Chapter 20: Genetic Variation and Evolution

Genetic Variation and Evolution

• Genetic Variation: This refers to the differences in alleles of genes within individuals of a population. It serves as the baseline requirement for evolution; without variation, a population cannot change in response to environmental pressures. Variation is typically measured by polymorphism, which is the presence of multiple alleles for a single gene locus.

• Population Genetics: This field studies the distribution and change in frequency of alleles within populations. In this context, evolution is defined as a measurable change in the genetic composition or allele frequencies of a population over successive generations.

• Hardy-Weinberg Principle: A mathematical model used to predict genotype frequencies in a population that is not evolving (serving as a null hypothesis).

  • The Equations: The principle uses two primary formulas:

  1. $p + q = 1$ (for allele frequencies, where $p$ is dominant and $q$ is recessive).

  2. $p^2 + 2pq + q^2 = 1$ (for genotype frequencies, where $p^2$ represents homozygous dominant, $2pq$ represents heterozygotes, and $q^2$ represents homozygous recessive).

  • Five Assumptions for Equilibrium:

  • No mutations occur to create new alleles.

  • No gene flow (no migration of individuals in or out).

  • Random mating (individuals do not choose mates based on specific traits).

  • An infinitely large population size (to prevent sampling error).

  • No natural selection (all genotypes have equal fitness).

Agents of Evolutionary Change

• Mutation: The ultimate and original source of all genetic variation. While mutation rates for individual genes are typically low (approximately $10^{-5}$ to $10^{-6}$ per locus per generation), the cumulative effect across genome is significant over time.

• Gene Flow: The transfer of alleles between populations through the movement of fertile individuals or gametes. This can introduce new alleles or alter existing frequencies, potentially counteracting the effects of local adaptation if it brings in maladaptive traits from other environments.

• Nonrandom Mating:

  • Assortative Mating: Individuals with similar phenotypes mate more frequently than expected by chance, which increases the proportion of homozygous individuals in a population.

  • Disassortative Mating: Individuals with different phenotypes mate, which increases the frequency of heterozygotes.

• Genetic Drift: Random fluctuations in allele frequencies that occur by chance rather than selection. This is most impactful in small populations.

  • Founder Effect: Occurs when a small number of individuals establish a new population, potentially carrying only a small, non-representative sample of the original population's genetic diversity.

  • Bottleneck Effect: A sharp reduction in population size due to environmental events (like floods or fires), leading to a loss of genetic variation in the surviving population.

• Selection: The only agent of evolutionary change that consistently leads to adaptation. Natural selection occurs when individuals with certain inherited traits produce more surviving offspring than others.

Natural Selection and Fitness

• Requirements for Natural Selection:

  1. Variation: Phenotypic differences must exist among individuals.

  2. Differential Reproductive Success: Some individuals must leave more offspring than others due to their traits.

  3. Inheritance: The advantageous traits must be genetically determined and transmissible to the next generation.

• Fitness: Defined as the relative reproductive success of a phenotype or genotype. It is multifaceted, involving survival (longevity), mating success (sexual selection), and the number of offspring produced per reproductive event.

Selection Types

• Directional Selection: Favors one phenotypic extreme, causing the population's trait distribution to shift in that direction.

  • Example: The evolution of antibiotic resistance in bacteria or the increase in beak size in Darwin's finches during periods of drought.

• Disruptive Selection: Favors both extremes of a trait while selecting against intermediate phenotypes. This can lead to a bimodal distribution and potentially speciation.

  • Example: Seedcracker birds having either very large beaks for hard seeds or very small beaks for soft seeds, with medium-beaked birds being less fit.

• Stabilizing Selection: Acts against extreme phenotypes and favors the intermediate types, which reduces phenotypic variation but maintains the mean.

  • Example: Human birth weight; infants significantly smaller or larger than the average ($3$ to $4$ kg) have higher mortality rates.

Maintenance of Genetic Variation

• Frequency-Dependent Selection: The fitness of a phenotype depends on how common it is in the population.

  • Negative Frequency-Dependent Selection: Rare phenotypes are favored, which maintains high levels of variation (e.g., predators looking for the most common prey type).

  • Positive Frequency-Dependent Selection: The most common phenotype is favored, which tends to eliminate variation.

• Oscillating Selection: Environmental changes cause selection to favor different phenotypes at different times, preventing any one allele from becoming fixed.

• Heterozygote Advantage: Occurs when individuals who are heterozygous at a particular locus have greater fitness than both types of homozygotes.

  • Example: In regions where malaria is prevalent, individuals heterozygous for the sickle-cell allele ($HbA/HbS$) are resistant to malaria and do not suffer from full sickle-cell disease, giving them a survival advantage over both homozygotes ($HbA/HbA$ and $HbS/HbS$).

Interactions and Limits

• Interactions Among Forces: Evolution is often the result of competing forces. For example, genetic drift or gene flow may introduce alleles that are less fit, which natural selection then works to remove. Mutation provides the variety that selection acts upon.

• Limits of Selection: Selection is constrained by historical accidents (it can only act on existing variations) and trade-offs. Additionally, many genes are pleiotropic, meaning a beneficial change in one trait might cause a detrimental change in another, limiting how far a trait can evolve.