Each of the two species is now free to evolve separately, free from contamination by the genes of the other, even though the original geographical barrier to such contamination is no more. Without the initial intervention of geographical barrriers (or some equivalent), species could never become specialised to particular diets, habitats or behaviour patterns. Notice that ‘intervention’ does not necessarily mean it is geography itself that made the active change—as when a valley floods or a volcano erupts.
