Chapter 6: The Ways of Change: Drift and Selection (Part 2)

Introduction

  • Overview presented by Faith Hall, a junior biology major with a minor in psychology.

  • Experience includes taking this course previously and achieving an A in Fall 2024.

  • Weekly meeting schedule for the Support Instructor (SI) sessions includes:

    • Tuesdays: 6:30 PM - 7:45 PM in HELD 118

    • Thursdays: 5:30 PM - 6:45 PM in HECC 202

  • Additional resources available on the instructor's website.

Objectives for the Course

  • Students should be able to:

    • Define population genetics.

    • Calculate allele frequencies within a population.

    • Determine Hardy-Weinberg equilibrium using allele frequencies.

    • Analyze the effects of genetic drift across different population sizes.

    • Predict outcomes of bottleneck and founder events on allelic diversity.

    • Compare measures of fitness between phenotypes and alleles.

    • Discuss pleiotropy's impact on selection acting on alleles.

    • Understand the limitations of natural selection on dominant alleles.

    • Evaluate inbreeding effects on individual fitness and population genetics.

Genetic Drift

  • Definition: Genetic drift is a mechanism of evolution caused by random sampling errors during reproduction, predominant in small populations.

    • Observation of Populations:

    • Populations can lose alleles over generations; examples with allele dynamics are visualized over multiple generations.

    • Examples of fixed alleles:

      • 100% with certain alleles (e.g., white-eyed flies), leading to fixation events where certain alleles disappear.

Impact of Genetic Drift

  • Reduced Genetic Variation:

    • Genetic drift leads to loss of diversity; some alleles become fixed while others are lost.

  • Bottleneck Effect:

    • A bottleneck occurs when population size is significantly reduced, resulting in a nonrepresentative allele set in subsequent generations.

    • Example: Rare alleles are more likely to be lost, their survival depends on their frequency before the bottleneck and the bottleneck's severity.

Founder Effect

  • Definition: A unique bottleneck resulting from a small number of individuals colonizing a new habitat.

  • Case Study of Norfolk Island:

    • Evidence of high incidence (25.5% of inhabitants) of migraine headaches linked to X chromosome allele due to founder effect.

Concept of Fitness

  • Fitness: It pertains to an individual's reproductive success and survival relative to others, linked to specific phenotypes.

    • Components of Fitness:

    • Survival to maturity

    • Mating success

    • Fecundity

    • Relative fitness (w): Comparison of individual contribution to overall population fitness.

Selection and Fitness

  • Average Excess Fitness:

    • Defined as the difference between the fitness of individuals with a specific genotype and the average fitness of the population:

    • Δp=p×(aA1ω)\Delta p = p \times \left( \frac{a_{A1}}{\omega} \right)

      • Where:

      • Δp\Delta p = change in allele frequency due to selection

      • pp = frequency of the A1 allele

      • ω\omega = average fitness of the population

      • aA1a_{A1} = average excess fitness for the A1 allele.

Dynamics of Natural Selection

  • Natural selection has different effectiveness based on population size.

    • Selection has more impact in large populations, whereas drift is more considerable in smaller ones.

  • Examples of Evolutionary Response:

    • Minor fitness advantages can lead to significant allele frequency changes over time.

Pleiotropy

  • Definition: A single mutation affecting multiple traits; differentiates into

    • Antagonistic Pleiotropy: Beneficial effects on one trait could harm another.

  • Example of Pesticide Resistance in Mosquitoes:

    • Increased Ester1 gene frequency leads to pesticide resistance but increased predation risk from spider populations.

Experimental Evolution

  • E. coli Long-Term Evolution Experiment (LTEE):

    • Initiated by Richard Lenski, tracking 12 genetically identical populations since 1988.

    • Populations have evolved with selection pressures resulting in increased fitness.

    • Each 500 generations, genome analysis provides data on mutation dynamics.

Allele Relationships

  • Bacterial genetics serve as a model due to haploidy simplifying interactions.

  • Different allele types:

    • Additive Alleles: Homozygotes showcase twice the phenotypic effect compared to heterozygotes.

    • Dominance: A dominant allele's presence can mask a recessive allele in a heterozygous condition.

Mutation Dynamics

  • Mutation Rates:

    • Lower rates for specific genes, but numerous new mutations arise yearly; e.g., in humans, approximately 7.9 billion new mutations annually.

  • Mutation-Selection Balance: A concept of equilibrium reached through the interplay of negative selection against disadvantageous alleles and the introduction of new mutations.

Balancing Selection

  • Definition: Type of selection favoring more than one allele, maintaining genetic diversity.

  • Mechanisms:

    • Negative Frequency-Dependent Selection: Rarer phenotypes favored over common ones.

    • Heterozygote Advantage: Heterozygotes display greater fitness compared to homozygotes, such as seen in the context of sickle-cell anemia facilitating malaria resistance.

Inbreeding

  • Inbreeding can impact genetic health and lead to increased homozygosity of rare recessive alleles.

  • Example: The Habsburg dynasty, notably Charles II of Spain, exhibited severe inbreeding effects leading to significant health issues.

Genetic Spatial Structure

  • Population Subdivision: Influences genetic diversity and connectivity based on physical landscape and organism mobility.

  • Gene flow acts as a counterbalance, homogenizing allele frequency amidst population divergence created by genetic drift.

Summary - Key Concepts

  • Genetic drift is a crucial evolutionary mechanism with greater effects in small populations, while larger populations exhibit stronger selection effects.

  • Genetic bottlenecks and inbreeding can lead to significant allele loss and decreased fitness.

  • Balancing selection and mutation provide avenues for maintaining genetic diversity and addressing challenges of fitness dynamics in populations.