The first graph is titled fur color distribution immediately after release, with y-axis labeled frequency. There are three bars representing gray, brown and black mice. The gray and black mice have very low bars at low frequency, and the brown mice have a tall bar with high frequency. The second graph is titled fur color distribution after twenty years, with y-axis labeled frequency. The gray mice have the highest frequency, the brown mice have a mid-level frequency, and the black mice have very low frequency.

A group of mice was released into a large field to which no other mice had access. Immediately after the release, a representative sample of the mice was captured, and the fur color of each individual in the sample was observed and recorded. The mice were then returned to the field. After twenty years, another representative sample of the mice was captured, and the fur color of each individual in the sample was again recorded. Which of the following best explains the change in the frequency distribution of fur color phenotypes in the mouse population, as shown in the figures above?

Question

The first graph is titled fur color distribution immediately after release, with y-axis labeled frequency. There are three bars representing gray, brown and black mice. The gray and black mice have very low bars at low frequency, and the brown mice have a tall bar with high frequency. The second graph is titled fur color distribution after twenty years, with y-axis labeled frequency. The gray mice have the highest frequency, the brown mice have a mid-level frequency, and the black mice have very low frequency.

A group of mice was released into a large field to which no other mice had access. Immediately after the release, a representative sample of the mice was captured, and the fur color of each individual in the sample was observed and recorded. The mice were then returned to the field. After twenty years, another representative sample of the mice was captured, and the fur color of each individual in the sample was again recorded. Which of the following best explains the change in the frequency distribution of fur color phenotypes in the mouse population, as shown in the figures above?

BlackTom AI · Accepted Answer

Natural selection favoring camouflage on light-colored soil explains the observed shift: gray mice became much more common because they had higher survival in that environment.

1. Relevant knowledge points and concepts
- Natural selection: differential survival and reproduction of phenotypes best suited to the environment causes allele-frequency changes in populations over generations.
- Adaptation and camouflage: coloration that reduces predation increases fitness.
- Mutation and genetic drift: mutation rates are generally low and unlikely to produce a large, directed change in phenotype frequencies in 20 years; drift is strongest in small populations.
- Sampling bias and sexual selection are alternate explanations that must be evaluated against the data and context.

2. Step-by-step reasoning
- Immediately after release the sample shows high frequency of brown, low frequencies of gray and black.
- After 20 years the sample shows gray at highest frequency, brown intermediate, black still rare.
- Because the field is closed to other mice, immigration/emigration cannot explain the change. Therefore the change must be due to internal processes: selection, mutation, drift, or sampling error.
- The pattern (increase of gray at the expense of brown) matches prediction for selection if gray mice are better camouflaged and thus less likely to be eaten by predators.

3. Calculations / intermediate steps
- No numeric calculation is required here; the logic uses frequency shifts qualitatively: brown → decreased, gray → increased, black → remains low. The magnitude and direction of change point to selective survival of gray phenotype.

4. Why the correct answer is correct
- Light-colored soil and little vegetation would make brown mice more visible and gray mice better camouflaged. Predators would disproportionately remove brown mice, so gray allele frequency rises over generations—exactly the observed outcome.

5. Why other options are incorrect
- Unstable allele/mutation: implausible to convert many brown alleles to gray or black in 20 years; mutation is random and rare.
- Sexual selection favoring black and brown would increase, not decrease, those phenotypes—contrary to data.
- Capture bias: both samples are described as representative; systematic under-capture of gray in the first sample would have to be extreme and coincidentally reversed in the second sample—less parsimonious than selection.

6. Conclusion
The change is best explained by camouflage-based natural selection: The field was composed primarily of light-colored soil and little vegetation, affording gray mice protection from predators. This matches the provided correct answer.

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