AP Biology: Unit 7, Natural Selection: Evolutionary Mechanisms (Drill 29)

Question 1. Based on the data in Table 1, what does the allele frequency shift between Year 1 and Year 3 in the lava flow zone most directly indicate?

• A) Random genetic drift has reduced the frequency of the m allele in both zones equally.
• B) Gene flow from the sandy wash zone has introduced dark-coated migrant mice, raising the M allele in the data set described.
• C) Differential survival of dark-coated mice in the lava flow zone has increased the M allele frequency over time. ✓
• D) The MM genotype confers a reproductive advantage unrelated to coat color in lava flow habitats.

Explanation: Correct answer: C. The M allele frequency in the lava flow zone rose from ~0.48 to 0.675, calculated as freq(MM) + 0.5·freq(Mm) = 0.47 + 0.5(0.41) = 0.47 + 0.205 = 0.675. Since predation by visually hunting owls is the primary selective agent, dark-coated mice (M_) survive to reproductive maturity at higher rates on dark lava substrate, directly increasing M allele frequency. (A) is incorrect because random drift would produce unpredictable changes in both zones and cannot explain the consistent directional shift toward M in lava and m in sandy wash. (B) is incorrect because gene flow between zones would tend to homogenize allele frequencies, not drive them in opposite directions. (D) is incorrect because the passage attributes the selective agent to owl predation based on visual contrast, not to a reproductive mechanism independent of coat color.

Question 2. In the sandy wash zone, which mice are most likely to survive to reproductive maturity, and why?

• A) Pale-coated (mm) mice, because their coat color matches the substrate and reduces detection by visual predators. ✓
• B) Dark-coated (MM) mice, because dominant alleles confer greater fitness in all environments.
• C) Heterozygous (Mm) mice, because heterozygosity always confers a survival advantage over homozygotes within this context.
• D) All genotypes survive equally, because natural selection acts only when food resources are limited.

Explanation: Correct answer: A. In the sandy wash zone, the substrate is pale tan. Owls hunting by visual contrast will detect dark-coated mice more easily against this pale background. Pale-coated (mm) mice are cryptic, reducing detection and predation risk, allowing them to survive to reproductive maturity at higher rates. The data support this: mm frequency reaches 0.72 in the sandy wash by Year 3. (B) is incorrect because dominance describes inheritance pattern, not inherent fitness; fitness is always environment-dependent. (C) is incorrect because heterozygote advantage is a specific mechanism that must be demonstrated; the passage provides no evidence for it here. (D) is incorrect because natural selection operates whenever heritable traits are associated with differential survival or reproduction, regardless of resource limitation.

Question 3. A researcher claims: "The divergence in coat color frequencies between the two zones after three years is more consistent with natural selection than genetic drift." Which of the following best evaluates this claim?

• A) The claim is weakened because the sample size of 120 mice per zone is too small to distinguish selection from drift.
• B) The claim is weakened because the passage does not measure reproductive success directly, only coat color genotypes.
• C) The claim is weakened because MC1R mutations can occur only in the sandy wash zone and are physically incapable of arising in mice living in the lava flow zone.
• D) The claim is strengthened because allele frequencies shifted in opposite and predictable directions in each zone, consistent with the identified selective agent operating differently in each habitat. ✓

Explanation: Correct answer: D. The key signature distinguishing natural selection from genetic drift is directionality and predictability. In the lava flow zone, M allele frequency increased (favoring dark coats on dark substrate); in the sandy wash, m allele frequency increased (favoring pale coats on pale substrate). These opposite, environment-matched shifts are consistent with directional selection driven by the identified selective agent. Genetic drift produces random, unpredictable changes that would not reliably match substrate in both zones simultaneously. (A) is incorrect because n = 120 is a reasonable sample size and both zones show consistent directional trends, making drift less likely as the sole explanation. (B) is incorrect because differential genotype frequencies over time in habitat-appropriate directions are valid indirect evidence for differential survival. (C) is incorrect because differing mutation rates would introduce variation randomly, not produce the directional, substrate-matched divergence observed.

Question 4. If a wall were built between the lava flow and sandy wash zones, completely preventing migration between them, what would be the most likely long-term outcome?

• A) Allele frequencies in both zones would eventually converge as recurrent mutation restores the lost variation and equalizes the populations over time.
• B) The two populations would accumulate independent genetic changes, and continued isolation could eventually contribute to reproductive isolation over many generations. ✓
• C) The populations would experience balancing selection and maintain identical allele frequencies in both zones.
• D) Genetic drift would immediately eliminate the less common allele in each zone within one generation.

Explanation: Correct answer: B. Preventing gene flow means the two populations evolve independently. Each continues to experience directional selection from owl predation, and mutations, genetic drift, and other selection pressures will accumulate separately. Over many generations the populations may diverge increasingly in genotype and potentially in additional traits. Increased divergence is the most certain near-term outcome; whether this divergence eventually reaches reproductive isolation depends on the degree and duration of isolation and the pace of other evolutionary changes; it is a possible longer-term consequence, not a guaranteed one. (A) is incorrect because mutation alone would not reverse strong directional selection or cause convergence between populations under different selective regimes. (C) is incorrect because balancing selection maintains variation within a population; these zones are under different directional pressures. (D) is incorrect because genetic drift eliminates alleles gradually, not immediately.

Question 5. The rock pocket mouse study is often cited as evidence for natural selection acting in observable time. Which feature of the study design most strengthens this interpretation?

• A) The study was conducted entirely on preserved museum specimens, so it could not track any allele-frequency change in a living population over time.
• B) The study documents allele frequency change in a known population over a defined time period with an identified selective agent, making the data consistent with ongoing natural selection. ✓
• C) The study involves a small, geographically isolated population where selection is stronger than in larger mainland populations.
• D) The study demonstrates that coat color is the only trait under selection in desert rodent populations.

Explanation: Correct answer: B. The strength of this study comes from combining several elements: measurable heritable variation (MC1R genotypes), documented allele frequency change across years, a defined time period, and an identified ecologically plausible selective agent (owl predation by visual contrast). Together these make the data consistent with directional selection operating in each habitat. Compared to fossil evidence, which shows change over time but cannot directly measure allele frequencies or identify selective agents in real time, this study provides a more complete mechanistic picture. (A) is incorrect because precision of measurement, while valuable, is not the key distinguishing feature. (C) is incorrect because the study is in the Sonoran Desert, not on an island, and population size affects drift, not the quality of the selection evidence. (D) is incorrect because the study does not claim coat color is the only selected trait, and overclaiming would weaken rather than strengthen the conclusions.