Allopatric speciation is defined as speciation driven by geographic isolation that prevents gene flow.

• A. Speciation driven by changes in climate within a local population.
• B. Speciation due to reproductive isolation within the same geographic area.
• C. Speciation caused by artificial selection in captivity.
• D. Speciation driven by geographic isolation that prevents gene flow.

Answer: (D)

Allopatric speciation happens when geographic barriers physically split populations, stopping gene flow between them. Once gene flow is interrupted, the separated groups experience different mutations, drift, and selective pressures in their distinct environments. Over time these differences accumulate, and the two groups may become reproductively isolated, even if the barrier were removed, leading to the emergence of separate species. The statement “speciation driven by geographic isolation that prevents gene flow” captures this defining process, making it the best description.

Other scenarios describe different paths to divergence but not the classic allopatric route: changes in climate within a local population can drive adaptation, but without geographic separation, interbreeding can continue; speciation within the same geographic area is typically sympatric; and artificial selection in captivity is human-directed rather than a natural, geographically driven interruption of gene flow.