Repoductive isolation could be any number of things as I already said, anything from sexual organs are not compatible, to chromosome number not compatible, to a river or mountain that separates the two communities. This usually results in the two species being genetically different because of breeding only within the immediate group.
A thousand years ago people in Africa, Europe and America would have been different species.
For people who believe that man originated from one Adam, could this not explain the differences among people groups?