Population Genetics & Hardy-Weinberg Vocabulary

Microevolution vs. Macroevolution

• Evolution = change in allele frequencies over time.
  • Microevolution
  • Small‐scale changes in allele frequencies within a single population.
  • DOES NOT produce a new species.
  • Example: shift in gene frequencies while a lineage still lives in water.
  • Macroevolution
  • Large‐scale changes that culminate in speciation (reproductive isolation).
  • Example: water-living ancestor → divergence into tetrapods that can live on land.

Gene Pools & Allele Frequency Basics

• Gene pool = total collection of alleles in a population.
• Frequency of each allele is expressed as a proportion (0–1 or 0–100 %).
• For a locus with two alleles (dominant A and recessive a):
  • p = frequency of A
  • q = frequency of a
  • Fundamental rule: p + q = 1
• Counting method illustrated with fish example (7 fish, 14 alleles)
  • Dominant alleles counted = 6 → 6/14 = 0.429 (42.9 %)
  • Recessive alleles counted = 8 → 8/14 = 0.571 (57.1 %)
  • Percentages must sum to 100 % (check for accuracy).

Evolutionary Forces That Shift Allele Frequencies

• Natural selection
  • Alleles improving survival or reproduction become more common.
• Sexual selection / non-random mating
  • Attractive traits → more mating opportunities → higher allele freq.
  • Instructor’s advice: “Do not mate randomly.”
• Mutation
  • New alleles appear via DNA error; provides raw material for evolution.
• Genetic drift
  • Random change, strongest in small populations (e.g., tornado bottleneck).
• Gene flow (migration)
  • Immigration/emigration moves alleles between populations; mixes gene pools.

Hardy–Weinberg Principle (H-W)

• Provides the “null model” for a NON-evolving population (genetic equilibrium).
• Under perfect equilibrium, genotype frequencies follow: p^2 + 2pq + q^2 = 1
  • p^2 = frequency of homozygous dominant (AA)
  • 2pq = heterozygous (Aa)
  • q^2 = homozygous recessive (aa)
• Purpose
  • Allows researchers to test whether real populations deviate → evidence of evolution.
  • Functions like a straw-man/null hypothesis: we try to FALSIFY H-W to show evolution.

Five Assumptions for Hardy–Weinberg Equilibrium

1. No natural selection.
2. Random mating (no sexual selection).
3. No mutation.
4. Infinitely large (or very large) population → no genetic drift.
5. No gene flow (no migration in/out).

Deviations & Their Evolutionary Implications

Worked Example: Ear-Wax Consistency (Human Trait)

• Alleles: W (wet earwax, dominant) & w (dry earwax, recessive).
• Field data: 9 % of 100 people have dry earwax phenotype → q^2 = 0.09.
  • Solve for q: q = \sqrt{0.09} = 0.30 (30 %).
  • Solve for p: p = 1 - q = 1 - 0.30 = 0.70 (70 %).
• Genotype frequencies predicted under H-W:
  • p^2 = 0.70^2 = 0.49 → 49 % WW.
  • 2pq = 2(0.70)(0.30) = 0.42 → 42 % Ww (wet earwax carriers of w).
  • q^2 = 0.09 → 9 % ww.
• Demonstrates connection between phenotype counts and underlying allele distribution.

Additional Example: 10-Individual Population

• Genotypes listed: 4 AA, 4 Aa, 2 aa (total alleles = 20).
• Dominant A alleles: 4\times2 + 4\times1 = 12 → 12/20 = 0.60 (p = 0.60).
• Recessive a alleles: 4 + 4 = 8 → 8/20 = 0.40 (q = 0.40).

Adaptations vs. Acclimations (Acquired Traits)

• Adaptation = inherited genetic trait enhancing fitness (e.g., bats’ large ears for echolocation).
• Acclimation (acquired trait) = learned/physiological adjustment, not genetic (e.g., bats avoid flying in rain).
• Both influence survival, but only adaptations are heritable and thus affect allele frequencies across generations.

Laboratory & Course Context Mentioned

• Previous lab activities once included PCR & gel electrophoresis; curriculum now simplified.
• Hardy–Weinberg math problems formerly common; current focus = CONCEPTS, 5 assumptions.
• Students still expected to:
  • Memorize the 5 H-W conditions.
  • Understand allele‐frequency calculations (even if not solving full equations on exams).
  • Complete transcription/translation worksheets & term projects (due before Monday to receive credit).

Ethical / Practical / Philosophical Angles Discussed

• H-W as a “no fun” model: impossible in nature, highlights how real life always deviates.
• “Hot surfer cousin” joke underscores gene flow’s role—social behaviors affect genetic makeup.
• Instructor’s gendered humor (“encourage nonrandom mating”) used to illustrate sexual selection in humans.

Key Equations & Symbols (All in LaTeX)

• Fundamental allele relationship: p + q = 1
• Genotype frequency expansion: p^2 + 2pq + q^2 = 1
• Solving for missing variable: q = \sqrt{q^2} ; p = 1 - q
• Heterozygote frequency: 2pq

Quick Study Checklist

• Know definitions: microevolution, macroevolution, allele, gene pool.
• Be able to describe & give examples of the five evolutionary forces.
• Memorize the 5 Hardy–Weinberg assumptions & explain why each matters.
• Practice allele counting & H-W calculations (use examples above as models).
• Differentiate adaptation (genetic) vs. acclimation (learned/physiological).
• Understand why Hardy–Weinberg is a null hypothesis and how deviation → evidence of evolution.