AP Biology Natural Selection — Worked Answer Explanations

Unit 7 · 8 questions explained

Below is a complete answer key for our AP Biology Natural Selection practice questions. For each question you'll find the correct choice, a full written explanation of how to get there, and — for every wrong answer — a short note on exactly why it's tempting and where it goes wrong. Reading these straight through is one of the fastest ways to find the gaps in a unit before exam day.

Prefer to test yourself first? Take the timed Natural Selection practice test and come back here to review, or head back to the Natural Selection unit overview.

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Question 1 · Easy
If allele frequencies are not changing across generations, the population is in:
- A
  Genetic drift.
  Why not A: Drift causes frequencies to change.
- B
  Hardy-Weinberg equilibrium.Correct
- C
  Speciation.
  Why not C: Speciation involves change.
- D
  Stabilizing selection.
  Why not D: Stabilizing selection eliminates extremes — frequencies still change.
Explanation
Hardy-Weinberg equilibrium occurs when allele and genotype frequencies remain constant across generations. The five conditions are: no mutation, no migration, no selection, infinite population, and random mating.

Key takeaway
Hardy-Weinberg = no change in allele frequencies; it's the null model for evolution.
Question 2 · Easy
Which of the following is NOT one of Darwin's four conditions for natural selection?
- A
  Variation among individuals.
  Why not A: Required.
- B
  Heritability of traits.
  Why not B: Required.
- C
  Differential reproductive success.
  Why not C: Required.
- D
  An external designer that selects favorable traits.Correct
Explanation
Darwin's four conditions: (1) variation, (2) heritability, (3) overproduction of offspring, (4) differential reproductive success. There is no external designer; selection arises from environmental pressures.

Key takeaway
Natural selection requires variation, heritability, overproduction, and differential reproductive success — no designer.
Question 3 · Easy
Antibiotic resistance in bacterial populations is best explained by:
- A
  Bacteria deciding to evolve when exposed to antibiotics.
  Why not A: Evolution is not directed or intentional.
- B
  Pre-existing variation in resistance, with selection favoring resistant bacteria.Correct
- C
  Lamarckian inheritance of acquired resistance.
  Why not C: Lamarckian inheritance is not the modern explanation.
- D
  Random luck without any selective pressure.
  Why not D: Without selection, resistant bacteria wouldn't dominate.
Explanation
Bacterial populations contain pre-existing genetic variation. When antibiotics are applied, susceptible bacteria die and resistant ones survive and reproduce. This shifts the population toward resistance — classical natural selection.

Key takeaway
Antibiotic resistance: variation already exists; selection just amplifies it.
Question 4 · Easy
Genetic drift has a stronger effect in:
- A
  Large populations.
  Why not A: Effects are diluted in large populations.
- B
  Small populations.Correct
- C
  Populations with high mutation rates.
  Why not C: Mutation is a different evolutionary force.
- D
  Populations under strong selection.
  Why not D: Selection masks drift effects.
Explanation
Genetic drift is random change in allele frequency due to sampling. In small populations, sampling error is proportionally larger, so drift dominates. In large populations, deviations average out.

Key takeaway
Drift dominates in small populations; selection dominates in large populations.
Question 5 · Easy
Two populations of finches are separated by a mountain range. Over thousands of generations they evolve different beak shapes and can no longer interbreed. This is an example of:
- A
  Sympatric speciation.
  Why not A: Sympatric requires no geographic barrier.
- B
  Allopatric speciation.Correct
- C
  Polyploidy speciation.
  Why not C: Polyploidy is a chromosome-doubling mechanism, common in plants.
- D
  Convergent evolution.
  Why not D: Convergent evolution makes different lineages similar.
Explanation
Allopatric speciation occurs when a geographic barrier separates populations, allowing them to diverge through accumulated genetic differences until reproductive isolation evolves.

Key takeaway
Allopatric = different geography; sympatric = same area. Both can produce speciation.
Question 6 · Easy
Which of the following BEST illustrates evidence for common ancestry?
- A
  Bird wings and bat wings have similar bone structures despite different evolutionary origins.
  Why not A: That illustrates convergent evolution; the bones happen to be similar despite separate evolution.
- B
  Whales, humans, and dogs all share the same set of bones in their forelimbs (homologous structures).Correct
- C
  Eyes evolved independently in vertebrates and cephalopods.
  Why not C: Convergent evolution again.
- D
  Sharks and dolphins both have streamlined bodies for swimming.
  Why not D: Convergent evolution due to similar selective pressures.
Explanation
Homologous structures (same underlying bones, different functions) indicate common ancestry. The same bone-by-bone arrangement evolving independently in unrelated lineages would be vanishingly unlikely.

Key takeaway
Homology indicates common ancestry; analogy/convergence indicates similar selective pressures.
Question 7 · Medium
In a Hardy-Weinberg population, the frequency of the recessive phenotype is $0.16$ . What is the frequency of the heterozygous genotype?
- A
  $0.16$
  Why not A: That's the recessive phenotype frequency.
- B
  $0.36$
  Why not B: That's the dominant homozygote frequency.
- C
  $0.48$ Correct
- D
  $0.64$
  Why not D: Confused with the dominant phenotype frequency.
Explanation
$q^{2} = 0.16$ , so $q = 0.4$ and $p = 0.6$ . Heterozygote frequency $= 2 pq = 2 (0.6) (0.4) = 0.48$ .

Key takeaway
Hardy-Weinberg: $p^2 + 2pq + q^2 = 1$. Find $q$ from $q^2$, then compute $2pq$.
Question 8 · Medium
Disruptive selection is a type of natural selection in which:
- A
  Both extreme phenotypes are favored over the intermediate.Correct
- B
  The intermediate phenotype is favored.
  Why not B: That's stabilizing selection.
- C
  Only one extreme phenotype is favored.
  Why not C: That's directional selection.
- D
  All phenotypes are equally favored.
  Why not D: That would not be selection at all.
Explanation
Disruptive selection drives the population toward two distinct phenotypes by selecting against the average. It can lead to sympatric speciation if combined with assortative mating.

Key takeaway
Three types: stabilizing (favors mean), directional (favors one extreme), disruptive (favors both extremes).