Rapaire - the answer is, over geological time. You get gradual changes all the time, like people getting taller over the generations, but for true speciation, you need not only a disaster that isolates the tail of a distribution, but also the lucky break that makes it the right (as in not wrong, not as in not left) tail for the new surroundings. That's why there are so many more extinctions than new species forming.

I'm trying to think of a good ritual to have for this holiday, other than sex, which is too obvious! Any ideas?