When part of a population of a species becomes geographically isolated from the remainder, they may over time evolve different characteristics from the parent population. This is particularly likely to happen if the isolated population is small, because of founder effects, or if they become isolated in an environment which differs makes new demands on them.

If there is sufficient genetic change following geographical isolation, then if the geographical barriers are removed (perhaps because of human agency), members of the two populations will be unable to mate with each other successfully. At this point, a new species has emerged. Geographical isolation is thus a key factor in speciation, the formation of new species.

However, it is more common for there to be considerable genetic and phenotypic change without the loss of the capacity for interbreeding - interbreeding is simply prevented by the geographical separation of populations. In this case the populations are normally regarded as subspecies.

The African Elephant for instance has always been regarded as a single species. Because of morphological and DNA differences some scientists classify the elephants into three subspecies. Researchers at the University of California, San Diego (UCSD) have argued that divergence due to geographical isolation has gone further, and the elephants of West Africa should be regarded as a separate species from either the savanna elephants of Central, Eastern and Southern Africa, or the forest elephants of Central Africa.

Other cases arise where two populations that are quite distinct morphologically, and are native to different continents, have been classified as different species; but when they members of one species are introduced into the other's range, they are found to interbreed freely, showing that they were in fact only geographically isolated subspecies. This was found to be the case, for example, when the Mallard Anas platyrhynchos was introduced into New Zealand; it interbred freely with the native Grey Duck , which had been classified as a separate species, Anas superciliosa; it is controversial whether its specific status can now be retained.

A synonym is: allopatry