Examples
The African Elephant has always been regarded as a single species but, because of morphological and DNA differences, some scientists classify them into three subspecies. Researchers at the University of California, San Diego 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 both the savanna elephants of Central, Eastern and Southern Africa, and the forest elephants of Central Africa. A similar situation exists with the Asian Elephant, which has four distinct living sub-species.
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 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 when the Mallard was introduced into New Zealand and interbred freely with the native Grey Duck, which had been classified as a separate species. It is controversial whether its specific status can now be retained. Mallards interbreed similarly and aggressively with the Southern African yellow-billed duck.
Read more about this topic: Allopatric Speciation
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