Species are normally subdivided into populations that are more or less scattered, that may be separated at one time, but may merge together again, and successfully reproduce, at a later time. Tattersall and DeSalle describe this process as

Asquirrel population lives in an area. over many years, a river in that area grows wider and stronger, eventually forming a canyon. the squirrel populations on the two sides of the canyon can no longer mate with each other and ultimately form distinct species. this is an example of