National Geographic : 2009 Feb
• 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 nch with a smaller beak is not a new species of nch, but Peter Grant reckons it might take only a few such episodes before a new species is estab- lished that would not choose to reproduce with its parent species. The variation seen among the Galápagos nches is a classic example of "adaptive radi- ation," each species evolving from a common ancestor to exploit a special kind of food. Another famous radiation took place on a di erent set of islands---islands of water rather than land. e lakes and rivers of Africa s Great Ri Valley contain some 2,000 species of cichlid sh that have evolved from a few ancestors, some in an instant of geologic time. For example, Lake Vic- toria, the largest of those lakes, was completely dry just 15,000 years ago. Its 500 diverse spe- cies of cichlid have all evolved since then from a handful of species of uncertain origin. Like the nches, cichlid sh species have adapted to diets in di erent habitats, such as rocky or sandy patches of lake beds. Some species eat algae and have densely packed teeth suited to scrap- ing and pulling plant matter, while others feed on snails and have thick, powerful jaws capable of crushing open their shells. And what gene is responsible for thickening those jaws? e gene for the protein BMP4---the same gene that makes the Galápagos ground nch s beak deep and wide. What better evidence for Darwin s belief in the commonality of all species than to nd the same gene doing the same job in birds and sh, continents apart? In e Origin of Species, Darwin tactfully le unspoken how his theory would extend that commonality to include humankind. A decade later he confronted the matter head-on in e Descent of Man. He would be delighted to know that a certain gene, called FOXP2, is critical for the normal development of both speech in In the beak of the finch and the fur of the mouse, we can see the hand of natural selection at work, molding and modifying individual genes to adapt the organism to its particular circumstances.