Which of the following best describes allopatric speciation?

Allopatric speciation is a fundamental mechanism of evolution that occurs when a population of a species becomes divided by a geographic barrier, such as mountains, rivers, or distance. This physical separation prevents individuals from mating with each other, leading to the isolation of populations. Over time, the separated groups experience different environmental conditions, selective pressures, and genetic drift, which can result in the accumulation of genetic differences. Eventually, these changes can lead to the formation of distinct species, as reproductive barriers develop that prevent interbreeding even if the populations come back into contact.

The other choices describe different aspects of evolutionary processes but do not align with the specific criteria of allopatric speciation. Behavioral changes refer to how species might adapt their mating or feeding behaviors, but this is more indicative of behavioral speciation. Divergence due to similar habitats pertains to sympatric speciation, where species evolve in the same geographic location without physical barriers. Environmental adaptation usually involves changes within an existing species rather than leading to the formation of new species through geographic isolation.

Thus, the defining characteristic of allopatric speciation is indeed the occurrence of geographic barriers that facilitate the evolution of new species through isolation, making the choice about geographic barriers the correct answer.