Final answer:
Allopatric speciation occurs when two populations of the same species are geographically separated, leading them to evolve independently and become unable to interbreed upon potential secondary contact.
Step-by-step explanation:
When two populations of the same species become geographically separated from one another and consequently do not interbreed, resulting in genetic divergence to the point that they can no longer interbreed even if residing in the same location, allopatric speciation has occurred. This process involves the isolation of populations which leads to genetic differences accumulating due to mechanisms such as natural selection, mutation, and genetic drift. These populations evolve independently with allele frequencies at various genetic loci becoming increasingly different due to environmental conditions, natural selection, and genetic drift leading to divergent adaptations in each group. Over time, if the genetic differences are substantial enough to prevent interbreeding, a new species has evolved from the parent species.