Geographic isolation is thought to be a major contributor to speciation. When populations have no significant contact with one another, they cannot interbreed and exchange genetic information. Over time, natural selection and genetic drift can produce significant differences in isolated populations.

This isolation can come about in several different ways:

Colonization of Distant Areas
When organisms move from the original range of a population to more distant areas, they are likely to be faced with different environmental conditions. As the new colony adapts, natural selection may favor different characteristics. If the colony is small, it may also have a different range of genetic variability than the parent population.

Extinction of Intermediate Populations
Many populations are so large that if you were to look at them across their range you would see that the organisms at either end have marked differences and may not even be able to interbreed. The intermediate populations essentially keep the two extremes connected, because they breed with the outliers and transfer genetic material between the distant populations. If the intermediates become extinct, then the two extremes lose all contact with one another.

Geographic Barriers
Geographic changes can effectively divide a population even when its members do not move. Changes in the course of a river or the uplift of mountains can split populations in two. For small or relatively immobile organisms, highways, fallen trees, and the creation of farm fields or urban tracts can all create geographical barriers. Once separated, unique selective pressures and chance events can cause each population to evolve in a different way.