Which scenario best illustrates allopatric speciation?

• A. A geographic barrier such as a river splits populations, leading to divergence
• B. Divergence due to different feeding habits within the same geographic area
• C. A polyploid event occurs within the same area
• D. Hybridization between two species in the same location

Answer: (A)

Allopatric speciation happens when populations are physically separated by a geographic barrier, which stops gene flow between them and lets each group evolve on its own path. A river that splits populations creates exactly that situation: the two groups experience different environmental pressures and random genetic changes, so they diverge over many generations. If the barrier remains or lasts long enough, these differences can become strong enough that the two populations can no longer interbreed, even if the barrier is removed later, which is how new species arise through geographic isolation.

The other scenarios don’t involve geographic separation. Divergence due to different feeding habits within the same area is a form of sympatric speciation driven by ecological differences, not by physical isolation. A polyploid event occurring in the same area is another route to speciation that often happens without geographic barriers, common in plants. Hybridization between two species in the same location involves mixing gene pools rather than splitting them into isolated populations, so it doesn’t illustrate the isolation aspect of allopatric speciation.