I always like to use the "Frog Pond" example. Here in north central Florida, about 5 miles west of Newberry, there's a small pond in the middle of large cattle operation and it's the only full time body of water for at least 2 square miles. The pond's been there for as long as I can remember and the thing is chock full of frogs. I couldn't even begin to guess how many generations of inbred frogs have lived and died in that pond. I was over there about three weeks ago and didn't see a single frog with 3 eyes or 5 legs.
Like Pit said, reptiles are completely different.