There have been many instances of species that have needed to adapt over time due to living in different geological conditions from other species members. When this occurs the species becomes genetically different from the "original" species member thus creating the subspecies. Subspecies are still generically similar enough that they are able to interbreed between species. However, when the genetic difference becomes so extreme the species will no longer be able to interbreed successfully.
A great example of a species that needed to evolve into subspecies due to geographic differences is the tiger. Currently there are six subspecies of tigers in existence. The Amur Tiger, Bengal Tiger, Indochinese Tiger, Malayan Tiger, South-China Tiger and Sumatran Tiger. In this subspecies we can also included the now extinct Bali Tiger, Caspian Tiger and Javan Tiger.
Johnson, G., Raven, P., (2005) Biology The Living Environment. Holt, Rinehart and WInston