June 20, 2000.
How the Galapagos Islands Changed Finches and the Finches Changed the World
This is Roger Payne speaking to you from the Odyssey, as we track sperm whales among the Galapagos Islands. As I mentioned last time, Darwin pondered long and hard trying to fit into his developing theory on the "transmutation of species " the fact that the Galapagos finches he had collected were not variants of the same species but true species in themselves.
Galapagos finches are examples of convergent evolution, and convergent evolution is always impressive. This may be the reason that it was the finches, rather than the giant tortoises that became the key for Darwin to develop his theory of evolution. Convergent evolution is a process by which natural selection shapes organisms that have similar lifestyles until they look almost alike, even though they may arrive at their final appearance along totally different evolutionary paths. For example, marsupial mammal moles from Australia and placental mammal moles from the rest of the world (the ones all of us are more familiar with) look virtually identical. They are of a similar shape, are blind, and have greatly exaggerated forelimbs which they use for burrowing. You and I might easily mistake either species for the other, yet they are no more closely related than bats are to kangaroos. They have arrived at their similar appearance because spending a life tunnelling through surface soil in search of grubs means that you must possess strong forelimbs to dig with, must be small (since all you can catch are relatively non motile grubs-anything faster than a grub easily evades you by scuttling away before you can arrive at it by digging), and you don't really need eyes (there is no light underground for vision). If you follow the mole lifestyle there is pretty much only one way to look: mole-like.
Convergent evolution is always striking and Darwin's finches are an excellent example of it. Darwin found Galapagos finches that live on seeds and have the typical thickened finch bill (of course they do, they are, after all, finches). But there are also Galapagos finches that live on insects and have the fine bill of a typical insectivore-most un-finch like-and others with still finer bills shaped by natural selection for feeding on flowers. Each looks very like the finches, insectivores and flower feeders with which Darwin was familiar elsewhere yet it was clear that the Galapagos insectivores and flower feeders had derived from finches. There is even a woodpecker finch with a somewhat woodpecker-like bill that it uses to excavate dead wood in order to expose insect larvae. But then, lacking the barbed tongue of true woodpeckers with which to stab the larva and remove it from its burrow, it has adopted the habit of picking up a thorn with which to impale the larva. And there is even a Galapagos finch which, like a vampire bat, laps up the blood of its victims (often seabirds in this case) after piercing their skin or feather shafts.
That's all from me tonight, more tomorrow.
© 2000 - Roger Payne