geographic isolation prevents cross breeding between the populations that have been separated by the isolation.
There are two populations of squirrels on either side of the grand canyon. These populations are considered two different . 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.
Without the separation the two diverging population would be brought closer together by the hybridization of members of the two diverging populations cross breeding.
In the Beak of the Finch Jonathan Weiner accounts how hybridization works against the of the Darwin Finches. The separation of different types of finches increases in dry years but decreases in wet years due to cross breeding. The net change in speciation is zero over the time the Finches were scientifically studied.