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.
Geographic isolation contribute to evolution in many ways. First it stops gene flow between different sub populations. Mating can only happen in this specific gene pool. Second new mutation may appear and act as an unique identity of the gene pool. Eventually the sub population would become very different from the original group. Thus, this prevents them from mating with the original group again.
