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Volume I · MMXXVI AP Biology
Library› AP Biology› Unit 7: Natural Selection
⁂   AP Biology · Unit 7

7. Natural Selection

13–20% of the AP exam. 9 key topics. 4 common errors analyzed.

13–20% exam weight 22 instructional periods

Unit 7: Natural Selection

Unit 7 covers natural selection — a core component of the AP Biology curriculum. Master these concepts to build the mechanistic vocabulary the FRQ rubrics demand.

Key topics in this unit

  1. Evidence for evolution: fossil record, comparative anatomy (homologous, analogous, vestigial structures), molecular homology, biogeography
  2. Mechanisms of natural selection: heritable variation, differential reproductive success, adaptation
  3. Types of natural selection: directional, stabilizing, disruptive (diversifying)
  4. Other evolutionary mechanisms: genetic drift (bottleneck and founder effects), gene flow, mutation
  5. Hardy-Weinberg equilibrium: conditions (large population, random mating, no mutation/migration/selection) and allele/genotype frequency calculations
  6. Hardy-Weinberg as a null model — deviation from equilibrium indicates evolution
  7. Speciation: allopatric vs. sympatric speciation; pre- and post-zygotic reproductive isolation mechanisms
  8. Phylogenetics: constructing and interpreting cladograms, parsimony principle, shared derived characters (synapomorphies)
  9. Molecular clocks and evidence from DNA/protein sequence comparisons

Common errors — what the rubric penalizes

These are the most frequently penalized mistakes on AP exam FRQs and MCQs. Review each before your next practice session.

Common error

Stating that individual organisms evolve rather than populations — natural selection acts on individuals but evolution occurs at the population level over generations

Common error

Miscalculating Hardy-Weinberg frequencies by omitting the 2pq term for heterozygotes or solving for q when the problem gives q² (requiring a square root step)

Common error

Confusing analogous structures (similar function, different evolutionary origin, e.g., bird wing and insect wing) with homologous structures (common ancestry, possibly different function)

Common error

Drawing cladograms with incorrect outgroup placement or misreading the most recent common ancestor from a branching diagram

Study checklist

  • Can you list all key topics from memory without this page?
  • Have you worked through at least one FRQ-style question per topic?
  • Can you explain each common error above — and why the correct answer differs?
  • Have you reviewed the relevant College Board CED section and formula sheet entries?
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