What factors contribute to allopatric speciation?

Allopatric speciation occurs when populations of a species become geographically isolated from each other, which may happen due to physical barriers such as mountains, rivers, or distance. These geographical barriers prevent gene flow between the isolated populations, allowing them to diverge genetically over time due to natural selection, mutation, and genetic drift. The separation creates different environments that can lead to adaptations specific to each population, ultimately resulting in the emergence of new species.

In the context of this question, geographical barriers are critical as they initiate the process of allopatric speciation by isolating gene pools. Without these barriers, populations would continue to interbreed, and speciation would be less likely to occur. While other factors, such as genetic mutations, can play a role in the divergence of populations once they are separated, they do not initiate the speciation process on their own. Thus, the presence of geographical barriers is essential for allopatric speciation to take place, making it the correct choice.