Well, there are many factors that can cause one species to divide into two. One of these is when populations get isolated from each other by something like a lake forming or forest being cleared. And there's another idea that as individuals adapt to their environment, this might have a knock-on effect on mate choice, a process called sensory drive speciation. Now this seems to occur in cichlid fish. They have shown that a female preference for either red or blue striped males only exists in clear water, where they are actually able to see.