Question

which of the following best describes what occurred during the periods indicated by the arrows?
a natural selection acted on variation that was present in both populations a and b, and the lack of gene flow eventually led to the formation of two new species.
b population b was separated for a longer time from the original population than population a was, and population b evolved into a more distinct new species than population a did.
c gene flow between populations a and b maintained sufficient variation in both gene pools to facilitate the development of two distinct species.
d in response to different environments, populations a and b experienced different mutation rates, leading to the emergence of two different gene pools and, over time, new species a and b.

Explanation:

• Option A: Natural selection acts on existing variation in populations. When gene flow is absent (e.g., due to geographic isolation), populations can diverge, leading to speciation. This aligns with evolutionary theory.
• Option B: The diagram (implied by the question) likely doesn't show Population B being separated longer, and "more distinct" is not a standard part of speciation logic based on time alone.
• Option C: Gene flow typically reduces divergence, so it wouldn't facilitate forming two distinct species.
• Option D: Mutation rates are not primarily driven by environment in a way that directly causes speciation; natural selection on existing variation (not different mutation rates) is key.

Answer:

A. Natural selection acted on variation that was present in both populations A and B, and the lack of gene flow eventually led to the formation of two new species.