By the time the Beagle left the waters of the remote Galapagos Islands, Charles Darwin had still not realized that the natural variations within a species would provide the means for its evolution. Later, the Galapagos finches furnished ideal support for his theory of natural selection, because after a long separation from the finches of the rest of the world, they had become highly inbred. Geographic isolation had restricted their gene pool, so that the subtle differences Darwin saw among the island finches must have resulted from genetic changes, or polymorphisms, that arose spontaneously in individual birds and were then . . .