of foxes on the Channel Islands evolved into the smaller island fox, which is now considered a different species from the larger gray fox. The gray fox is still found on the mainland.

Figure 22.8The evolution of an endemic island species from a mainland species. (a) The smaller island fox found on the Channel Islands evolved from (b) the gray fox found on the California mainland.a: Kevin Schafer/Getty Images; b: Prisma Bildagentur AG/Alamy stock photo

Concept Check: Explain how geography played a key role in the evolution of the island fox. Answer

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The smaller size of the island fox is an example of island dwarfing, a phenomenon in which the size of large animals on an isolated island shrinks dramatically over many generations. It is the result of natural selection in which a smaller size provides a survival and reproductive advantage, probably because of the limited availability of food and other resources.

Isolated Continents

The evolution of major animal groups is also correlated with known changes in the distribution of landmasses on the Earth. The first mammals arose approximately 200 mya, when the area that is now Australia was still connected to the other continents. However, the first placental mammals, which have a long internal gestation and give birth to well-developed offspring, evolved much later, after continental drift had separated Australia from the other continents (look ahead to Figure 26.5). Except for a few species of mammals that have been introduced into Australia more recently, the natural wildlife of Australia lacks any of the larger, terrestrial placental mammals. How do biologists explain this observation? It is consistent with the idea that placental mammals first arose somewhere other than Australia and the barrier of a large ocean prevented most terrestrial placental mammals from migrating there.

On the other hand, Australia has more than 100 species of kangaroos, koalas, and other marsupials, most of which are not found on any other continent. Marsupials are a group of mammal species whose young are born in a very immature condition and then develop further in the mother’s abdominal pouch, which covers the mammary glands. Evolutionary theory is consistent with the idea that the existence of these unique Australian species is due to their having evolved in isolation from the rest of the world for millions of years.

Convergent Evolution Suggests Adaptation to the Environment

The process of natural selection is also evident in the study of plants and animals that have similar characteristics, even though they are not closely related evolutionarily. This similarity is the result of convergent evolution, in which two species from different lineages have independently evolved similar characteristics, called convergent traits, because they occupy similar environments.

• Both the ruby-throated hummingbird (Archilochus colubris) and the hummingbird moth (Hemaris thysbe) beat their wings rapidly as they hover next to flowers and obtain nectar with a long, thin beak or long proboscis, respectively (Figure 22.9a). Both species independently evolved these adaptations that enable them to feed on flowers. The ruby-throated humming bird is a vertebrate, whereas the hummingbird moth is an invertebrate, so they are not closely related evolutionarily.

• Another example of convergent evolution involves aerial rootlets found in vines such as English ivy (Hedera helix) and wintercreeper (Euonymus fortunei) (Figure 22.9b). Based on differences in their structures, these aerial rootlets appear to have developed independently as an effective means of clinging to the support on which a vine attaches itself.

• A third example of convergent evolution is revealed by the molecular analysis of fishes that live in very cold water. Antifreeze proteins enable certain species of fishes to survive the subfreezing temperatures of Arctic and Antarctic waters by inhibiting the formation of ice crystals in body fluids. Researchers have determined that these fishes are an interesting case of convergent evolution (Figure 22.9c). Among different species of fishes, one of five different genes has independently evolved to produce antifreeze proteins. For example, in the sea raven (Hemitripterus americanus), the antifreeze protein is rich in the amino acid cysteine, and the secondary structure of the protein is a β pleated sheet. In contrast, the antifreeze protein in the longhorn sculpin (Trematomus nicolai) is encoded by an entirely different gene. The antifreeze protein in this species is rich in the amino acid glutamine, and the secondary structure of the protein is largely composed of α helices.

Figure 22.9Examples of convergent evolution. The species in each of the pairs shown in this figure are not closely related evolutionarily but occupy similar environments, suggesting that natural selection results in similar adaptations to a particular environment.a (left): Sari ONeal/Shutterstock; a (right): Sue Bishop/Shutterstock; b (left): mm88/Getty Images; b (right): 2003 Steve Baskauf/bioimages.vanderbilt.edu; c (left): WaterFrame_eda/Alamy Stock Photo; c (right): Andrew J. Martinez/Science Source

Concept Check: Can you think of another example in which two species that are not closely related have a similar adaptation? Answer

The similar characteristics in the examples shown in Figure 22.9—for example, the long, thin beak of the ruby-throated hummingbird and the long, thin proboscis of hummingbird moth—are called analogous structures, or convergent traits. They represent cases in which characteristics have arisen independently, two or more times, because different species have evolved similar structures for survival—in this case, feeding on nectar—in a given environment.

Fossils Show Successive Evolutionary Change

As discussed in Chapter 26, fossils are the preserved remains of past life. The fossil record has provided biologists with evidence of the history of life on Earth. Today, scientists have access to a far more extensive fossil record than was available to Darwin and other scientists of his time. Even though the fossil record is still incomplete, the many fossils that have been discovered provide detailed information regarding evolutionary change in a series of related organisms. When fossils are compared according to their age, from oldest to youngest, successive evolutionary change becomes apparent. Let’s consider a couple of examples in which paleontologists have observed evolutionary change.

Evolution of Terrestrial Tetrapods from Fish

In 2005, fossils of Tiktaalik roseae, nicknamed fishapod, were discovered by paleontologists Ted Daeschler, Neil Shubin, and Farish Jenkins. The discovery of the fishapod illuminated one of several steps that led to the evolution of tetrapods, which are animals with four legs. T. roseae is called a transitional form, because it represents an intermediate state between an ancestral form and the form of its descendants (Figure 22.10). 

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Fishapod is a transitional form between fishes, which have fins for locomotion, and tetrapods, which are four-limbed animals. Unlike a true fish, T. roseae had a broad skull, a flexible neck, and eyes mounted on the top of its head like a crocodile. Its interlocking rib cage suggests it had primitive lungs. Perhaps the most surprising discovery was that its pectoral fins (those on the side of the body) revealed the beginnings of a primitive wrist and five finger-like bones. These appendages would have allowed T. roseae to support its body on shallow river bottoms and lift its head above the water to search for prey and perhaps even move out of the water for short periods. During the Devonian period (417–354 mya), this could have been an important advantage in the marshy floodplains of large rivers.

Figure 22.10A transitional form in the tetrapod lineage. This figure shows two early tetrapod ancestors, a Devonian fish and the transitional form Tiktaalik roseae, as well as one of their descendants, an early amphibian. An analysis of the fossils shows that T. roseae, also known as a fishapod, had both fish and amphibian characteristics, so it was likely able to survive for brief periods out of the water.(middle): Corbin17/Alamy Stock Photo

Core Skill: Modeling  The goal of this modeling challenge is to propose a model that represents a transitional form between dinosaurs and birds.

Modeling Challenge: Maniraptora includes a group of dinosaurs that is thought to have given rise to modern birds. An example of a maniraptora is Falcarius utahensis, shown below. Propose a model that represents a transitional form between dinosaurs and birds. As in Figure 22.10, place the more recent species at the top and the earlier form at the bottom. In this case, copy and paste an image of a turkey at the top and F. utahensis at the bottom. In the middle, draw a model for a transitional form. Next to your model, list four key characteristics of the transitional form.

The early tetrapods gave rise to three main groups of modern tetrapods:

• Amphibians

• Birds and modern reptiles

• Mammals

Most tetrapods occupy a terrestrial environment. However, some, such as frogs and hippopotamuses, are semi-aquatic, spending a large amount of time in the water. Interestingly, tetrapod evolution has produced new species that are aquatic and no longer have hindlimbs. For example, whales evolved from terrestrial mammals, as described next.

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Evolution of Whales from Terrestrial Mammals

Cetacea is an order of aquatic animals that includes whales, dolphins, and porpoises. The closest living relatives of cetaceans are the hippos. Figure 22.11 presents a hypothesis for the evolution of whales based on the fossil record.

• The genus Pakicetus comprised the earliest known whales. However, these animals did not bear a striking resemblance to modern whales. They were wolflike meat eaters that spent some of their time in fresh water and ate fish. Their skull had a long shape like that of modern whales. The eyes were positioned close together and high on the skull, which is characteristic of aquatic animals that peer out of the water. A particularly striking trait was a thick, bony wall around the middle ear, which is found in modern whales but not in other mammals.

• The genus Ambulocetus consisted of semi-aquatic whales of brackish (slightly salty) waters. They were roughly the size of a male sea lion and had short and powerful legs. The tail vertebrae were particularly large, suggesting that the tail was very muscular and possibly used for swimming. The eyes were more toward the sides but still high on the skull.

• The members of the genus Remingtonocetus were similar to those of Ambulocetus but with a longer snout and a fat pad in the jaw that aided in underwater hearing. They lived in saltwater habitats.

• In members of the genus Rodhocetus, the eyes were on the side of the head, and the nasal opening was beginning to shift away from the tip of the snout. The forelimbs had five fingers, and the hindlimbs had only four toes, suggesting the degeneration of the hindlimbs.

• The genus Dorudon was composed of whales that were completely aquatic animals. The nasal opening was shifted back toward the eyes to form a blowhole. The forelimbs became flippers, and the hindlimbs were very tiny. The tail was modified at the end to form a fluke.

• Odontoceti and Mysticeti are suborders of the order Cetacea, which includes many extinct species as well as all modern species of whales, dolphins, and porpoises. These animals show a complete loss of the hindlimbs in the adult. The nasal opening is the blowhole seen in modern whales. In odontocetes, echolocation is used for hunting. In mysticetes, baleen is used for filtering food.

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Figure 22.11Evolution of whales. Pakicetus, Ambulocetus, Remingtonocetus, Rodhocetus, and Dorudon are extinct genera of whales. Odontocetes and Mysticetes are suborders of the order Cetacea, which includes all modern species of whales, dolphins, and porpoises. This simplified representation of whale evolution is a type of diagram called a phylogenetic tree, which is explained in Chapter 25. (Note: The genera described in this phylogenetic tree are not depicted as direct ancestors to modern whales, but they all shared common ancestors.)

Core Concept: Structure and Function This diagram shows the morphological (structural) changes that occurred in the evolution of whales that made them better suited to an aquatic environment.

Click the arrowhead to expand.

Taken together, the changes documented in the fossil record of whales reveal a progression over the past 50 million years from a terrestrial tetrapod to aquatic animals that lack hind limbs and have many adaptations that are beneficial in an aquatic environment.

A Comparison of Homologies Shows Evolution of Related Species from a Common Ancestor

Let’s now consider other widespread observations of the process of evolution among living organisms. In biology, the term homology refers to a similarity that occurs due to descent from a common ancestor. Two species may have a similar trait because the trait was originally found in a common ancestor. As described next, such homologies may involve anatomical, developmental, or molecular features.

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Anatomical Homologies

As noted by Theodosius Dobzhansky, many observations regarding the features of living organisms simply cannot be understood in any meaningful scientific way except as a result of evolution. A comparison of vertebrate anatomy is a case in point. An examination of the limbs of modern vertebrate species reveals similarities that indicate the same set of bones has undergone evolutionary changes, becoming modified to perform different functions in different species. 

As seen in Figure 22.12, the forelimbs of vertebrates have a strikingly similar pattern of bone arrangements. These are termed homologous structures—structures that are similar to each other because they are derived from a structure in a common ancestor. The forearm has developed different functions among various vertebrates, including grasping, walking, flying, swimming, and climbing. The theory of evolution explains how these animals have descended from a common ancestor and how natural selection has resulted in modifications to the structure of the original set of bones in ways that ultimately allowed them to be used for several different functions.

Figure 22.12An example of anatomical homology: homologous structures found in vertebrates. The same set of bones is found in the human arm, turtle arm, bat wing, and whale flipper, although their relative sizes and shapes differ significantly. This homology suggests that all of these animals evolved from a common ancestor.

Core Concept: Structure and Function These homologous sets of bones have evolved into somewhat different structures that provide functions that differ among humans, turtles, bats, and whales.

Click the arrowhead to expand.

Another result of evolution is the phenomenon of vestigial structures, anatomical features whose function is reduced or absent but that resemble structures of their presumed ancestors (Table 22.3). An interesting case is found in humans. People have a complete set of muscles for moving their ears, even though most people are unable to do so. By comparison, many modern mammals can move their ears, and presumably this was an important trait in a distant human ancestor.

Why would organisms have structures that are no longer useful? Within the context of evolutionary theory, vestigial structures are evolutionary relics. Organisms having vestigial structures share a common ancestry with organisms in which the structure is functional. Natural selection maintains functional structures in a population of individuals. However, if a species changes its lifestyle so that the structure loses its purpose, the selection that would normally keep the structure in a functional condition is no longer present. When this occurs, the structure may degenerate over the course of many generations due to the accumulation of mutations that limit its size and shape. Natural selection may eventually eliminate such traits due to the inefficiency and energy cost of producing unused structures.

Developmental Homologies

Another example of homology is the way that animals undergo embryonic development. Species that differ substantially at the adult stage often bear striking similarities during early stages of embryonic development. These temporary similarities are called developmental homologies. 

In addition, evolutionary history is revealed during development in certain organisms, such as vertebrates. For example, if we consider human development, several features are seen in the embryo that are not present at birth. Human embryos have rudimentary gill ridges like a fish embryo, even though human embryos receive oxygen via the umbilical cord. The presence of gill ridges indicates that humans evolved from an aquatic species that had gill slits. A second observation is that every human embryo has a bony tail. It is difficult to see the advantage of such a structure in utero but easier to understand its presence assuming that an ancestor of the human lineage possessed a tail. These observations, and many others, illustrate that closely related species share similar developmental pathways.

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Molecular Homologies

Our last examples of homologies involve molecular studies. Similarities between organisms at the molecular level due to descent from a common ancestor or an interrelated group of ancestors are called molecular homologies. For example, all living species use DNA to store information and rely on the genetic code to translate mRNA into proteins. Furthermore, certain biochemical pathways are found in all or nearly all species, although minor changes in the structure and function of proteins involved in these pathways have occurred. For example, all species that use oxygen, which constitute the great majority of species on our planet, have similar proteins that make up an electron transport chain and ATP synthase (refer back to Figure 7.8). In addition, nearly all living organisms can break down glucose via a metabolic pathway that is described in Chapter 7. How do we explain these types of observations? Taken together, they indicate that such molecular phenomena arose very early in the origin of life and have been passed to all or nearly all modern forms.

Compelling molecular-level evidence indicating that modern life-forms are derived from an interrelated group of common ancestors is revealed by analyzing genetic sequences. The same type of gene is often found in diverse organisms. Furthermore, the degree of similarity between genetic sequences from different species reflects the evolutionary relatedness of those species. 

As an example, let’s consider the p53 gene, which encodes the p53 protein—a checkpoint protein of the cell cycle (refer back to Figure 15.14). Figure 22.13 shows a short amino acid sequence that makes up part of the p53 protein in a variety of species, including five mammals, one bird, and three fish. The top sequence is the human p53 sequence, and the right column gives the percentage of amino acids within each animal’s entire sequence that are identical to those in the entire human sequence. Amino acids in the other species that are identical to those in humans are highlighted in orange. The sequences from the two monkeys are the most similar to those in humans, followed by the other two mammalian species (rabbit and dog). The three fish sequences are the least similar to the human sequence, but the fish sequences tend to be similar to each other.

Figure 22.13An example of genetic homology: a comparison of a short amino acid sequence within the p53 protein from nine different animals. This figure compares a short region of the p53 protein, a checkpoint protein that plays a role in preventing cancer. Amino acids are represented by three-letter abbreviations. The orange-colored amino acids in the sequences are identical to those in the human sequence. The numbers in the right column give the percentage of amino acids within the whole p53 protein of each species that is identical with the human p53 protein, which is 393 amino acids in length. For example, 95% of the amino acids, or 373 of 393, are identical between the p53 sequences found in humans and in rhesus monkeys.

Taken together, the data shown in Figure 22.13 illustrate two critical points about gene evolution. First, specific genes are found in a diverse array of species such as mammals, birds, and fishes. Second, the sequences of closely related species tend to be more similar to each other than they are to the sequences of distantly related species. The mechanisms underlying this second observation are discussed in the next section.

The Molecular Processes That Underlie Evolution

Learning Outcomes:

1. Explain how paralogs and orthologs are produced.

2. Distinguish between vertical evolution and horizontal gene transfer.

Historically, the study of evolution was based on comparing the anatomies of extinct and modern species to identify similarities between related species. However, the advent of molecular approaches for analyzing DNA sequences has revolutionized the field of evolutionary biology. Now researchers can analyze how changes in the genetic material are associated with changes in phenotype. In this section, we will examine some of the molecular changes in the genetic material that reveal evolutionary change.

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Homologous Genes Are Derived from a Common Ancestral Gene

Two or more genes derived from the same ancestral gene are called homologous genes. The analysis of homologous genes reveals evidence of evolutionary change at the molecular level. How do homologous genes arise? As an example, let’s consider a gene that encodes a transport protein involved in the uptake of metal ions into the cells of two different species of bacteria (Figure 22.14). Homologous genes that are found in different species are termed orthologs. Millions of years ago, the two species of bacteria had a common ancestor. Over time, the common ancestor diverged into additional species, eventually evolving into Escherichia coli, Clostridium acetylbutylicum, and many other species. Since this divergence, the gene encoding the transport protein has accumulated mutations that altered its sequence, though the similarity between the E. coli and the C. acetylbutylicum genes remains striking. In this case, the two sequences are similar because they were derived from the same ancestral gene, but they are not identical due to the independent accumulation of different random mutations.

Figure 22.14The evolution of orthologs, homologous genes from different species. After the two bacterial species diverged from each other, the genes accumulated random mutations that resulted in similar, but not identical, gene sequences called orthologs. These orthologs in E. coli and C. acetylbutylicum encode metal ion transporters. Only one of the two DNA strands is shown from each gene. Bases that are identical between the two genes are shown in orange.

Concept Check: Why do these orthologs have similar gene sequences? Why aren’t the sequences identical? Answer

Core Concept: Evolution

Gene Duplications Produce Gene Families

Evidence of evolutionary change is also found within a single species. Two or more homologous genes within a single species are termed paralogs of each other. Rare gene duplication events produce multiple copies of a gene and ultimately lead to the formation of a gene family—a set of paralogs within the genome of a single species. A well-studied example of a gene family is the globin gene family in humans, which is composed of 14 genes that are hypothesized to be derived from a single ancestral globin gene (refer back to Figure 21.15). According to an evolutionary analysis, the ancestral globin gene first duplicated between 500 and 600 mya. Since that time, additional duplication events and chromosomal rearrangements have produced the current number of 14 genes on three different human chromosomes.

What is the advantage of a gene family? Even though all of the globin polypeptides are subunits of proteins that play a role in oxygen binding, the accumulation of changes in the various family members has produced globins that differ in the timing of their expression and in their functional properties. The various globin genes are expressed at different stages of development in humans. The functional differences of the globin proteins correlate with the oxygen transport needs of humans during the embryonic, fetal, and postpartum stages of life (refer back to Figure 14.3).

What is the evolutionary significance of the globin gene family regarding adaptation? On land, egg cells and small embryos are very susceptible to drying out if they are not protected in some way. Species such as birds and reptiles lay eggs with a protective shell around them. Most mammals, however, have become adapted to a terrestrial environment by evolving internal gestation. The ability to develop young internally has been an important factor in the survival and proliferation of humans and other mammals. The embryonic and fetal forms of hemoglobin allow the embryo and fetus to capture oxygen from the bloodstream of the mother.

Horizontal Gene Transfer Contributes to the Evolution of Species

At the molecular level, the type of evolutionary change depicted in Figures 22.13 and 22.14 is called vertical evolution. In these cases, new species arise from pre-existing species by the accumulation of genetic changes, such as gene mutations and gene duplications. Vertical evolution involves genetic changes in a series of ancestors that form a lineage. 

In addition to changing via vertical evolution, species accumulate genetic changes by horizontal gene transfer—a process in which an organism incorporates genetic material from another organism without being the offspring of that organism. Horizontal gene transfer can involve the exchange of genetic material between members of the same species or different species.

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How does horizontal gene transfer occur? Figure 22.15 illustrates one possible mechanism for horizontal gene transfer. In this example, a paramecium, which is a eukaryotic microorganism, has engulfed a bacterial cell. During the degradation of the bacterium in a phagocytic vesicle, a bacterial gene escapes to the nucleus of the eukaryotic cell, where it is inserted into one of the chromosomes. In this way, a gene has been transferred from a bacterial species to a eukaryotic species. 

Figure 22.15Horizontal gene transfer from a bacterium to a eukaryote. In this example, a bacterium is engulfed by a paramecium (a ciliated protist), and a bacterial gene is transferred to one of the paramecium’s chromosomes.

Core Skill: Connections Look back at Table 19.3. What are three mechanisms of gene transfer that could result in horizontal gene transfer between two different bacterial species? Answer

Click the arrowheads to expand.

 Table 19.3

By analyzing gene sequences among many different species, researchers have discovered that horizontal gene transfer is a common phenomenon. This process can occur from bacteria and archaea to eukaryotes, from eukaryotes to bacteria and archaea, between different species of bacteria and archaea, and between different species of eukaryotes. Therefore, our view of evolution should not only focus on one species evolving into one or more new species via the accumulation of random mutations. It also involves the horizontal transfer of genes among different species, enabling those species to acquire new traits that foster the evolutionary process.

Horizontal gene transfer among bacterial species is relatively widespread. As discussed in Chapter 19, bacterial species carry out three natural mechanisms of gene transfer known as conjugation, transformation, and transduction. By analyzing the genomes of bacterial species, scientists have determined that many genes within a given bacterial genome are derived from horizontal gene transfer. Genome studies have suggested that as much as 20–30% of the variation in the genetic composition of modern bacterial species can be attributed to this process. The roles of the genes acquired by horizontal gene transfer are quite varied, though they commonly involve functions that are beneficial for survival and reproduction. These include genes that confer antibiotic resistance, the ability to degrade toxic compounds, and pathogenicity (the ability to cause disease).

Evolution at the Genome Level Involves Changes in Chromosome Structure and Number

Thus far, we have considered several ways a species might acquire new genetic variation. These include mutations within pre-existing genes, gene duplications that produce gene families, and horizontal gene transfer. Evolution also involves changes in chromosome structure and number. Comparisons of chromosomes of closely related species have revealed that changes in chromosome structure and/or number are common.

As an example, Figure 22.16 compares the banding patterns of the three largest chromosomes in humans and the corresponding chromosomes in chimpanzees, gorillas, and orangutans. (Refer back to Figure 16.1 for an example of chromosome banding.) The banding patterns in the chromosomes are strikingly similar because these species are closely related evolutionarily. Chromosome 1 looks very similar in all species. Even so, you can see that some of the other chromosomes show interesting differences. Humans have one large chromosome 2, but this chromosome is divided into two separate chromosomes in the other three species. This explains why human cells have 23 pairs of chromosomes, whereas cells of chimpanzees, gorillas, and orangutans have 24. The fusion of the two smaller chromosomes during the development of the human lineage may have caused this difference in chromosome number. Another interesting change in chromosome structure is seen in chromosome 3. The banding patterns among humans, chimpanzees, and gorillas are very similar, but the orangutan has a large inversion that flips the order of the bands in the centromeric region. As discussed in Chapter 24, changes in chromosome structure and number may affect the ability of two organisms to breed with one another. Such changes have been important in the establishment of new species.

Figure 22.16Evolutionary changes in chromosome structure and number found in primates. This figure compares the three largest human chromosomes and the corresponding chromosomes in the chimpanzee, gorilla, and orangutan. It is a schematic drawing of Giemsa-stained chromosomes. The differences between these chromosomes illustrate the changes that have occurred during the evolution of these related primate species.

Concept Check: Describe two changes in chromosome structure that have occurred among these chromosomes

Summary of Key Concepts

• Evolution is a heritable change in one or more characteristics of a population from one generation to the next.

22.1 Overview of Evolution

• Darwin and Wallace proposed the theory of evolution based on their understanding of geology and population growth and their observations of species in their natural settings. Darwin's voyage on the Beagle, during which he studied many species, including finches on the Galápagos Islands, was particularly influential in the development of his ideas (Figure 22.1, Table 22.1).

• Darwin and Wallace expressed the theory of evolution as descent with modification through variation and natural selection. As a result of natural selection, genetic variation changes from generation to generation to produce populations of organisms with traits (adaptations) that favor greater reproductive success (Figure 22.2).

• The Grants’ research on finches showed how differences in beak size (a heritable trait) were driven by natural selection (Figures 22.3, 22.4).

22.2 Evidence of Evolutionary Change

• Evidence of evolutionary change is found in studies of natural selection, selective breeding, biogeography, convergent evolution, the fossil record, and homologies (Table 22.2).

• Selective breeding, the selecting and breeding of organisms having desired traits, is a human-driven form of selection (Figures 22.5, 22.6, 22.7).

• Biogeography provides information on the geographic distribution of related species. When populations become isolated on islands or continents, they often evolve into new species (Figure 22.8).

• In convergent evolution, independent adaptations result in similar characteristics, called analogous structures, because different species occupy similar environments (Figure 22.9).

• Fossils provide evidence of evolutionary change in a series of related organisms. The fossil record often reveals transitional forms that link past ancestors to modern species (Figures 22.10, 22.11).

• Homologies are similarities that occur due to descent from a common ancestor. The set of bones in the forearms of vertebrates is an example of an anatomical homology. Homologies can also be seen during embryonic development and at the molecular level (Figures 22.12, 22.13).

• Vestigial structures, structures that were functional in an ancestor but no longer have a useful function in modern species, are evidence of evolutionary change (Table 22.3).

22.3 The Molecular Processes That Underlie Evolution

• Molecular evolution is the process of evolution at the level of genes and proteins. Molecular processes that underlie evolution include the formation of orthologs and paralogs, horizontal gene transfer, and changes in chromosome structure and number.

• Orthologs are homologous genes in different species that have accumulated random mutations over time (Figure 22.14).

• Paralogs are homologous genes in the same species that are produced by gene duplication events. Gene duplication can result in the formation of a gene family such as the globin gene family, which supported the evolutionary adaptation of internal gestation.

• Another mechanism that produces genetic variation is horizontal gene transfer, in which genetic material is transferred from one organism to another organism that is not its offspring. Such genetic changes are subject to natural selection (Figure 22.15).

• Molecular evolution can also involve changes in chromosome structure and number

CHAPTER 1: AN INTRODUCTION TO BIOLOGY

1.1 Principles of Biology and the Levels of Biological Organization (pp. 2-3)

Things to ponder:

What are the two primary characteristics of a population?

1. Size (the number of individuals in a population)

2. Genetic composition (the variety of genes within the population)

What properties changes as the levels of organization increase?

• Complexity (how intricate or detailed something is)

• Functionality (how well something works)

• Interactions (how things work together)

Living Organisms Can Be Studied at Different Levels of Organization

[Fig. 1.3 The levels of biological organization]

• Atoms:

• Atom, the smallest unit of matter that makes up everything

• Example: Hydrogen (H), Oxygen (O)

• Molecules and macromolecules:

• Molecules : two or more atoms bonded (joined) together

• Example: Water (H₂O), Carbon dioxide (CO₂)

• Macromolecules, large molecules made up of smaller molecules

• Example: Proteins, DNA (genetic material)

• Cells: 

• Cell (the basic unit of life that can carry out all life processes):

• Tissues:

•  Tissue, a group of similar cells working together for a specific function

• Organs: 

• Organ,  structure made up of different types of tissues working together

• Organisms:

•  Organisms, a living being that can function on its own

• Population:

• Population, a group of the same species (type of organism) living in the same area)

Example: A group of deer in a forest

• Community:

• Community, different populations of species living together in the same area

Example: Deer, trees, and birds living together in a forest

• Ecosystem:

• Ecosystem, all living and non-living things in an area interacting together

Example: A forest with animals, plants, water, and sunlight

• Biosphere:

• Biosphere, all the ecosystems on Earth combined

• Example: Earth

CHAPTER 22: AN INTRODUCTION TO EVOLUTION

• Evolution (the process of change in living things over time, used to describe a heritable change in one or more characteristics of a population from one generation to another):

• Example: How humans evolved from early ancestors

• Microevolution(small changes within a population over a short time. Involves changes in a single gene or in allele frequencies in a population over time):

• Example: A population of birds developing slightly larger beaks over a few generations

• Macroevolution(big changes that happen over a long period and create new formation of species or groups of related species):

• Example: Dinosaurs evolving into birds

• Species(a group of related organisms that share a distinctive form and can interbreed):

• Example: Dogs are one species, while cats are another

• Population (a group of the same species living in a certain place):

• Example: All the wolves in a forest

• Molecular evolution(how genes and DNA and protein change over time. Comparisions of gene or protein sequences in different organisms can reveal evolutionary relationships ):

• Example: How human DNA has changed compared to early humans

22.1 Overview of Evolution (pp. 459-464)

Things to ponder:

Who was Alfred Russel Wallace? A scientist who was inspired by Charles Darwin and sent his work to him. He came to the same conclusion as Charles so Charles published his findings before him even though he was scared and was going to originally release it after his death because he didn't want Alfred to take credit

What are the differences between the mechanisms of evolution proposed by Lamarck and Darwin?  Lamarck suggested that giraffes developed their elongated necks and front legs by feeding on the leaves at the top of the trees. The exercise of stretching up to the leaves altered the neck and legs, and Lamarck thought that these characteristics were then given to the offspring. MEANWHILE Darwins idea of evolution was about variation. You have both short and long necked giraffes then short die off because they cant reach trees thus making more long necked giraffe. We will see more long necked giraffes because the short ones wont reproduce as much.

Empirical thought:

relies on observation to form an idea or hypothesis rather than trying to understand life from a nonphysical or spiritual point of view

The Work of Several Scientists Set the Stage for Darwin’s Ideas

• Classification: John Ray developed an early classification system for plants and animals based on anatomy and physiology. Established the modern concept of a species, saying that organisms of one species do not interbreed with members of another, and used it as the basic unit of his classification system. Carolus Linnaeus then expanded on Ray ideas. Neither proposed that evolutionary change promotes the formation of new species. however their systematic classification of plants and animals helped scholars of this period see the similarities and differences among living organisms. 

      Early proponents of Evolution:

• George Buffon, proposed that populations of living things change through time

• Jean Baptiste Lamarck (Inheritance of acquired characteristics), around same time of Buffon, Lamarck suggested an intimate relationship between variation and evolution. By examining fossils, he realized that some  species had remained the same over the millennia but others had changed. Popularized view that species change over the course of many generation by adapting to new environments. He thought that behavioral changes could modify traits and hypothesized that these modified traits were inherited by offspring. Called this the idea of Inheritance of acquired characteristics.

Darwin Suggested That Existing Species Are Derived from Pre-Existing Species

[Fig. 22.1 Charles Darwin and the voyage of the Beagle, 1831-1836]

[Table 22.1 A Comparison of Beak Type and Diet Among the Galápagos Finches Darwin Studied]

Hypotheses about Geology

• Geological processes:   

• Catastrophism [Baron Georges Cuvier]: suggests that Earth's geological features and the history of life were shaped by sudden, short-lived, violent events, such as natural disasters (e.g., floods, volcanic eruptions). According to Cuvier, these catastrophic events caused mass extinctions of species, after which new organisms appeared, either through migration or creation. Many of his work hypothesized that Earth was about 6000 years ago. fit well with religious teachings, however catastrophism fit better with a much older age for earth

• Uniformitarianism [James Hutton & Sir Charles Lyell]: the scientific principle that the Earth's geological features were formed by continuous and uniform processes over long periods of time. This concept suggests that the same natural forces and gradual changes, such as erosion, sedimentation, and volcanic activity, that shape the Earth today have been operating in a consistent manner throughout Earth's history. The phrase often associated with uniformitarianism is "the present is the key to the past."James Hutton (late 18th century) first introduced the concept, arguing that geological processes occur slowly over vast time scales.Sir Charles Lyell (19th century) further developed and popularized the theory in his book Principles of Geology, providing extensive evidence that geological changes happen gradually rather than through sudden catastrophic events.

Example:
The formation of the Grand Canyon is often used as an example of uniformitarianism

Population Growth

• Essay on the Principle of Population by Thomas Malthus

Population Growth Potential (Exponential):

• Humans have the ability to reproduce rapidly. If each generation has more children, the population can double or even grow faster over time.

• For example, if two parents have four children, and those children each have four children, the population grows very quickly—this is called exponential growth, meaning the numbers keep multiplying (2, 4, 8, 16, etc.).

2. Resource Growth (Linear):

   • However, the things we need to survive, like food and land, do not increase as fast.

   • We can improve farming and find new land, but these improvements happen gradually and steadily, adding a little bit at a time (e.g., 1, 2, 3, 4).

   • This kind of growth is called linear growth, meaning resources grow by the same amount over time rather than multiplying.

The Problem:
Since the population grows much faster than resources, at some point, there won't be enough food or space for everyone. This leads to problems like famine, war, and disease, which Malthus believed would naturally control the population by reducing its size.

Voyage of the Beagle

• Darwin’s voyage aboard the Beagle, 1831-1836 

Purpose of the Voyage:

• The HMS Beagle was a survey ship tasked with mapping the coastline of South America and collecting oceanographic data.

Darwin’s Role:

• Served as the ship's naturalist, recording observations on:

  • Weather and climate.

  • Geological features (rocks, minerals, fossils).

  • Plants and animals.

  • Indigenous cultures.

Key Observations:

• Differences between island and mainland species, suggesting species adapt to their environments over time.

• These observations, especially in the Galápagos Islands, contributed to the foundation of his evolutionary theory.

• Galápagos finches

Darwin’s Initial Thoughts:

• He originally classified the finches as different types of birds (blackbirds, grosbeaks, etc.).

John Gould’s Contribution:

• A British ornithologist later identified them as distinct finch species.

Key Differences:

• Finch species had unique beak shapes and sizes adapted to different food sources (e.g., seeds, insects, cactus).

Evolutionary Significance:

• Beak adaptations provided advantages for survival and reproduction based on available food sources.

• These variations demonstrated the concept of adaptive radiation—one species evolving into many to fill different ecological roles.

• Initially, the finches likely had similar diets, but over time, competition within their own population led to natural selection favoring individuals with specialized beak shapes suited for different food types. This allowed them to minimize competition by adapting to new niches.

• Grassquit finch (Tiaris obscurus); common ancestor of the Galápagos finches

Origin of Galápagos Finches:

• All finches evolved from a single ancestral species, the dull-colored grassquit finch from South America.

Evolutionary Process:

• When the finches arrived on the Galápagos Islands, different food sources and environments led to natural selection, shaping their traits over generations.

Key Idea:

• The concept of descent with modification, where species gradually change from a common ancestor.

• Alfred Russell Wallace

His Work:

• Conducted research in the Malay Archipelago (Indonesia, Malaysia, Singapore).

• Studied species variation across different islands, similar to Darwin’s observations in the Galápagos.

Key Publication (1855):

• On the Law Which Has Regulated the Introduction of New Species—discussed patterns in species distribution and variation.

Parallel Discoveries:

• Wallace independently arrived at the idea of natural selection, based on environmental pressures shaping species.

• His work prompted Darwin to publish On the Origin of Species sooner than planned.

• The Origins of Species 

  Key Concepts Introduced by Darwin:

• Natural selection as the driving force of evolution.

• Species evolve over long periods through small, inherited changes.

• Survival of the fittest—organisms best suited to their environment survive and reproduce.

Evidence Presented:

• Observations from the Beagle voyage.

• Comparative anatomy and fossil records.

Natural Selection Changes Populations from Generation to Generation

[Fig. 22.2 Evolutionary adaptation to a new environment via natural selection]

• Basis of evolution: 

1. Existence of variation in a given species 

2. Environmental pressure 

• Natural selection: Individuals with heritable traits that make them better suited to their native environment tend to flourish and reproduce. certain traits that favor reproductive success become more prevalent in population over time

• Adaptation: refers to the process where species become better suited to their environment due to natural selection acting on advantageous traits over generations.

• Genetic change

• Positive: Traits that enhance survival and reproduction in a given environment. For example, camouflage in animals to avoid predators.

• Negative: Traits that decrease an organism's ability to survive or reproduce. These traits are less likely to be passed on and may decrease in frequency over time.

• Neutral: Traits that do not significantly affect survival or reproduction, and may not be subject to natural selection. These traits can remain in the population without much change. Cavefish with no eye

22.2 Evidence of Evolutionary Change (pp. 465-473)

[Table 22.2 Evidence of Biological Evolution] 

Things to ponder:

Why are transitional forms rare in the fossil record? We tend to find more end-stage fossils because fully evolved species lasted longer, had larger populations, and had a greater chance of being fossilized. Transitional forms, on the other hand, were often in small, evolving populations and existed for a shorter time, so they are less likely to be preserved in the fossil record.

How does the presence of vestigial structures argue against the supposition of divine creation/intelligent design? The presence of vestigial structures suggests that organisms are the result of evolutionary processes that adapt features over time, and that some features lose their function as the environment or the species changes. This runs counter to the idea of intelligent design, which implies a perfectly efficient, purposeful creation. Instead, vestigial structures provide evidence of imperfection and evolution, supporting the theory that organisms change and adapt over time rather than being created in their current form.

Fossils Show Successive Evolutionary Change [Fig. 22.5 A transitional form in the tetrapod lineage; Fig. 22.6 Evolution of whales]

• Transitional forms: fossils or organisms that show intermediate traits between an ancestral form and its descendants. They provide evidence of evolutionary change over time by displaying characteristics that bridge the gap between older and more recent species. In simpler terms, transitional forms are "in-between" stages in evolution, showing how species gradually changed and adapted over millions of years.

• Tiktaalik roseae: a transitional form between fish and early amphibians, showing features of both. Was likely able to survive for brief periods out of water.

• Pakicetus to Odontoceti and Mysticeti: transitional form between land-dwelling mammals and modern whales, showing how whales evolved from walking animals. MAIN POINT: in order for evolution to happen for something like flying you need limbs to be able to make wings but something like a worm wont be able to cause no limbs.

Biogeography Indicates That Specie in a Given Area Have Evolved from Pre-Existing Species [Fig. 22.7 The evolutionary of an endemic island species from a mainland species]

•  Biogeography: the study of how and why different species of plants and animals are distributed across the Earth. It looks at where organisms live, how they got there, and how their environments influence their evolution over time.

• Endemic: means that a plant, animal, or other living thing is found naturally in only one specific place and nowhere else in the world. (Kangaroo)

• Island fox (Urocyon littoralis): evolved from the mainland gray fox which is much larger. during the last ice age, the santa barbara channel was frozen and narrow enough for ancestors of the mainland gray fox to cross over to the channel islands. when the ice age ended the ice melted and sea levels rose causing the foxes to be cut off from the mainland. over the 16 k years the population of foxes on the channel islands evolved into smaller island fox which is now considered a different species form the larger gray fox. the gray fox is still found on the mainland

• Australia’s marsupial mammals: A long time ago, Australia was connected to other continents. But when it broke away and became isolated, the animals living there were separated from the rest of the world. At that time, marsupials (mammals that carry their babies in pouches, like kangaroos) were common. On other continents, placental mammals (like lions, bears, and humans, which give birth to fully developed babies) became more successful and took over. But in Australia, because it was cut off from other lands, placental mammals couldn't reach it. This allowed marsupials to survive and evolve in their own unique way without competition.

Convergent Evolution Suggests Adaptation to the Environment [Fig. 22.8 Examples of Convergent evolution]

• Convergent evolution:  is the process where different, unrelated species evolve similar traits because they live in similar environments or face similar challenges. Over time, bats (mammals) and birds (reptiles) evolved the ability to fly because flying helps them survive. This confuses scientists because they think some organisms are similiar due to their similiar characteristics so put them in same place but they are unrelated. It’s evidence of selection

• Analogous structures = convergent traits: are the result of convergent evolution. These are the physical features that look similar and serve the same function but evolved independently in different species. Their wings are analogous structures because they serve the same purpose (flying) but evolved from different ancestors.

Selective Breeding Is a Human-Driven Form of Selection [Fig. 22.9 Common breeds of dogs that have been obtained by selective breeding Fig. 22.10 Crop plants developed by selective breeding of the wild mustard plant; Fig. 22.11 Results of selective breeding for oil content in corn plants]

• Selective breeding = artificial selection: when humans intentionally breed plants or animals to develop specific traits. Unlike natural selection, where traits evolve due to environmental pressures, selective breeding is controlled by humans to enhance desirable characteristics. artificial selection is like a faster version of natural selection, but instead of nature deciding which traits are useful for survival, humans make the choices. 

• Dogs:  Humans selectively bred different dog breeds from the same species (Canis lupus familiaris), leading to vast differences in size and appearance, such as greyhounds and dachshunds. In natural selection, such drastic changes would take much longer and occur due to environmental pressures rather than human choice. Some bred to hunt rats because they competed with humans for food.

• Crops: The wild mustard plant (Brassica oleracea) was selectively bred over thousands of years to produce different vegetables, such as broccoli and cabbage. In contrast, natural selection would have shaped the plant based on environmental factors like drought resistance or pest avoidance.

A Comparison of Homologies Shows Evolution of Related Species from a Common Ancestor [Fig. 22.12 An example of anatomical homology: homologous structures found in vertebrates; Fig. 35.20 Feature of the bird wing and feather; Table 22.3 Examples of Vestigial Structures; Fig. 22.13 An example of genetic homology: a comparison of a short amino acid sequence within the p53 protein from nine different animals]

• Homology: refers to similarities between organisms due to shared ancestry, and it can be seen in several ways: anatomical, developmental, and molecular.

Anatomical Homologies 

• Homologous structures (vertebrate limbs): These are body parts in different species that have a similar structure because they evolved from a common ancestor. Example: Vertebrate limbs (like a human arm, a bat wing, and a whale flipper) have similar bone structures, even though their functions differ (grasping, flying, swimming). When they have the same bones from the same ancestory 

• Analogous structures (bat vs. bird wing): These are body parts in different species that have similar functions but evolved independently, not from a common ancestor. Example: The wings of a bat and a bird both serve the purpose of flight, but their structures are different, and they evolved separately (bat wings are made of skin stretched over bones, bird wings are made of feathers). They don’t have the same bones maybe only one so that one bone is homologous but as a whole they aren’t homologous. Not Inherited from the same ancestor. For example insect wing and bird wing. Different lineage.

• Vestigial structures (e.g. muscles for moving ears in humans): These are body parts that no longer serve a function but are remnants of structures that had a purpose in ancestral species. Example: Muscles for moving the ears in humans, which are now largely useless but were functional for moving the ears in some of our ancestors. Mutations might limit the size and erations. Natural selection may eliminate such traits due to the inefficiency and energy cost of producing unused structures. If a species changes its lifestyle so that the structure loses its purpose, the selection that would normally keep the structure in a functional condition is no longer present.

Developmental Homologies 

• Humans: embryonic gill slits & tails: when different species are babies (embryos), they sometimes look similar in certain ways, which suggests they share a common ancestor.

For humans, the two main examples of developmental homologies are:

Embryonic Gill Slits:When humans are developing as embryos (tiny babies inside the womb), they have tiny, temporary structures that look like gill slits (like fish have for breathing underwater). Humans don't use these slits for breathing (we don't have gills), but these slits are similar to the gill slits of fish. This shows that humans share a common ancestor with fish, because fish and humans both had gill slits when they were embryos. Embryonic Tail:When humans are embryos, they also have a tail-like structure. This tail-like feature doesn’t last long and eventually disappears as we develop, so we don’t have a tail as adults.However, this tail-like structure is a sign that humans share a common ancestor with animals that had tails, like monkeys, dogs, or even fish. In short, these similarities in early development show that humans share a common ancestor with fish, amphibians, and other animals that had gills and tails. The gill slits and tail-like structure are just temporary features that remind us of our evolutionary past.

Molecular Homologies 

• p53 protein sequence: refer to similarities in the genetic material (DNA) or proteins of different species, suggesting that they share a common ancestor. The p53 protein is a critical protein in humans and many other organisms. It helps regulate the cell cycle (the process by which cells grow and divide) and can prevent cancer by stopping the growth of cells with damaged DNA.The p53 protein is found in many different species, not just humans. In fact, the sequence of amino acids (the building blocks of proteins) that makes up the p53 protein is very similar across different species. This means that the gene that codes for p53 has been conserved (stayed similar) through evolution. For example, humans and mice have very similar p53 proteins, showing that both species share a common ancestor that had this protein. The similarities in the protein sequences indicate that this protein has been important for millions of years in helping cells stay healthy.The p53 protein is found in many species, and the similarity in the protein's structure and function across species (like humans, mice, and other vertebrates) suggests that all these species evolved from a common ancestor that had this protein. The fact that the protein is so similar shows how important it has been for the survival of these species.