AP Biology: Unit 7, Natural Selection & Continuing Evolution (Drill 33)

Question 1. Using the breeder's equation (R = h² × S) and the data in Table 1, what is the predicted response to selection (R)?

• A) 0.54 mm
• B) 0.79 mm
• C) 0.43 mm ✓
• D) 1.33 mm

Explanation: Correct answer: C. R = h² × S = 0.79 × 0.54 = 0.4266 ≈ 0.43 mm. This is the predicted increase in mean beak depth in the offspring generation relative to the pre-drought population. (A) 0.54 mm is S, the selection differential. (B) 0.79 is h², the heritability. (D) 1.33 mm is approximately h² + S, incorrect; the breeder's equation multiplies, not adds.

Question 2. The heritability estimate (h² = 0.79) for beak depth in this population indicates which of the following?

• A) Approximately 79% of the variation in beak depth in this population is attributable to additive genetic differences among individuals, making beak depth highly responsive to selection. ✓
• B) 79% of all G. fortis individuals have a beak depth greater than the population mean.
• C) The beak depth trait will increase by 79% in every generation under selection.
• D) Beak depth is 79% determined by genetics in every finch species, so the same heritability value applies universally across all populations of Darwin's finches.

Explanation: Correct answer: A. Narrow-sense heritability (h²) is the proportion of phenotypic variance attributable to additive genetic variance. A value of 0.79 means ~79% of observed variation in beak depth is due to genetic differences transmitted from parent to offspring, making the trait highly responsive to selection. (B) is incorrect; heritability is a ratio of variances, not a frequency distribution around the mean. (C) is incorrect; the response to selection is calculated via the breeder's equation, not directly from h². (D) is incorrect; heritability is population- and environment-specific and does not apply universally across species.

Question 3. A researcher claims: "The 1977 drought demonstrates that natural selection can produce measurable evolutionary change within a single generation." Which of the following, if true, would most strongly support this claim?

• A) The researchers measured beak depth in the same individuals before and after the drought.
• B) The 1978 survivors of the drought were randomly selected with respect to beak depth, with no association to survival.
• C) Beak depth is not heritable in G. fortis populations on other islands.
• D) Offspring born in 1978–1979 had a higher mean beak depth than the 1976 population mean, consistent with the breeder's equation prediction. ✓

Explanation: Correct answer: D. The claim is about evolutionary change, a heritable shift transmitted across generations, not just a change in survivor composition. The breeder's equation predicts offspring mean beak depth should be ~0.43 mm greater than pre-drought. Confirming this in actual offspring data would demonstrate that the selection differential was real, the trait is heritable, and the change was passed to the next generation, all requirements for evolution by natural selection. The critical distinction is between selection within a generation (change in survivor mean) and evolution across generations (change transmitted to offspring). (A) is incorrect; measuring survivors before and after documents selection, not inheritance. (B) is incorrect; random survival with respect to beak depth would mean no selection occurred. (C) is irrelevant to what occurred on Daphne Major.

Question 4. After the drought ended and normal rainfall returned, researchers observed that mean beak depth gradually declined back toward pre-drought values. Which of the following best explains this pattern?

• A) Directional selection continued to favor large beaks even after seed size returned to normal, yet the mean beak depth still drifted downward over time.
• B) Selection shifted to favor smaller or more intermediate beak sizes once the diverse seed community recovered, because individuals with very large beaks were less efficient at handling the small, soft seeds that became re-available. ✓
• C) Genetic drift reversed the drought-induced change as the population resampled allele frequencies randomly.
• D) Mutation pressure produced new alleles coding for smaller beaks in the post-drought population.

Explanation: Correct answer: B. When the diverse seed community recovered, the fitness landscape changed. Very large, deep beaks may be inefficient for handling small, soft seeds; intermediate or smaller beak sizes perform better across the full range of available food. Selection thus shifted to favor smaller or more intermediate sizes, pulling the population mean back toward pre-drought values. This return could be described as selection favoring smaller beaks (directional, toward the prior mean) or as selection favoring intermediate sizes around the historic optimum, both framings are consistent with the data. (A) is incorrect; beak depth declined after normal conditions returned, contradicting continued directional selection for large beaks. (C) is incorrect; the consistent, directional return across multiple years is more consistent with selection than random drift. (D) is incorrect; mutation rates are too low to produce consistent directional phenotypic change across a population within a few generations.

Question 5. The G. fortis beak study is often cited as evidence that evolution is ongoing. Which feature of the study most directly supports this interpretation?

• A) The study was conducted on an isolated island, minimizing confounding effects of immigration.
• B) The researchers measured adult beak depth only once, at a single time point, so the study could not track any heritable trait change across generations.
• C) The study documents a measurable, heritable shift in a quantitative trait in a living population, tracked continuously over multiple years in response to an identified environmental change. ✓
• D) The study demonstrates that beak depth is controlled by a single gene with high penetrance.

Explanation: Correct answer: C. Demonstrating ongoing evolution requires showing that a heritable trait changed measurably in response to an identifiable selective agent and that the change is predicted to propagate to offspring. This study supports that conclusion: beak depth is heritable (h² = 0.79), it shifted measurably within one generation (+0.54 mm in survivors), and the breeder's equation predicts an offspring-generation shift of ~0.43 mm, linking the change to a documented selective pressure (drought and altered seed availability). The study supports ongoing evolution because it connects a measurable, heritable trait shift to an identified selective pressure in a living population tracked over time. (A) is incorrect; island isolation helps control for gene flow but does not by itself show evolution is ongoing. (B) is incorrect; sample size affects statistical confidence, not whether evolution is occurring. (D) is incorrect; beak depth is a polygenic quantitative trait, not single-gene.