National Geographic : 2009 Feb
beach islands formed less than 6,000 years ago. Darwin s greatest idea was that natural selec- tion is largely responsible for the variety of traits one sees among related species. Now, in the beak of the nch 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 organ- ism to its particular circumstances. Darwin, who assumed that evolution plod- ded along at a glacially slow rate, observable only in the fossil record, would be equally delighted by another discovery. In those same Galápagos nches, modern Darwins can watch evolution occur in real time. In 1973, Peter and Rosemary Grant, now of Princeton University, began annual obser vations of the nch popula- tions on the tiny Galápagos island of Daphne Major. ey soon discovered that the nches in fact evolved from one year to the next, as condi- tions on the island swung from wet to dry and back again. For instance, Daphne Major ini- tially had only two regularly breeding ground nches, one of which was the medium ground nch (G. fortis) that fed on small seeds. When severe drought struck the island in 1977 and small seeds became scarce, the medium nches were forced to switch to eating bigger, harder seeds. ose with larger beaks fared better and survived to pass on the trait to their o spring. Another shift took place after a competi- tor 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 nches remained To understand the story of evolution---both its narrative and its mechanism--- modern Darwins don't have to guess. They consult genetic scripture. Matt Ridley s latest book is Francis Crick: Discoverer of the Genetic Code. Lynn Johnson photographed "Village Health Workers" in December 2008.