What type of speciation occurs when a population is divided by a geographical barrier?

Allopatric speciation occurs when a population is geographically separated by an external barrier, such as a mountain range, river, or other significant physical obstacles. This geographical isolation prevents gene flow between the divided populations, leading to different evolutionary paths due to distinct environmental pressures and genetic drift over time. As each subgroup adapts to its specific environment, they can undergo genetic changes that may eventually result in the formation of new species.

In contrast, sympatric speciation happens when new species arise from a population without geographical barriers, often through mechanisms like polyploidy or behavioral changes. Adaptive radiation involves the rapid evolution of multiple species from a common ancestor as they adapt to various ecological niches. Parapatric speciation occurs when populations are partially separated by a geographical barrier but still have some level of interaction. Each of these alternatives represents different mechanisms of speciation that do not fit the scenario of a divide caused explicitly by a geographical barrier.