Allopatry and sympatry are terms used in biogeography to describe the comparative distributions of populations and species. Species with sympatric distributions overlap in their geographical range to some degree. In contrast, allopatric species do not overlap in their distributions. Evolutionary biologists are particularly interested in these sorts of distributions, because they are believed to be one of the most important factors affecting the evolution of distinct, new populations and species.

Ecologists and evolutionary biologists define a population as: "a group of individuals of the same species, which are capable of interbreeding with each other and producing fertile offspring." When populations of a particular species become physically isolated, they are effectively prevented from breeding with each other. Such an allopatric distribution might occur because the populations live on different islands, or because they inhabit opposite sides of a mountain chain. In either case, because of...