ch23

Flashcards covering key concepts from Chapter 23: The Evolution of Populations to review evolution, Hardy-Weinberg, drift, selection, mutation, recombination, and gene flow.

What is the modern definition of evolution?

Evolution is defined as a change in allele frequency within a population over time. This fundamental process drives the diversification of life, meaning that the genetic makeup of a population is altered across generations, leading to new traits, adaptations, and ultimately, new species.

What is an 'allele frequency' in the context of evolution?

Allele frequency refers to the proportion of a specific allele (a variant form of a gene) within a population's gene pool. For example, if a gene has two alleles, A and a, the frequency of A is the proportion of all A alleles in the population, relative to the total number of alleles for that gene.

What does the Hardy-Weinberg Theorem describe?

The Hardy-Weinberg Theorem provides a mathematical description of a hypothetical population that is not evolving. In such a population, both allele and genotype frequencies remain constant across generations. Specifically, genotype frequencies can be predicted directly from allele frequencies using the formulas: p^2 for homozygous dominant, 2pq for heterozygous, and q^2 for homozygous recessive. It serves as a crucial null hypothesis for evolutionary change, meaning any significant deviation from its predictions indicates that evolutionary forces are at play.

What is the significance of the Hardy-Weinberg Theorem as a null hypothesis in evolutionary biology?

As a null hypothesis, the Hardy-Weinberg Theorem provides a baseline for a non-evolving population. When real population data deviates from Hardy-Weinberg equilibrium, it signals that one or more of the five evolutionary forces (genetic drift, gene flow, mutation, non-random mating, or natural selection) are acting on the population, causing it to evolve. This allows scientists to identify and study these evolutionary mechanisms.

In Hardy-Weinberg equilibrium, what do p and q represent?

In the Hardy-Weinberg model, p represents the frequency of the dominant allele (e.g., A) in a population, while q represents the frequency of the recessive allele (e.g., a). Together, these two frequencies account for all alleles for that specific gene in the population, so their sum must equal one: p + q = 1. These allele frequencies are the foundation for calculating genotype frequencies.

What is the Hardy-Weinberg genotype frequency equation and what do its components represent?

The Hardy-Weinberg genotype frequency equation is: p^2 + 2pq + q^2 = 1.

• p^2: Represents the frequency of the homozygous dominant genotype (AA). It is the probability of an individual inheriting two copies of the dominant allele.
• 2pq: Represents the frequency of the heterozygous genotype (Aa). It is the probability of an individual inheriting one dominant and one recessive allele.
• q^2: Represents the frequency of the homozygous recessive genotype (aa). It is the probability of an individual inheriting two copies of the recessive allele.

The sum of these genotype frequencies must equal one, reflecting that all possible genotypes for that gene are accounted for in the population.

If p = 0.6 and q = 0.4, what are the expected Hardy-Weinberg genotype frequencies?

Given p = 0.6 and q = 0.4, the Hardy-Weinberg genotype frequencies are calculated as follows:

• Frequency of homozygous dominant (AA) = p^2 = (0.6)^2 = 0.36
• Frequency of heterozygous (Aa) = 2pq = 2(0.6)(0.4) = 0.48
• Frequency of homozygous recessive (aa) = q^2 = (0.4)^2 = 0.16

These frequencies represent the expected proportions of genotypes in a population where no evolutionary forces are acting, and can be used to compare with observed frequencies to detect evolution.

What is the assumption of Hardy-Weinberg equilibrium regarding population size?

One key assumption of Hardy-Weinberg equilibrium is that the population size is effectively infinite. This large size is crucial to prevent genetic drift, meaning that allele frequencies will not randomly fluctuate from one generation to the next due to chance events. In a very large population, random sampling effects on allele frequencies are negligible.

What is the assumption of Hardy-Weinberg equilibrium regarding mating?

Another assumption is that mating is random (panmixia). This means that every individual in the population has an equal chance of mating with any other individual, regardless of their genotype or phenotype. If mating is non-random (e.g., individuals preferentially mate with others that share similar traits), it can alter genotype frequencies, thus violating Hardy-Weinberg equilibrium.

What is the assumption of Hardy-Weinberg equilibrium regarding selection?

The Hardy-Weinberg principle assumes that natural selection does not act on any alleles. This implies that all genotypes have equal fitness, meaning they have equal rates of survival and reproduction. If certain genotypes have a selective advantage or disadvantage, their frequencies will change over time, and the population will evolve, deviating from equilibrium.

What is the assumption of Hardy-Weinberg equilibrium regarding mutation?

A critical assumption is that no new mutations occur. Mutations introduce new alleles or change existing ones, directly altering allele frequencies and immediately violating the equilibrium. While mutation rates are often low, they are the ultimate source of all genetic variation and are fundamental for long-term evolutionary change, even if they aren't a strong immediate force for disequilibrium in a single generation.

What is the assumption of Hardy-Weinberg equilibrium regarding migration?

Hardy-Weinberg equilibrium assumes no migration or gene flow (neither immigration nor emigration). Gene flow is the movement of alleles between populations. If individuals (and thus their alleles) move in or out of a population, it can introduce new alleles, remove existing ones, or change the relative frequencies of alleles, causing the population to deviate from equilibrium.

What is genetic drift?

Genetic drift is an evolutionary force defined as changes in allele frequency due to random chance. Unlike natural selection, drift is not driven by fitness advantages but by purely stochastic events. It is particularly pronounced and consequential in small populations, where random events can cause significant shifts in allele proportions. Genetic drift can lead to a loss of genetic variation within a population and can result in the random fixation (frequency of 100%) or loss (frequency of 0%) of alleles, regardless of whether those alleles are beneficial, neutral, or deleterious. This mechanism is one of the ways populations deviate from Hardy-Weinberg equilibrium.

How does population size influence genetic drift?

Genetic drift is inversely proportional to population size. In small populations, random chance plays a much larger role; for instance, a few individuals failing to reproduce due to accident (not selection) can have a dramatic impact on allele frequencies in the next generation. In large populations, the effects of random chance tend to average out, and genetic drift has a much weaker influence on allele frequencies, making them less likely to deviate significantly from one generation to the next unless other forces are at play.

What is the founder effect?

The founder effect is a specific type of genetic drift that occurs when a new population is established by a very small number of individuals (the 'founders') from a larger source population. Because the founders carry only a small, and often unrepresentative, sample of the original population's genetic variation, the allele frequencies in the newly founded, isolated population may differ significantly from the original population simply due to this random sampling. This reduced genetic diversity and altered allele frequencies can have long-lasting effects on the new population's evolutionary trajectory.

What is the bottleneck effect?

The bottleneck effect is another form of genetic drift characterized by a sharp reduction in the size of a population due to environmental events (like natural disasters, disease outbreaks, or habitat loss) or human activities (like overhunting). This drastic reduction in population size leads to a dramatic reduction in genetic diversity because many alleles are lost by chance as individuals die off. The allele frequencies of the surviving, smaller population are often very different from the pre-bottleneck population, again due to random sampling, making the gene pool of the survivor population unrepresentative of the original.

What is non-random mating?

Non-random mating describes any mating pattern where individuals do not have an equal chance of mating with any other individual in the population. This violates a key Hardy-Weinberg assumption. Non-random mating can be influenced by preferences (e.g., sexual selection) or proximity (e.g., inbreeding). While it consistently alters genotype frequencies (e.g., by increasing homozygosity), it generally does not directly change allele frequencies on its own, meaning it does not directly cause evolution in the sense of changing the overall proportion of alleles in the gene pool. However, by changing genotype frequencies, it can expose alleles to selection or drift in different ways.

What is inbreeding?

Inbreeding is a specific type of non-random mating that involves mating between close relatives. This practice significantly increases the proportion of homozygotes across the entire genome and reduces overall heterozygosity within a population. A major consequence is inbreeding depression, where a reduction in fitness (e.g., lower fertility, higher susceptibility to disease, reduced survival) occurs due to the increased probability of offspring inheriting and expressing two copies of deleterious recessive alleles that were rare in the population but present in common ancestors.

What is selfing, and how does it relate to inbreeding?

Selfing, or self-fertilization, is an extreme form of inbreeding where an individual produces offspring with itself. This process very rapidly increases homozygosity and decreases heterozygosity within a population over generations. Selfing is common in many plant species and some invertebrates, and while it guarantees reproductive success for isolated individuals, it can lead to severe inbreeding depression due to the rapid exposure of recessive deleterious alleles.

What is assortative mating?

Assortative mating occurs when individuals do not mate randomly but instead choose partners based on their phenotypes. There are two types:

1. Positive assortative mating: Individuals mate with partners who are more similar to themselves for a particular trait than would be expected by chance. For example, tall individuals preferentially mate with other tall individuals. This increases homozygosity specifically for the genes influencing the chosen trait.
2. Negative assortative mating: Individuals mate with partners who are more dissimilar to themselves for a particular trait. For example, individuals with a rare trait might seek partners with a common trait, which can increase heterozygosity for that particular locus.

Assortative mating affects genotype frequencies for the selected traits but usually has little direct impact on overall allele frequencies.

What are the three primary modes of natural selection, and how do they generally differ?

The three primary modes of natural selection are fundamentally distinguished by how they affect the phenotypic distribution within a population over time:

1. Directional selection: Favors one extreme phenotype, shifting the population mean.
2. Disruptive/Diversifying selection: Favors both extreme phenotypes over intermediate ones, creating a bimodal distribution.
3. Stabilizing selection: Favors intermediate phenotypes, reducing phenotypic variation around the mean.

What is directional selection?

Directional selection is a mode of natural selection that favors individuals at one extreme end of a phenotypic range. This selection pressure causes the population's average (mean) trait value to shift over time towards that favored extreme. Examples include the evolution of antibiotic resistance in bacteria (favoring resistant strains) or the increase in body size in animal populations facing colder environments. This type of selection leads to a consistent change in allele frequencies toward beneficial alleles that confer the extreme phenotype.

What is disruptive/diversifying selection?

Disruptive or diversifying selection is a mode of natural selection that favors individuals at both extreme ends of the phenotypic range over intermediate phenotypes. This pattern typically occurs in heterogeneous environments where different extreme traits are advantageous in different niches or with different resources. Over time, this can lead to a bimodal distribution of traits within a population, with two distinct peaks. It is a powerful force that can contribute to speciation by promoting genetic divergence between sub-populations that exploit different resources or habitats.

What is stabilizing selection?

Stabilizing selection is a mode of natural selection that favors intermediate variants in a population, acting against individuals with extreme phenotypes. This type of selection results in reduced phenotypic variation and often maintains the status quo for a particular trait, keeping the population mean relatively stable. A classic example is human birth weight, where intermediate-weight babies tend to have higher survival rates than very small or very large babies, thus the average birth weight is maintained over generations.

What is sexual selection and how does it drive evolutionary changes?

Sexual selection is a specific type of natural selection driven by differential mating success, meaning some individuals reproduce more than others due to their ability to attract mates. It leads to changes in allele frequency that enhance an individual's reproductive opportunities, often at the expense of survival. Sexual selection frequently results in secondary sexual dimorphism, where males and females of a species exhibit distinct phenotypic differences (e.g., elaborate plumage in male birds, large antlers in male deer). This process manifests in two main forms:

1. Intra-sexual selection: Competition among individuals of the same sex (usually males) for mates.
2. Inter-sexual selection: Individuals of one sex (usually females) choose mates of the opposite sex based on specific traits.

Sexual selection highlights a potential trade-off between maximizing reproductive success and maximizing individual survival.

What is mutation, and what is its ultimate role in evolution?

Mutation refers to any change in the DNA sequence. It is the ultimate source of all new genetic variation in a population. Without mutations, there would be no new alleles for natural selection, genetic drift, or gene flow to act upon. While individual mutations are often random and can be harmful, neutral, or beneficial, their cumulative effect over vast periods provides the raw material for evolutionary processes. Mutations encompass various forms, including point mutations, insertions/deletions (indels), gene duplication, chromosomal changes, and genome duplication.

What is recombination and how does it contribute to genetic variation?

Recombination is the process by which genetic material is rearranged, resulting in new combinations of alleles in offspring. It primarily occurs during meiosis through crossing over, where homologous chromosomes exchange segments of DNA. This shuffles existing alleles into new combinations on a chromosome. Unlike mutation, which creates new alleles, recombination creates new combinations of alleles from already existing ones. This increased genetic diversity within a population provides more raw material for natural selection to act upon, allowing populations to adapt to changing environments more effectively without creating entirely new alleles.

What is a point mutation, and what are its potential consequences?

A point mutation is a type of gene mutation that involves a change of a single nucleotide base within the DNA sequence. Despite being a small alteration, it can have significant effects on the resulting protein:

• Silent mutation: The change in nucleotide does not alter the amino acid sequence, often because of redundancy in the genetic code.
• Missense mutation: The change in nucleotide results in a different amino acid being incorporated into the protein, potentially altering protein function (as seen in sickle-cell anemia).
• Nonsense mutation: The change in nucleotide creates a premature stop codon, leading to a truncated and typically non-functional protein. Point mutations are a common source of new alleles.

What is an indel mutation, and why can it be particularly harmful?

An indel mutation is a type of gene mutation characterized by the insertion or deletion of one or more nucleotide bases in a DNA sequence. Indels are particularly harmful if the number of inserted or deleted bases is not a multiple of three. This is because DNA is read in codons (groups of three bases), so a non-multi-of-three indel causes a frameshift mutation. A frameshift drastically alters the entire reading frame downstream of the mutation, leading to a completely different amino acid sequence, often resulting in a premature stop codon and typically a severely non-functional protein.

What is gene duplication, and what is its evolutionary significance?

Gene duplication is a chromosomal alteration where a segment of DNA, including one or more genes, is copied. This results in an organism having multiple copies of the same gene. Its evolutionary significance is profound: the duplicated copy is redundant, meaning the original gene can still perform its function. This redundancy allows the duplicated copy to accumulate mutations over time without harming the organism, potentially leading to the acquisition of novel functions. Thus, gene duplication is a significant source of new genetic material upon which natural selection can act, contributing to the evolution of new genes and complex biological systems.