One of the most striking sequences in Carl Sagan's classic PBS series Cosmos was his illustration of how evolutionary selection works: the bit about the Heike crabs in Japan. These are crabs whose shells bear features that look strikingly like the face of a Samurai warrior, so the fishermen tend to throw these crabs back into the water, in remembrance of a legendary battle that took place in that particular region many centuries ago. Over centuries of such behavior on the part of the fishermen, a sort of "artificial selection" took place:
How does it come about that the face of a warrior is incised on the carapace of a crab? The answer seems to be that humans made the face. The patterns on the crab's shell are inherited. But among crabs, as among people, there are many different hereditary lines. Suppose that, by chance, among the distant ancestors of this crab, one arose with a pattern that resembled, even slightly, a human face. Even before the battle of Danno-ura, fishermen may have been reluctant to eat such a crab. In throwing it back, they set in motion an evolutionary process: If you are a crab and your carapace is ordinary, the humans will eat you. Your line will leave fewer descendants. If your carapace looks a little like a face, they will throw you back. You will leave more descendants. Crabs had a substantial investment in the patterns on their carapaces. As the generations passed, of crabs and fishermen alike, the crabs with patterns that most resembled a samurai face survived preferentially until eventually there was produced not just a human face, not just a Japanese face, but the visage of a fierce and scowling samurai. All this has nothing to do with what the crabs want. Selection is imposed from the outside. The more you look like a samurai, the better are your chances of survival. Eventually, there come to be a great many samurai crabs.
To this day, that is the most elegant illustration of external selection in nature that I've ever read. So where's the weirdness?
Well, Sagan's "Tale of the Heike Crabs" is really, really elegant and haunting. The "Tale of the Barbie Lobsters", though, is really, really weird.
(via Paul Riddell)