Thursday, March 23, 2006


The Killer Rat-Kangaroo's Tooth

"Natural selection didn't come up with the best design; it just made the best of what was available.

The 'hand' of the Giant Panda (Ailuropoda melanoleuca) has six 'digits'. In processing its staple diet of bamboo, the Giant Panda drags the stalks between its sixth 'digit' and its paw to strip off the leaves. This sixth 'digit' or 'thumb' is a curious device. It is not, as one might expect, simply an additional finger of the type sometimes produced through congenital defect. In fact, the Panda's 'thumb' is not a real digit at all, but a greatly enlarged and specialised wrist bone called the radial sesamoid that lacks much of the flexibility of true digits. Why, one might wonder, did the Panda evolve a 'thumb' out of a wrist bone when it already had a 'real' thumb?

Evolution can be a fickle and opportunistic process. Often the end result can appear surprisingly imperfect, even 'sloppy'. The bottom line is that, at any juncture in its evolution, a species is constrained by accidents of history. When 'fashioning' new adaptations, natural selection can only work with what it's got. Because the real thumb (the first digit) of the Giant Panda was already modified and in use for another task (for walking on), evolution could only work with what was available, in this case, a radial sesamoid bone. It is this very fact of imperfection that underpins the reality of natural selection."

A pdf version is available here

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