Each generation has the chance of changing a little bit from the previous one. A George W. Bush joke won’t get passed on very much these days; but if it changes into an Obama joke, it may. It’s the survival of the funniest. Now, ask yourself this question. Where did chickens come from? Was there a day somewhere in the past when no chickens existed, and then the next day there were chickens? No. Chickens, like all animals, came into being by a process of gradual change. They emerged from a previous species–let’s call it the proto-chicken–that changed a little bit with each generation. Like better jokes being passed on, birds with more useful traits tended to survive and reproduce. Eventually a new species altogether emerged. Just in time to cross the road.