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AP Biology Unit 7 Practice Questions: Natural Selection

10 original exam-style questions on Natural Selection. Answer each one to see the explanation — no account needed.

Question 1 of 10 · Speciation Models

Which of the following scenarios would MOST likely lead to sympatric speciation?
  1. A. A river forms through the middle of a squirrel population's habitat, physically separating two groups.
  2. B. A volcanic eruption reduces a bird population to 20 survivors on an isolated island.
  3. C. Two subpopulations of a plant species develop different flowering times due to disruptive selection on flowering date, preventing gene flow between them.
  4. D. A glaciation event separates a salamander population into two mountain ranges for 50,000 years.
Show answer and explanation

Correct answer: C

Sympatric speciation occurs within a single geographic area without physical barriers; disruptive selection on flowering time can create reproductive isolation between subpopulations in the same location if early-flowering individuals predominantly mate with other early-flowering individuals. Choices A, B, and D all involve geographic separation, which describes allopatric speciation.

Question 2 of 10 · Hardy-Weinberg Equilibrium

A population biologist monitors a population of field mice over 10 years. The mice carry two alleles at a coat color locus: B (dark brown, dominant) and b (tan, recessive). Field conditions shift from grassland to open farmland at year 5, reducing cover for dark-colored mice.

Allele and genotype frequencies in a field mouse population
YearPopulation SizeFreq(B)Freq(b)BB countBb countbb count
08000.600.40294390116
107600.410.59127497136
At Year 10, the Bb genotype count (497) is substantially higher than the Hardy-Weinberg expectation of 2(0.41)(0.59)×760≈3682(0.41)(0.59) \times 760 \approx 368. Which explanation would best account for this excess of heterozygotes?
  1. A. Heterozygote advantage (overdominance) favored Bb individuals with higher survival or reproduction
  2. B. Genetic drift eliminated rare alleles, reducing heterozygosity toward zero
  3. C. Non-random mating caused BB males to preferentially mate with bb females
  4. D. Mutation pressure converted BB individuals to Bb at a high rate
Show answer and explanation

Correct answer: A

An excess of heterozygotes relative to H-W expectations is the hallmark of heterozygote advantage (overdominance), where Bb individuals have a fitness advantage over both homozygotes. Genetic drift (B) would more likely reduce heterozygosity by fixing one allele, not elevate it above H-W expectations.

Question 3 of 10 · Balancing Selection and Heterozygote Advantage

In a population under balancing selection for a gene with two alleles (A1 and A2), heterozygotes (A1A2) have higher fitness than either homozygote. What is the expected long-term outcome for allele frequencies?
  1. A. A1 will reach fixation because it is the dominant allele.
  2. B. Both A1 and A2 will be maintained at stable intermediate frequencies in the population.
  3. C. A2 will be eliminated because it reduces fitness in homozygotes.
  4. D. Allele frequencies will fluctuate randomly as genetic drift overwhelms selection.
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Correct answer: B

Heterozygote advantage (overdominance) is a form of balancing selection that maintains both alleles at stable equilibrium frequencies — eliminating either allele would reduce mean population fitness because heterozygotes have the highest fitness. This mechanism explains the persistence of the sickle-cell allele in malaria-endemic regions as a classic example.

Question 4 of 10 · Natural Selection at the Molecular Level

A population of stickleback fish in a freshwater lake has significantly fewer lateral body plates than an ancestral ocean population. Researchers find that the freshwater population shows reduced expression of the Ectodysplasin (EDA) gene. Which of the following best explains this change?
  1. A. The fish intentionally reduced plate production to improve swimming efficiency in fresh water.
  2. B. Natural selection favored individuals with lower EDA expression because reduced plates provided a fitness advantage in the freshwater environment.
  3. C. Random mutation of the EDA gene increased its expression, reducing plate number through negative feedback.
  4. D. Genetic drift eliminated the high-EDA allele when the population colonized the lake.
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Correct answer: B

In multiple independent freshwater stickleback populations, reduced lateral plates have evolved repeatedly, strongly implicating natural selection at the EDA locus; reduced armor may reduce energy costs or predation in freshwater where the selective regime differs from the ocean. Individual fish cannot intentionally regulate gene expression in response to environment (choice A confuses developmental plasticity with intentional adaptation).

Question 5 of 10 · Hardy-Weinberg Equilibrium

A population geneticist surveys a population of 500 individuals for the MN blood group, which is controlled by a single gene with two codominant alleles, LML^M and LNL^N. The genotype counts are shown below.

MN blood group genotype frequencies in a population of 500 individuals
GenotypeNumber of IndividualsObserved Frequency
LMLML^M L^M1800.36
LMLNL^M L^N2400.48
LNLNL^N L^N800.16
Under Hardy-Weinberg equilibrium, what is the expected number of LMLNL^M L^N heterozygotes in this population of 500?
  1. A. 180
  2. B. 220
  3. C. 240
  4. D. 300
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Correct answer: C

Using p=0.60p = 0.60 and q=0.40q = 0.40, the expected heterozygote frequency is 2pq=2(0.60)(0.40)=0.482pq = 2(0.60)(0.40) = 0.48, giving 0.48×500=2400.48 \times 500 = 240. Because the observed frequency matches the H-W expectation exactly, this population is not detectably evolving at this locus.

Question 6 of 10 · Modes of Natural Selection

In a population of rock pocket mice, dark-furred individuals survive at higher rates on lava flows, while light-furred individuals survive at higher rates on sandy substrates. If a new lava flow covers most of the remaining sandy habitat, which of the following best explains the expected change in allele frequencies over several generations?
  1. A. Directional selection will shift the population toward higher frequencies of the dark fur allele as dark substrate becomes dominant
  2. B. Stabilizing selection will favor intermediate fur coloration as the mixed terrain creates intermediate selective pressures
  3. C. Disruptive selection will intensify, increasing the frequencies of both extreme phenotypes relative to intermediate ones
  4. D. Genetic drift will become the primary evolutionary mechanism as the effective population size decreases with habitat loss
Show answer and explanation

Correct answer: A

As lava (dark substrate) comes to dominate the habitat, dark fur provides a survival advantage across an increasing proportion of the environment, shifting allele frequencies in a single direction — the hallmark of directional selection. Choice C (disruptive selection) would apply only if both extreme environments were maintained at comparable proportions simultaneously.

Question 7 of 10 · Speciation and Reproductive Isolation

Two populations of squirrels are separated by the formation of a large river that neither population can cross. Over thousands of generations the two populations diverge genetically, and eventually individuals from the two populations can no longer interbreed even if reunited. Which of the following terms most accurately describes the mechanism of speciation that occurred?
  1. A. Allopatric speciation driven by geographic isolation and independent genetic divergence
  2. B. Sympatric speciation driven by polyploidy within a shared geographic range
  3. C. Artificial selection imposed by an environmental change in food availability
  4. D. Stabilizing selection maintaining a common phenotypic optimum in both populations
Show answer and explanation

Correct answer: A

Allopatric speciation occurs when a geographic barrier prevents gene flow between populations; independent mutation, genetic drift, and natural selection accumulate differences until reproductive isolation is complete. Sympatric speciation (B) occurs without geographic separation, typically through polyploidy in plants, and is not applicable to this squirrel scenario.

Question 8 of 10 · Vestigial Structures

Vestigial structures, such as the pelvic bones in whales or the appendix in humans, are best interpreted as evidence of which evolutionary phenomenon?
  1. A. Convergent evolution, because similar structures arise independently in unrelated lineages.
  2. B. Descent with modification from ancestors in which the structure served a functional role.
  3. C. Adaptive radiation, because ancestral species rapidly diversified into all modern forms.
  4. D. Lamarckian inheritance, because individuals passed reduced structures to offspring after disuse.
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Correct answer: B

Vestigial structures are reduced, non-functional remnants of structures that were functional in ancestors; they persist because natural selection has not completely eliminated them, and they provide clear evidence that modern species are modified descendants of ancestors with different body plans. Convergent evolution (choice A) explains the independent origin of similar functional structures, the opposite concept.

Question 9 of 10 · Bottleneck Effect

A population of beetles living in a bottle-neck event is reduced to 10 individuals for several generations before recovering. Which of the following evolutionary consequences is most likely?
  1. A. Increased genetic diversity due to elevated mutation rates during the bottleneck.
  2. B. Reduced genetic diversity due to genetic drift during the small-population phase.
  3. C. Increased fitness due to selection removing all deleterious alleles during the bottleneck.
  4. D. Unchanged allele frequencies because natural selection maintains equilibrium.
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Correct answer: B

A population bottleneck drastically reduces effective population size, allowing genetic drift to randomly eliminate many alleles; the surviving population carries only the alleles present in those 10 individuals, greatly reducing overall genetic diversity. Bottlenecks do not reliably purge all deleterious alleles because selection cannot act on rare recessives when they are mostly in heterozygotes.

Question 10 of 10 · Disruptive Selection

A population of moths has a bimodal distribution of wing color: many individuals are very dark and many are very light, but few have intermediate shading. Which type of natural selection is most likely acting on this population?
  1. A. Stabilizing selection, which favors the intermediate phenotypes and eliminates the extremes.
  2. B. Directional selection, which shifts the entire distribution toward lighter wing color.
  3. C. Disruptive selection, which favors both extreme phenotypes and selects against intermediates.
  4. D. Sexual selection, which favors darker males as more attractive to females.
Show answer and explanation

Correct answer: C

Disruptive (diversifying) selection is defined by selection against intermediate phenotypes, producing a bimodal distribution with high frequencies of both extremes; this pattern results when two distinct microhabitats (e.g., dark bark and light lichen) each favor a different extreme. Stabilizing selection produces the opposite — a narrow unimodal distribution centered on the intermediate value.

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