I'm not 100% sure, but I think it mainly happened for the reason of surviving. One area wasn't comfortable for some species, so some members left and started living in a new area. While the rest adapted to their previous environment and evolved. Kind of like how the Polar bear changed its fur color to white, but the grizzly bear is still brown and black. But all in all its the doing of God himself looking out for his creatures and spreading them all around the world.