AP Bio
Highest weight: 13–20%

Natural Selection

The single highest-weight unit. Hardy-Weinberg, drift, gene flow, mutation, selection, speciation, phylogenetics. Spend extra time here.

Must-know content

  • Darwin's mechanism: heritable variation + differential reproductive success → evolution by natural selection.
  • Evidence for evolution: fossil record, biogeography, comparative anatomy (homologous vs. analogous structures, vestigial organs), embryology, molecular sequence similarity.
  • Hardy-Weinberg equilibrium — null model of no evolution. Allele frequencies stay constant when:
    1. No mutation
    2. Random mating
    3. No gene flow
    4. Very large population (no drift)
    5. No selection
Hardy-Weinberg
p + q = 1 p² + 2pq + q² = 1
p² = AA homozygote frequency · 2pq = Aa heterozygote frequency · q² = aa homozygote frequency. Solve for q first: q = √(q²), then p = 1 − q, then 2pq.
  • Microevolution mechanisms:
    • Natural selection — directional, stabilizing, disruptive.
    • Genetic drift — bottleneck, founder effect.
    • Gene flow — migration mixes alleles.
    • Mutation — ultimate source of new alleles.
    • Non-random mating — assortative, sexual selection.
  • Speciation:
    • Allopatric — geographic isolation.
    • Sympatric — same area; e.g., polyploidy in plants.
    • Reproductive isolating mechanisms — prezygotic (habitat, temporal, behavioral, mechanical, gametic) and postzygotic (reduced hybrid viability/fertility).
  • Phylogenetics: cladograms based on shared derived characters; molecular clocks; common ancestry.
  • Origin of life: abiotic synthesis (Miller-Urey), RNA world hypothesis, endosymbiosis (eukaryote origin).
  • Extinction: mass extinctions reshape biodiversity; current human-caused extinction event.

Example questions

MCQ In a population, 16% of individuals show a recessive trait. Assuming HW equilibrium, what fraction is heterozygous? (A) 0.16 (B) 0.24 (C) 0.36 (D) 0.48

Answer: D. q² = 0.16 → q = 0.4; p = 0.6; 2pq = 2(0.6)(0.4) = 0.48.

FRQ A small island population of lizards is decimated by a hurricane; only 12 individuals survive. Identify the evolutionary force at work and predict how it will affect genetic diversity in the next generation.

Answer: This is genetic drift — specifically a bottleneck effect. The 12 survivors carry only a subset of the original gene pool, so allele frequencies in subsequent generations reflect that reduced sample, with overall lower genetic diversity. Some alleles may have been lost entirely; others may now be at much higher frequency than in the original population. The population is more vulnerable to inbreeding and to selection because fewer alleles remain available for adaptation.

MCQ Two species of fireflies share a habitat but never interbreed because they flash at different times of night. This is an example of: (A) Mechanical isolation (B) Temporal isolation (C) Behavioral isolation (D) Gametic isolation

Answer: C (behavioral). Some sources also classify timing-of-mating-behavior as temporal — accept B or C depending on framing; the AP CED leans behavioral when courtship signals (flash patterns) differ.

Drill flashcards

Unit 7 Natural selection Tap / Space to flip
Unit 7 Differential survival and reproduction based on heritable variation. Drives adaptation.
Unit 7 Fitness Tap / Space to flip
Unit 7 Relative reproductive success — the number of offspring that themselves reproduce.
Unit 7 Genetic drift Tap / Space to flip
Unit 7 Random change in allele frequencies due to chance. Strongest in small populations.
Unit 7 Bottleneck effect Tap / Space to flip
Unit 7 Drastic population reduction reduces genetic diversity by chance (e.g., overhunted cheetahs).
Unit 7 Founder effect Tap / Space to flip
Unit 7 A small group colonizes a new area, carrying only a subset of the original gene pool.
Unit 7 Hardy-Weinberg Tap / Space to flip
Unit 7 Null model of no evolution. p² + 2pq + q² = 1 holds when 5 conditions are met (no mutation/migration/drift/selection, random mating).
Unit 7 Directional selection Tap / Space to flip
Unit 7 Favors one phenotypic extreme; shifts the population mean (e.g., antibiotic resistance).
Unit 7 Stabilizing selection Tap / Space to flip
Unit 7 Favors intermediate phenotypes; reduces variance (e.g., human birth weight).

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