There is no difference between geographic isolation and speciation. Geographic isolation leads to reproductive isolation and then speciation. Geographic isolation prevents cross-breeding between the populations that have been separated by the isolation.For example, there are two populations of squirrels on either side of the river. These populations are considered two different species. There are distinct differences between the two populations of squirrels. It is reasonable to assume these two populations were once one population. Isolation allowed the two populations to drift apart.