Speciation is the formation of a new species by dividing one species into two. Two species are considered to be separate if the individuals cannot reproduce to produce viable offspring (those that are fertile and cannot produce offspring of their own). Thus, if two individuals are reproductively isolated, they are considered different species.

Speciation often begins when members of one species are geographically isolated from each other. Geographical isolation can occur because of physical barriers such as a mountain, river, forest, desert, or island preventing movement of individuals and genetic mixing of members of the species. For example, a population of fish in a large lake might be split in two during a great drought. When water evaporated and two separate smaller lakes formed, some individuals would be isolated from the rest of the population. The two populations of fish would be geographically isolated from each other.

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