Populations in nearby areas may separate by preferential choice. This is parapatric speciation.
The North American threespine stickleback is typical of parapatric speciation. Some spend their time feeding near the surface (limnetics), while others prefer the lake floor (benthics). As sunlight filters through water depth, these 2 habitats have different lighting. Over time, the deep and shallow females respectively chose to mate with males at their depth, leading to reproductive isolation.
A similar reproductive isolation has occurred with sea lions in the Galápagos Islands. Central island sea lions prefer feeding in shallower waters, while those near the northwestern islands feed deeper. These sea lions are ostensibly the same species and could interbreed, as they live in overlapping foraging ranges, but they choose not to.