Notes on Evolutionary Theory: Selection, Variation, and Balancing Selection

Intensity of selection and the role of genetic variation

  • Intensity of selection reflects the rate of change in real time; visualized with generation time on the x-axis and allele frequency on the y-axis.
  • Why genetic variation matters: if a population has little genetic variation, selection has little material to act on. With two alleles at a locus (p and q, where q = 1 - p), higher standing variation generally enables a stronger or faster response to selection.
  • Selection coefficient s as a measure of the strength of selection; higher s means stronger selection and a more rapid shift in allele frequencies.
  • Response to selection depends on both the intensity of selection and the level of genetic variability in the population.
  • Summary takeaway: adaptation requires both a beneficial allele and enough genetic variation for selection to act upon.

Dominance and allele-frequency trajectories

  • Dominance relationships shape how quickly beneficial alleles spread through a population.
  • When the beneficial allele (A2) is not recessive and has favorable dominance, allele frequencies rise rapidly.
  • If there is no dominance (additive effect), the trajectory is intermediate.
  • If the beneficial allele is recessive (relative to the alternative), the allele takes longer to increase in frequency because selection acts only on the homozygotes; heterozygotes do not display the full advantageous phenotype.
  • Intuition: the A2 allele gains traction mainly through homozygotes; as a result, initial frequency is crucial for how fast fixation occurs.
  • In short: dominance accelerates adaptation; recessivity slows it, especially when the advantageous allele starts at a low frequency.

Examples of adaptation: lactose tolerance and skin color

  • Lactose tolerance in mammals: historically infants could digest milk (lactase enzyme), but adults largely could not. With domestication of livestock (milk production across ages), natural selection favored lactase persistence into adulthood.
  • Lactose tolerance is a classic example of adaptation driven by a nutritional/environmental shift (dietary changes due to human practices).
  • Light skin color evolution: in the Northern Hemisphere, populations in temperate/boreal regions show lighter skin color, correlated with UV exposure and vitamin D synthesis; this is another example of adaptation to environmental conditions.
  • These examples illustrate how selective pressures can drive allele-frequency changes across populations in response to real-world contexts.

Adaptation, mutation, and drift

  • When a beneficial allele is rare, it can be lost by random genetic drift or segregation even if it is advantageous.
  • Random fluctuations in allele frequencies (drift) can override selection when population size is small; in large populations, selection is more effective.
  • This creates a balance between selection and drift: strong selection dominates in large populations; drift dominates in small populations.

Genetic correlations, pleiotropy, and evolutionary trade-offs

  • Genetic correlations: a single gene can affect multiple traits (pleiotropy); selection on one trait can have correlated effects on others.
  • Example given: body height and foot size are genetically correlated; taller individuals tend to have larger feet.
  • Evolutionary triggers and trade-offs: selection for a beneficial allele can be constrained if it is linked to detrimental effects on reproduction or other fitness components.
  • A classic trade-off example: increased gain (e.g., greater size or color) may come at a cost to reproduction or cancer risk, illustrating why adaptation is not always unidirectionally beneficial.
  • Textbook discussion (Soay sheep) emphasizes that trade-offs can maintain variation in a population due to differing fitness across genotypes and environments.

Linkage, selective sweeps, and standing genetic variation

  • When a new beneficial mutation arises and experiences hitchhiking (linkage) with nearby alleles, a selective sweep can occur: the target locus and neighboring alleles increase in frequency together, reducing genetic diversity in the surrounding region.
  • This results in a characteristic signature: a sweep of a haplotype around the beneficial allele.
  • Standing genetic variation (preexisting variation): the beneficial allele is already present on multiple genetic backgrounds. In this case, the signature is different; there isn’t a single sweeping haplotype, and surrounding regions may retain more diversity.
  • Key contrast: new mutations can produce strong, localized hitchhiking signals; standing variation often yields more complex patterns of variation around the selected locus.

Balancing selection and heterozygote advantage

  • Balancing selection maintains multiple alleles in a population, often because heterozygotes have higher fitness than either homozygote (heterozygote advantage).
  • Classic example: sickle cell trait in malaria-endemic regions. Heterozygotes (HbA HbS) have higher fitness than either homozygote (HbA HbA or HbS HbS) due to partial protection against malaria while avoiding severe sickle-cell disease.
  • Equilibrium under overdominance (heterozygote advantage): when WAA, WAa, and W_aa denote fitnesses, the stable allele frequencies can be derived.
  • Fitness parameterization: for two alleles A and a with
    • W_{AA} = 1 - s
    • W_{Aa} = 1
    • W_{aa} = 1 - t
      where s and t are selection against the homozygotes.
  • Under overdominance, the equilibrium frequencies are:
    • p^* = rac{t}{s + t}
    • q^* = rac{s}{s + t}
      with p^* + q^* = 1
  • In real-world terms, heterozygote advantage can maintain both alleles in populations, especially when there is a pathogenic environment (e.g., malaria) that differentially affects homozygotes and heterozygotes.
  • The concept explains why, despite directional pressures, diversity at a locus can be preserved rather than fixed.

Soay sheep: a case of trade-offs and heterozygote advantage

  • Soay sheep on an island off the coast of Scotland have shown patterns consistent with balancing selection and trait-trade-offs.
  • Observation described: individuals with certain high-hormone phenotypes show higher fitness, suggesting a trade-off between maintenance (physiological cost) and survival/reproductive success.
  • The summary indicates heterozygotes can exhibit higher overall fitness than either homozygote, illustrating a classic heterozygote advantage scenario and its role in maintaining genetic diversity.
  • The associated graph described (wild-type HO, heterozygote, and mutant) suggests that fitness is greatest in heterozygotes, consistent with balancing selection.
  • Takeaway: real-world systems like Soay sheep exemplify how trade-offs and heterozygote advantage can maintain polymorphisms and contribute to long-term persistence of multiple alleles.

Connections to foundational principles and real-world relevance

  • This material ties directly to fundamental population genetics concepts: natural selection, genetic variation, dominance, genetic drift, mutation, and recombination.
  • It links theoretical models (fitness schemata, overdominance) to observable patterns in nature (lactase persistence, skin color adaptation, malaria resistance, Soay sheep polymorphisms).
  • Practical implications: understanding how dominance and variation shape adaptation helps explain why some traits spread quickly while others remain polymorphic, and why some populations maintain diversity despite selective pressures.
  • Ethical/philosophical note: balancing selection demonstrates that multiple genetic strategies can be advantageous depending on environment, underscoring that there is often not a single “optimal” genotype across all contexts.

Quick reference: key equations and concepts

  • Allele frequency relationships:
    • p + q = 1, ext{ with } q = 1 - p
  • Fitness configuration for overdominance (heterozygote advantage):
    • W{AA} = 1 - s,\n W{Aa} = 1,
      W_{aa} = 1 - t
    • Stable equilibrium frequencies under overdominance:
    • p^* = rac{t}{s + t},\n q^* = rac{s}{s + t}
  • Conceptual distinctions:
    • Directional (positive) selection: allele frequency moves toward fixation; genetic variation tends to decrease.
    • Balancing selection: maintains multiple alleles; heterozygotes have fitness advantage; allele frequencies stabilize at a polymorphic equilibrium.
  • Key mechanisms discussed:
    • Dominance effects on the speed of adaptation.
    • Selective sweeps due to new mutations and hitchhiking of linked alleles.
    • Standing genetic variation and its different signatures compared to new mutations.
    • Genetic correlations and trade-offs (pleiotropy).
    • Real-world examples: lactose tolerance, skin color adaptation, sickle cell trait, Soay sheep