How does geographic isolation contribute to evolution?
Geographic isolation of a group of organisms eventually stops gene flow from other groups of same species. Thus isolated group evolves by accumulating new mutations not to be found in members of related groups.
All members of a species form a single population. But all organisms of a single species would not be confined in a single geographic location though they may interact and interbreed occasionally.
In nature, organisms are distributed in different areas, forming separate subpopulations. Each subpopulation thus maintains a gene pool. When members of different subpopulations migrate and interbreed genes are exchanged between gene pools. Higher the gene flow, more alike are the two subpopulations.
Hence geographical isolation stops gene flow between subpopulations. New mutations may appear in such isolated subpopulation giving them unique identity.
Eventually a geographically isolated subpopulation would become genetically very different. As a result members may fail to successfully interbreed with members of related parental group. Thus long geographical isolation may lead to reproductive isolation. In such case, the isolated subpopulation gets elevated to designation of a species.