what about mutations (e.g., due to cosmic rays) or genetic drift? what about the meteor that seems to have wiped out the dinosaurs?
According to Gould, in the Burgess Shale, there were all sorts of different creatures, some with more or less than 5 fingers (or toes) on their limbs. Only some survived, and part of their survival was just luck.
In web-footed or handed critters (ducks, seals), the number of fingers or toes is pretty arbitrary, since the key thing is to have enough fingers or toes to hold the web. -- Jim Devine "Segui il tuo corso, e lascia dir le genti." (Go your own way and let people talk.) -- Karl M., paraphrasing Dante A.