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In another set of islands, off the Gulf Coast of Florida, beach mice have paler coats than mice living on the mainland. This camouflages them better on pale sand: owls, hawks, and herons eat more of the poorly disguised mice, leaving the others to breed. Hopi Hoekstra, also at Harvard, and her colleagues traced the color difference to the change of a single letter in a single gene, which cuts down the production of pigment in the fur. The mutation has occurred since the beach islands formed less than 6,000 years ago.

Darwin's greatest idea was that natural selec­tion is largely responsible for the variety of traits one sees among related species. Now, in the beak of the finch and the fur of the mouse, we can actually see the hand of natural selection at work, molding and modifying the DNA of genes and their expression to adapt the organism to its particular circumstances.

Darwin, who assumed that evolution plodded along at a glacially slow rate, observable only in the fossil record, would be equally delighted by another discovery. In those same Galápagos finches, modern Darwins can watch evolution occur in real time. In 1973, Peter and Rosemary Grant, now of Princeton University, began annual observations of the finch populations on the tiny Galápagos island of Daphne Major. They soon discovered that the finches in fact evolved from one year to the next, as conditions on the island swung from wet to dry and back again. For instance, Daphne Major initially had only two regularly breeding ground finches, one of which was the medium ground finch (G. fortis) that fed on small seeds. When severe drought struck the island in 1977 and small seeds became scarce, the medium finches were forced to switch to eating bigger, harder seeds. Those with larger beaks fared better and survived to pass on the trait to their offspring.

Another shift took place after a competitor arrived in 1982: the large ground finch (G. magnirostris), which also eats large, tough seeds. For many years the two species coexisted, and in 2002 both became unusually abundant. But then drought struck, and by 2005, only 13 large and 83 medium ground finches remained alive. Remarkably, instead of adjusting to the drought by eating bigger seeds as they had 28 years before, the surviving medium finches experienced a marked reduction in the size of their beaks, as in competition with their larger cousins they struggled to carve out a niche by surviving on very small seeds. A finch with a smaller beak is not a new species of finch, but Peter Grant reckons it might take only a few such episodes before a new species is established that would not choose to reproduce with its parent species.

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