Ch 24,25, pt 1 of 26

Vocabulary flashcards covering key terms and concepts from the lecture notes on species concepts, barriers to reproduction, modes of speciation, fossil dating, and phylogeny.

What is the biological species concept?

The biological species concept defines a species as a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring. This means a species is reproductively cohesive, and its boundaries are defined by what populations can mate and achieve successful reproduction. It emphasizes the importance of gene flow within a species but prevention of gene flow between species, highlighting reproductive isolation as a key factor in maintaining species distinctiveness.

What does 'reproductively cohesive' mean in the biological species concept?

To be 'reproductively cohesive' means that individuals within a population or species are capable of interbreeding with each other and producing offspring that are not only alive (viable) but also able to reproduce themselves (fertile). This cohesiveness through successful reproduction maintains gene flow within the group, integrating the population as a single species and distinguishing it from other groups that cannot interbreed effectively.

What is reproductive isolation and why is it important?

Reproductive isolation refers to the existence of biological barriers that prevent members of different species from interbreeding and producing viable, fertile offspring. These barriers effectively block gene flow between populations, serving as the mechanisms that define and maintain species boundaries under the biological species concept. It's crucial for speciation, as the establishment of these barriers leads to the divergence of populations into distinct species.

What is a prezygotic barrier?

A prezygotic barrier is a type of reproductive barrier that acts before the formation of a zygote (fertilized egg). These barriers typically prevent mating attempts between different species or, if mating occurs, prevent the successful fertilization of an egg. They are crucial for maintaining species distinctions by stopping gene flow at the earliest stages of reproduction.

What is habitat isolation?

Habitat isolation is a type of prezygotic barrier where two species that could potentially interbreed live in the same geographic area but occupy different habitats. Because they rarely or never encounter each other, they don't have the opportunity to mate. For example, one species of garter snake lives mainly in water while another species lives on land, preventing their interaction.

What is temporal isolation?

Temporal isolation is a prezygotic barrier where species that might otherwise interbreed are prevented from doing so because they mate at different times. This can involve differences in the time of day they are most active, different seasons for breeding, or even different years for cycle-based reproduction. For instance, two species of skunks may have overlapping ranges but one mates in late winter and the other in late summer.

What is behavioral isolation?

Behavioral isolation is a prezygotic barrier where different species have distinct courtship rituals or other behaviors that are required for mate recognition and successful reproduction. These unique signals or displays, such as specific songs, dances, or chemical scents, are essential for attracting mates within their own species but are ineffective or unattractive to members of other species, thereby preventing interbreeding. For example, blue-footed boobies perform a high-stepping dance that is only recognized by other blue-footed boobies.

What is mechanical isolation?

Mechanical isolation is a prezygotic barrier due to anatomical incompatibility between different species. This means that structural differences in their reproductive organs or other body parts physically prevent successful mating or the transfer of gametes. A classic example is the differing shell spirals in some snail species, where the shells coil in opposite directions, making copulation impossible.

What is gametic isolation?

Gametic isolation is a prezygotic barrier where the sperm of one species is unable to fertilize the eggs of another species. This incompatibility can be due to a variety of factors, including molecular differences on the surface of the egg and sperm that prevent fusion, or chemical signals exchanged between gametes that are species-specific. This is a crucial barrier, especially for species that release their gametes into the environment, such as many marine invertebrates, where specific recognition proteins on the gametes are essential for successful fertilization.

What is a postzygotic barrier?

A postzygotic barrier is a type of reproductive barrier that acts after fertilization has occurred and a zygote has formed. These barriers reduce the viability or fertility of hybrid offspring, preventing them from contributing to the gene pool of either parent species. They are less efficient than prezygotic barriers as energy and resources are already invested in creating the hybrid, but they still ensure the reproductive isolation between species.

What is hybrid inviability (or reduced hybrid viability)?

Hybrid inviability is a postzygotic barrier where the hybrid offspring formed from the mating of two different species either fail to develop past early embryonic stages or are born but have significantly reduced chances of survival to reproductive age. For example, hybrid salamanders may complete development but are frail and do not survive long enough to reproduce, effectively preventing gene flow between the parent species.

What is reduced hybrid fertility?

Reduced hybrid fertility is a postzygotic barrier where hybrid offspring are viable and may even be robust, but they are sterile or have significantly reduced fertility. This means they cannot produce viable gametes themselves, or their offspring are non-viable. A well-known example is the mule, which is the offspring of a horse and a donkey. Mules are strong and viable but are sterile because their chromosomes (from two different species) do not pair up properly during meiosis, preventing the formation of functional sperm or eggs.

What is hybrid breakdown?

Hybrid breakdown is a postzygotic barrier that typically manifests in later generations of hybrids. In this scenario, first-generation hybrids (F1) may be viable and fertile, but when these F1 hybrids mate with each other or with either parent species, their offspring (F_2 or subsequent generations) suffer from reduced fitness, viability, or fertility. This progressive deterioration ensures that gene flow from the original parent species is eventually halted after a few generations, preventing the long-term establishment of hybrid lineages.

What is allopatric speciation?

Allopatric speciation is a mode of speciation where populations become geographically or physically separated, leading to the interruption of gene flow. This geographic barrier prevents members of the two populations from interbreeding, allowing them to evolve independently due to different selective pressures, mutations, and genetic drift. Over time, these genetic differences accumulate to the point where, even if the barrier is removed, the populations can no longer interbreed, thus forming two distinct species. An example is the formation of new species after land masses separate or rivers change course.

What is sympatric speciation?

Sympatric speciation is a mode of speciation that occurs when populations diverge into new species while inhabiting the same geographic area. In contrast to allopatric speciation, there is no physical barrier separating them. Instead, gene flow within the population is reduced by other mechanisms, often through genetic changes such as polyploidy, sexual selection, or habitat differentiation. This leads to the emergence of reproductive barriers even when populations are in close proximity.

What is allopolyploidy?

Allopolyploidy is a specific type of polyploidy that leads to sympatric speciation, particularly common in plants. It arises from the hybridization of two different species. When an individual from species A mates with an individual from species B, and their hybrid offspring undergoes a chromosomal doubling event, the resulting allopolyploid individual has multiple sets of chromosomes from both parent species. This new allopolyploid organism can often self-fertilize or breed with other allopolyploids but is reproductively isolated from either parent species because of the differing chromosome numbers, effectively creating a new species rapidly.

What is polyploidy?

Polyploidy is a condition where an organism has more than two complete sets of chromosomes in its somatic cells. Normally, organisms are diploid (2n), meaning they have two sets. Polyploidy can result in organisms being triploid (3n), tetraploid (4n), or even higher. It's a significant mechanism of sympatric speciation, especially in plants, where it can lead to immediate reproductive isolation from the diploid parent species.

What is autopolyploidy?

Autopolyploidy is a type of polyploidy where an individual has more than two chromosome sets, all derived from a single species. This typically occurs when an error in cell division (like nondisjunction during meiosis or mitosis) leads to a doubling of chromosomes within a single sexually reproducing lineage. For example, a diploid organism (2n) might produce unreduced gametes (2n instead of n), and if two such gametes fuse, a tetraploid (4n) offspring results. This new tetraploid individual would be reproductively isolated from its diploid parent species because of incompatible chromosome numbers, representing a form of sympatric speciation.

What is a zygote?

A zygote is the single diploid cell that results from the fusion of two haploid gametes—typically an egg and a sperm—during sexual reproduction. It contains the complete genetic material from both parents and represents the very first cell of a new organism's development. In humans, the zygote is the start of embryonic development.

What is the Law of Superposition?

The Law of Superposition is a fundamental principle in geology and paleontology stating that, in undisturbed sequences of sedimentary rock layers, the oldest layers are at the bottom and progressively younger layers are found on top. This principle allows scientists to determine the relative ages of fossils and rock strata: fossils found in lower layers are generally older than those found in higher layers, providing a geological framework for understanding evolutionary history.

What is relative dating?

Relative dating is a method used to determine the sequential order of past events, particularly the age of fossils or geological strata, without necessarily determining their precise numerical age. It relies on principles like the Law of Superposition, assuming that deeper layers are older. It tells us whether one fossil or rock layer is older or younger than another, providing a chronological sequence of life forms on Earth, but not a specific number of years.

What is absolute dating?

Absolute dating refers to methods that assign actual, numerical ages to geological formations, rocks, and fossils, expressed in years. Unlike relative dating which only provides a sequence, absolute dating provides a specific measurable age range. The most common and accurate techniques for absolute dating involve radiometric methods, which measure the decay of radioactive isotopes within samples, allowing for a precise estimation of time since formation.

What is radiometric dating?

Radiometric dating is a sophisticated absolute dating technique that utilizes the constant and predictable decay of radioactive isotopes within rocks and organic materials to determine their age. Radioactive isotopes (parent isotopes) decay into stable isotopes (daughter isotopes) at a known, constant rate, expressed by their half-life. By measuring the ratio of parent to daughter isotopes in a sample, scientists can calculate how many half-lives have passed and thus estimate the absolute age of the sample. This method is incredibly precise for dating both very old geological events and more recent organic remains.

What is Carbon-14 dating?

Carbon-14 ($^{14}\text{C}$) dating is a specific type of radiometric dating used to determine the age of organic materials (e.g., bones, wood, cloth) up to about 50,000 to 75,000 years old. Living organisms continuously absorb carbon, including a small proportion of radioactive $^{14}\text{C}$. Once an organism dies, it stops taking in carbon, and the $^{14}\text{C}$ begins to decay into Nitrogen-14 ($^{14}\text{N}$) with a known half-life of 5,730 years. By measuring the remaining $^{14}\text{C}$ to $^{12}\text{C}$ ratio in a dead organism, scientists can calculate how long ago it died. It's not suitable for very ancient fossils because most of the $^{14}\text{C}$ would have decayed by then.

What is the Iridium anomaly and its significance?

The Iridium anomaly refers to an unusually high concentration of the rare element iridium found in a thin layer of rock dating back to about 66 million years ago, specifically at the Cretaceous–Paleogene (K–Pg) boundary. Iridium is much more abundant in meteorites and asteroids than in Earth's crust. Its discovery globally across this geological boundary provided strong evidence supporting the hypothesis that a large extraterrestrial impact (like an asteroid or comet) was responsible for the mass extinction event that marked the end of the Cretaceous period and the demise of the non-avian dinosaurs.

What is the Chicxulub Crater and its connection to the K-Pg extinction?

The Chicxulub Crater is a massive impact crater located beneath the Yucatán Peninsula in Mexico, spanning roughly 180 kilometers in diameter. It is dated to approximately 66 million years ago, precisely coinciding with the Cretaceous–Paleogene (K–Pg) boundary and the Iridium anomaly. Scientific consensus points to the impact event that formed this crater as the primary cause of the K–Pg mass extinction. The impact would have triggered widespread devastation, including tsunamis, global wildfires, and a 'nuclear winter' effect due to ejected dust and aerosols, leading to a catastrophic collapse of ecosystems worldwide.

What was the Permian extinction?

The Permian extinction, also known as the 'Great Dying,' was the most severe mass extinction event in Earth's history, occurring approximately 252 million years ago. This catastrophic event led to the extinction of about 90\% of all marine species and 70\% of terrestrial vertebrate species. Its causes are thought to be complex, possibly involving massive volcanic eruptions in Siberia leading to significant climate change, ocean acidification, and widespread anoxia (lack of oxygen in the oceans).

What was the Cretaceous–Paleogene (K–Pg) extinction?

The Cretaceous–Paleogene (K–Pg) extinction, which occurred about 66 million years ago, was a major global mass extinction event that famously led to the disappearance of the non-avian dinosaurs, pterosaurs, and large marine reptiles, along with a significant portion of plankton and plant species. It profoundly reshaped life on Earth. Strong evidence, including the Iridium anomaly and the Chicxulub Crater, supports the theory that a large asteroid impact was the primary trigger, causing widespread environmental upheaval that led to the collapse of ecosystems.

What is 'descent with modification'?

Descent with modification is Charles Darwin's fundamental concept of evolution. It posits that all organisms are descended from common ancestors ('descent'), but over vast spans of time, they have accumulated various genetic and morphological changes ('modification') that lead to the diversity of life observed today. This process involves the inheritance of variations over successive generations, driven by mechanisms such as natural selection, genetic drift, and mutation, explaining the unity and diversity of life.

What is phylogeny?

Phylogeny is the study of the evolutionary history and genealogical relationships among organisms or groups of organisms. It seeks to understand how different species, populations, or other taxonomic units are related to each other through common ancestry. Phylogenetic studies, often depicted visually in phylogenetic trees, help illuminate the patterns and processes of evolution and classification, revealing the 'tree of life.'

What is the Linnean hierarchy in taxonomy?

The Linnean hierarchy is a system for classifying and organizing organisms into a nested series of increasingly inclusive categories or ranks. Developed by Carl Linnaeus, this hierarchical structure helps biologists categorize and name species based on shared characteristics. The main ranks, from broadest to most specific, are: Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species. This system provides a standardized way to communicate about the diversity of life and reflect evolutionary relationships.

What is 'descent of modification'?

'Descent of modification' is essentially synonymous with 'descent with modification,' which is Charles Darwin's evolutionary theory. It describes the process by which species living today are derived from ancestral species through a gradual accumulation of changes over vast periods of time. This concept highlights that all life shares a common ancestor, and the diversity of life has arisen through modifications passed down through generations, leading to new forms and species.

What is a domain in taxonomy?

A domain is the highest and most inclusive taxonomic rank in the Linnean hierarchy, positioned above the Kingdom level. It is a fundamental classification that groups all forms of life into three overarching categories based on fundamental genetic and cellular differences: Bacteria, Archaea, and Eukarya. This classification reflects a deep evolutionary divergence early in the history of life.

What is allometry in evolutionary development?

Allometry refers to the differential growth rates of various body parts or organs during an organism's development. This means that different parts of an organism grow at different rates, leading to changes in the overall size and shape of the organism as it matures. Evolutionary changes in allometry can significantly alter adult morphology between species, explaining differences in proportion, such as the elongated skull of a bat compared to a mouse, even if they share similar developmental pathways.

What is heterochrony in evolutionary development?

Heterochrony refers to evolutionary changes in the timing or rate of developmental events, leading to significant morphological differences between descendant and ancestral species. It can involve an acceleration or deceleration of growth processes or a delay in the onset or offset of developmental stages. For example, if sexual maturity is achieved earlier while somatic development remains the same, an adult organism might retain juvenile features—a phenomenon called paedomorphosis or neoteny. Such shifts play a crucial role in macroevolutionary changes.

What are homeotic genes?

Homeotic genes are master regulatory genes that control the overall body plan and the development of specific anatomical structures in an organism. They determine the identity of body segments or structures during embryonic development, essentially telling cells 'what to be' (e.g., a leg, a wing, an antenna). Small changes in the expression or regulation of these powerful genes can lead to significant morphological transformations, playing a pivotal role in evolutionary development (evo-devo).

What are homeobox genes?

Homeobox genes are a family of highly conserved regulatory genes that encode transcription factors containing a specific DNA-binding domain known as the homeodomain (encoded by the homeobox sequence). These genes play a critical role in patterning the body axis and segment identity during embryonic development across a wide range of eukaryotes, from fungi to humans. They are a subset of homeotic genes, acting as genetic switches that control the expression of other genes, thereby orchestrating the development of complex structures.

What is Pitx1 and its role in evolutionary development?

Pitx1 is an example of a homeobox gene that illustrates how regulatory changes in gene expression, rather than changes in the coding sequence of the gene itself, can drive significant morphological evolution. In stickleback fish, research has shown that changes in the regulatory enhancer regions linked to the Pitx1 gene, not the gene's protein-coding sequence, are responsible for the presence or absence of pelvic spines. The loss of these spines in freshwater stickleback populations is an adaptation to predators, demonstrating how genetic switches controlling gene expression can lead to rapid and pronounced evolutionary changes.

What is a stickleback and why is it important in evolutionary studies?

The stickleback fish (e.g., three-spined stickleback) is a widely used model organism in evolutionary biology and evolutionary developmental biology (evo-devo). It is particularly valuable for studying rapid evolutionary changes, especially in traits related to adaptation (like the presence or absence of pelvic spines, as controlled by the Pitx1 gene) as populations transition between marine and freshwater environments. Its ability to colonize diverse new habitats and its clear genetic basis for phenotypic variation make it an excellent system for understanding how evolution works at the genetic level.

What is the juvenile ape hypothesis (or neoteny in humans)?

The juvenile ape hypothesis suggests that human development exhibits characteristics reminiscent of juvenile stages of apes. More broadly, this relates to the concept of neoteny, a type of heterochrony where the adult form of descendants retains features that were juvenile in their ancestors due to a slower rate of development or delayed onset of sexual maturity. In humans, traits like a large head-to-body ratio, flatter face, and delayed skeletal maturation are considered neotenous when compared to the developmental trajectory of other great apes, possibly contributing to our prolonged period of learning and brain development.

What is a mule, and what does it illustrate about hybrid fitness?

A mule is the sterile hybrid offspring resulting from the mating of a female horse (Equus caballus) and a male donkey (Equus asinus). Mules are renowned for their strength, endurance, and calm temperament, making them valuable working animals. However, they are a classic example of reduced hybrid fertility, a postzygotic barrier. Horses have 64 chromosomes, while donkeys have 62. The mule, therefore, has 63 chromosomes, an odd number. During meiosis, these chromosomes cannot pair up properly, leading to the production of non-functional gametes and thus sterility. This reproductive barrier ensures that horses and donkeys remain distinct species and prevents the flow of genes between them.

What was the Drosophila carbon-source experiment?

The Drosophila carbon-source experiment is a classic study that demonstrated allopatric speciation in a controlled laboratory setting. Researchers divided a single population of fruit flies (Drosophila melanogaster) into two groups and raised them for many generations on different food sources—one on starch-based medium and the other on maltose-based medium. After several generations, when flies from the two populations were brought together, they showed a strong preference for mating with individuals that had been raised on the same food source. This experiment illustrated how divergent selection pressures (the different carbon sources), even without a full geographic barrier, led to reproductive isolation and the initiation of speciation 'in parallel' between populations adapted to different resources.