chapter 13 Evolutionary Processes: Gene Flow, Genetic Drift, Non-Random Mating, Sexual Selection, and Maintenance of Variability

Herbicide/Pesticide Resistance & Natural Selection

• Any chemical control (herbicides, pesticides) imposes immediate selection pressure.

  • Individuals susceptible to the chemical die; resistant genotypes survive and reproduce.

  • Agriculture must therefore keep changing or rotating chemicals because populations evolve resistance quickly.

• Illustration of basic evolutionary rule: organisms not suited to an environment are eliminated; the rest reproduce.

Gene Flow and Genetic Divergence

• Gene flow = movement of alleles among populations.

  • Example: individuals from Central mating with Clemson, Seneca, Anderson → larger combined gene pool.

• Consequences:

  • Increases total genetic variation within each population.

  • Reduces divergence between populations (they share more alleles).

• If gene flow stops, populations can diverge so much that they become reproductively isolated—the first step in speciation.

Reproductive Isolation & Species Concepts

• Biological Species Concept (Ernst Mayr)

  • A species = group capable of interbreeding & producing fertile offspring; reproductively isolated from others.

• Limitations: cannot apply to fossils or asexual organisms.

  • Morphological Species Concept (fossils, museum specimens) – based on physical traits.

  • Ecological Species Concept – based on unique ecological niche.

• Course focus: reproductive isolation as the critical first step toward the formation of new species.

Genetic Drift

• Definition: random changes in allele frequencies most pronounced in small populations.

• Key property: drift is not adaptive; survival may be random, not linked to fitness.

• Mechanism: disproportionate sampling; some alleles lost, others become fixed (frequency =1).

Bottleneck Effect

• Sudden reduction in population size (natural disaster, habitat loss, etc.).

  • Forest‐fire example with light/dark moths (graphic: initially ~10 % light; post-fire frequencies changed).

• Habitat fragmentation → forest corridors shrink → limited migration → local alleles may fix.

  • California salamanders: neighbouring populations hybridise; distant north vs south are now reproductively isolated.

• Stronger impact in small populations: majority fail to reproduce, next generation is a random subset of alleles.

Founder Effect

• A new population founded by a few individuals; allele mix reflects chance.

  • Historical example: people on the Mayflower; modern examples in isolated religious communities.

  • Recessive traits can reach high frequency (e.g., hemophilia in European royalty due to inbreeding among nobles).

• Outcomes:

  • Formerly rare alleles may become common or absent.

  • Inbreeding increases → rare recessive disorders more frequent.

Non-Random Mating (Assortative Mating)

• Individuals choose mates with similar phenotype or reject dissimilar phenotypes.

  • Human example: race-based mate choice (now declining but historically significant).

• Consequences:

  • Increases homozygosity at the loci involved.

  • Notable case: Island of Pingelap—non-random mating raised frequency of color-blindness allele.

Sexual Selection

• Type of natural selection favoring traits that enhance mating success rather than direct survival.

Male Perspective

• Males produce abundant sperm → compete to inseminate many females.

  • Anecdote: captive white-tailed deer program—“Fred,” a sterilized teaser buck, aggressively fought for access to does.

• Traits: combat structures (antlers), size, displays.

Female Choice

• Eggs are costly; females are choosy.

  1. Good-Genes Hypothesis

  • Choose males with traits signaling health & survival potential (bright plumage in ducks).

  1. Runaway Hypothesis

  • Choose males with exaggerated ornamental traits (peacock train).

• Leads to sexual dimorphism—males more ornate/large than females.

Maintenance of Genetic Variability

• Populations with low variability may fail to adapt to change; diversity is advantageous.

Levels of Diversity

• Genetic diversity – DNA variation within species.

• Species diversity – number of species in a community.

• Ecosystem diversity – variety of ecosystems in a region (e.g., abrupt cornfield → Rocky Mountains in Colorado).

Selective Constraints & Imperfect Adaptations

• Adaptations are constrained; cannot be perfect.

  • Tropical trees with huge, largely inedible fruits; effective dispersers (megafauna) may be extinct.

  • Loss of orangutans in Indonesia → reduced seed dispersal, forest regeneration declines.

Disruptive Selection & Polymorphism

• Disruptive selection: environment favors extreme phenotypes; selects against the mean.

• Populations occupying wide geographic range may split into subspecies via differing local conditions & niche specialization.

Heterozygote Advantage & Allele Sheltering

• Heterozygotes mask lethal recessive alleles, allowing them to persist.

• Classic case: Sickle-cell disease

  • Mutation: single nucleotide change → single amino-acid substitution in hemoglobin.

  • \text{Hb}^s allele lethal in homozygotes, but heterozygotes resistant to malaria (Plasmodium).

  • Example of pleiotropy (one gene, many effects) & balancing selection.

Hardy–Weinberg Principle (Context for Drift & Mating)

• In an ideal population (large, random mating, no selection, mutation, migration) allele frequencies remain constant:
  (p^2 + 2pq + q^2 = 1)

• Violations (small size, non-random mating, selection, migration) produce evolution via mechanisms above.

Key Takeaways

• Evolutionary mechanisms are interconnected:

  • Selection (natural & sexual) = non-random differential reproduction.

  • Gene flow homogenizes populations.

  • Genetic drift and founder/bottleneck effects create random divergence.

  • Non-random mating alters genotype frequencies directly.

• Reproductive isolation is the gateway to speciation; understanding mechanisms that reduce gene flow is essential for explaining biodiversity.