AP Biology: Unit 7, Natural Selection: Types & Fitness (Drill 28)

Question 1. Based on the data, which of the following best describes the pattern of change in body mass across the four years?

• A) Mean body mass decreased while standard deviation increased, suggesting disruptive selection.
• B) Mean body mass remained stable while standard deviation decreased, suggesting stabilizing selection.
• C) Mean body mass increased while standard deviation decreased, consistent with directional selection favoring larger individuals. ✓
• D) Mean body mass increased while standard deviation increased, suggesting the population is diverging into two distinct size classes.

Explanation: Correct: (C) The data show mean body mass increasing from 22.4 g to 24.8 g over four years while standard deviation decreases from 3.2 g to 2.7 g. An increasing mean paired with decreasing variation is consistent with directional selection favoring larger individuals: the population is shifting toward one phenotypic extreme while variation around the new mean narrows as smaller phenotypes are preferentially removed by predation. Incorrect: (A) The data show an increasing mean, not a decreasing mean, and decreasing standard deviation, not increasing. Disruptive selection would produce increasing variance and potentially a bimodal distribution. Incorrect: (B) The mean did not remain stable; it increased by 2.4 g over four years. Stabilizing selection maintains a stable mean while reducing variance around it. Incorrect: (D) Standard deviation decreased, not increased. A diverging population undergoing disruptive selection would show increasing variance and potentially bimodality, not the narrowing distribution observed here.

Question 2. The passage states that hawks preferentially capture smaller voles. Which of the following best explains how this predation pattern could produce the observed shift in mean body mass over four years?

• A) Smaller voles have lower survival rates due to predation, so larger voles disproportionately survive, reproduce, and pass alleles for larger body mass to offspring. ✓
• B) Hawks cause smaller voles to develop larger body mass over their lifetimes in response to the stress of predation pressure.
• C) Predation reduces overall population size, which allows the remaining voles to access more food resources and grow to larger body masses within a single generation.
• D) Hawks selectively remove larger voles that are easier to spot in open grassland, creating space for smaller voles to become the dominant phenotype.

Explanation: Correct: (A) Natural selection operates through differential survival and reproduction. If hawks preferentially capture smaller voles, smaller individuals have lower survival rates and contribute fewer offspring to subsequent generations. Because body mass is heritable (heritability approximately 0.45), larger voles that survive pass alleles contributing to larger body mass to their offspring, gradually shifting the population mean upward across generations. This is the mechanism of directional selection. Incorrect: (B) Individual voles do not develop larger body mass in response to predation pressure during their lifetimes. This describes a Lamarckian view of inheritance, which is not how evolution works. Acquired characteristics are not heritable. Incorrect: (C) While reduced competition for food could increase individual body mass within a generation, this describes a phenotypic plastic response, not heritable evolutionary change. Because body mass is heritable, differential survival of larger voles can produce evolutionary change across generations; this is more consistent with selection than with purely individual growth responses. Incorrect: (D) The passage states hawks preferentially capture smaller voles, not larger ones. This distractor reverses the selective pressure described in the stimulus.

Question 3. A researcher proposes that the observed increase in mean body mass resulted from genetic drift rather than natural selection. Which of the following observations from the data provides the strongest evidence against the genetic drift hypothesis?

• A) The population size fluctuated slightly across years, which is consistent with drift occurring in a small population.
• B) The mean body mass increased in every single year, which could occur by chance in a small population over four years.
• C) The population size of approximately 120 individuals is small enough for drift to produce consistent directional change over four years.
• D) The standard deviation decreased consistently alongside the increasing mean, a pattern that reflects differential survival by phenotype rather than random allele frequency changes. ✓

Explanation: Correct: (D) Genetic drift produces random changes in allele frequencies and would not be expected to consistently shift the mean in one direction while simultaneously narrowing phenotypic variation. The coordinated increase in mean body mass and reduction in spread across all four years is more consistent with phenotype-specific survival, smaller voles being preferentially removed, than with purely random drift events. Drift alone does not reliably produce this kind of coordinated directional pattern. Incorrect: (A) Slight population size fluctuation is consistent with many scenarios and does not specifically support drift over selection. This observation neither confirms nor rules out drift. Incorrect: (B) Consistent directional change in the same direction across all four years actually supports the selection hypothesis more than the drift hypothesis. Drift would be expected to produce variable, non-directional changes across years rather than the consistent unidirectional trend observed. Incorrect: (C) While populations of approximately 120 can experience drift, drift produces random fluctuations in both directions, not consistent unidirectional change over four years. The consistent direction is more parsimoniously explained by a persistent directional selective force.

Question 4. A colleague suggests that the shift in mean body mass may reflect phenotypic plasticity, individual voles growing larger in response to reduced competition for food, rather than heritable genetic change. Which of the following experimental designs would best distinguish between heritable genetic change and phenotypic plasticity as explanations for the observed shift?

• A) Weigh a larger sample of voles each year to determine whether the trend in mean body mass continues at the same rate.
• B) Raise offspring from Year 4 voles in a controlled laboratory environment with standardized food access, and compare their body mass to offspring from Year 1 voles raised under identical conditions. ✓
• C) Remove hawks from the habitat and monitor whether mean body mass reverts to Year 1 values over the following four years.
• D) Measure food availability in the habitat each year and determine whether years with lower food availability correlate with lower mean body mass.

Explanation: Correct: (B) A common-garden experiment eliminates environmental variation by raising offspring from both time periods under identical controlled conditions. If offspring from Year 4 parents are consistently larger than offspring from Year 1 parents despite identical food access, the difference is genetic rather than environmental. This directly tests whether the body mass shift is heritable (genetic change) or attributable to the environmental conditions experienced by individuals (phenotypic plasticity). Incorrect: (A) A larger sample size improves statistical confidence in the trend but does not address whether the trend reflects genetic change or environmental response. This tests whether the pattern continues, not what caused it. Incorrect: (C) Removing hawks tests whether relaxed selection allows reversion, but if mean body mass reverts it could reflect either relaxed selection on a genetic trait or restoration of plastic responses to competition. This does not cleanly distinguish genetic from plastic causes. Incorrect: (D) Correlating food availability with body mass tests whether food is a proximate cause of body mass variation but does not determine whether the multi-year shift reflects genetic change or plastic response. This addresses the food hypothesis but not the heritability question.

Question 5. Assume the increase in mean body mass reflects genuine heritable genetic change driven by directional selection. If hawk predation continues at the same intensity for many additional generations, which of the following best describes the most likely long-term outcome for the vole population?

• A) The rate of change in mean body mass will slow as genetic variation for body mass is reduced by selection, eventually approaching a new equilibrium mean. ✓
• B) Mean body mass will continue increasing at the same rate indefinitely because natural selection always produces constant rates of phenotypic change when selective pressure is constant.
• C) The population will rapidly go extinct because directional selection always eliminates enough individuals to cause population collapse.
• D) Mean body mass will oscillate between high and low values each generation because directional selection alternates direction in response to environmental feedback.

Explanation: Correct: (A) Directional selection reduces genetic variation for the selected trait over time by removing alleles associated with the disfavored phenotype. As genetic variation decreases, the response to selection slows because there is less heritable variation for selection to act upon. The population will approach a new equilibrium at a higher mean body mass where further selection produces diminishing returns. This is a foundational prediction of quantitative genetics: the response to selection depends on available heritable variation, which selection itself erodes. Incorrect: (B) Natural selection does not produce constant rates of phenotypic change indefinitely. As the data show, standard deviation is already decreasing, indicating reduced variation. As the population loses genetic variation for the trait, the rate of change must slow regardless of how constant the selective pressure remains. Incorrect: (C) Directional selection does not inevitably cause population extinction. It reduces fitness variation by removing less-fit phenotypes, but individuals with sufficient fitness survive and reproduce. Extinction from predation requires predation intensity to exceed reproductive capacity, a different scenario. Incorrect: (D) Directional selection does not inherently alternate direction. Oscillating selection occurs when environmental conditions change, but the scenario specifies constant hawk predation pressure, which maintains the same selective direction.