Speciation in the Fast Lane.
Here is an interesting article about speciation. While geographic isolation and the resulting genetic drift is not the only mechanism for this process, the article gives an interesting insight into why tropical regions have such a high biological diversity. It also helps illustrate that once the divergence of two groups of organisms goes beyond a tipping point where mating with the other population produces less well adapted offspring, there is suddenly a selective pressure against those who are willing to jump the mating barrier. (Jumping the mating barrier? I don't know why but that sounds like more fun than I meant it to.) Speciation becomes self-reinforcing.