Allopatric speciation is the formation of one or more new species through geographical isolation. One of the criteria that defines a species is that its members are capable of interbreeding. But if members of a species become separated by a geographic barrier, such as an ocean or a mountain range, they can no longer interbreed (the separated groups are allopatric) and in time may diverge so much from each other that they are no longer even capable of interbreeding. They have therefore become different species.