Species

The most common definition of a species is a collection of organisms in which any adult male-female pair can interbreed and produce fertile offspring. For example, mice and oak trees clearly cannot interbreed, so they belong to separate species; horses and donkeys can interbreed, but the offspring (mules) are infertile, so horses and donkeys are also separate species. Of course, the definition doesn't work when applied to organisms that do not reproduce sexually, and is problematic in other respects as well.

A new species typically arises when an existing species is split into two geographically separated populations for a long period, during which the populations evolve in divergent directions until they can no longer be defined as a single species.