Allopatric speciation primarily occurs due to:

Allopatric speciation primarily occurs due to the geographic isolation of populations. This process involves a physical barrier, such as a mountain range or a body of water, that separates a species into two or more distinct populations. Over time, these isolated populations evolve independently of one another, leading to the development of new species. The isolation prevents gene flow between the groups, allowing genetic divergence caused by mutations, natural selection, and genetic drift to accumulate.

While behavioral differences, reproductive isolation mechanisms, and natural selection can play significant roles in speciation, they are typically associated with other types of speciation, such as sympatric speciation. In allopatric speciation, the initial step is the physical separation of populations, which is crucial for the independent evolutionary pathways that lead to speciation.