What is a plausible outcome when there is geographic isolation combined with population structure that reduces gene flow?

• A. Allopatric speciation is facilitated, leading to divergence between isolated populations.
• B. Gene flow continues unimpeded, preventing divergence.
• C. Mutation alone causes immediate speciation without isolation.
• D. All populations merge into a single species.

Answer: (B)

Geographic isolation with reduced gene flow allows populations to diverge over time. When groups are separated, they no longer exchange genetic material regularly, so allele frequencies can change independently in each population due to genetic drift, natural selection in different environments, and new mutations. Over many generations, these differences can accumulate to the point where the two groups can no longer interbreed, even if they come back into contact. This is the process of allopatric speciation—the formation of new species driven by geographic separation.

If gene flow remained unimpeded, the populations would continue mixing their genes, which tends to homogenize differences and prevent divergence, undermining the buildup of reproductive barriers. The idea that mutation alone would cause immediate speciation without any isolation, or that all populations would simply merge into one species, doesn’t fit the scenario where isolation and reduced gene flow are key forces shaping divergence.

So the most plausible outcome is divergence between isolated populations, potentially leading to the emergence of new species through allopatric speciation.