The variation seen among the Galápagos finches is a classic example of "adaptive radiation," each species evolving from a common ancestor to exploit a special kind of food. Another famous radiation took place on a different set of islands—islands of water rather than land. The lakes and rivers of Africa's Great Rift Valley contain some 2,000 species of cichlid fish that have evolved from a few ancestors, some in an instant of geologic time. For example, Lake Victoria, the largest of those lakes, was completely dry just 15,000 years ago. Its 500 diverse species of cichlid have all evolved since then from a handful of species of uncertain origin. Like the finches, cichlid fish species have adapted to diets in different habitats, such as rocky or sandy patches of lake beds. Some species eat algae and have densely packed teeth suited to scraping 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? The gene for the protein BMP4—the same gene that makes the Galápagos ground finch's beak deep and wide. What better evidence for Darwin's belief in the commonality of all species than to find the same gene doing the same job in birds and fish, continents apart?
In The Origin of Species, Darwin tactfully left unspoken how his theory would extend that commonality to include humankind. A decade later he confronted the matter head-on in The 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 people and song in birds. In 2001 Simon Fisher and his colleagues at the University of Oxford discovered that a mutation in this gene causes language defects in people. He later demonstrated that in mice, the gene is necessary for learning sequences of rapid movement; without it, the brain does not form the connections that would normally record the learning. In human beings, presumably, FOXP2 is crucial to learning the sophisticated flicker of lips and tongue with which we express our thoughts.
Constance Scharff of the Free University of Berlin then discovered that this very same gene is more active in a part of the brain of a young zebra finch, just when the bird learns to sing. With fiendish ingenuity, her group infected finches' brains with a special virus, carrying a mirror-image copy of part of the FOXP2 gene, which stifled the gene's natural expression. The result was that birds not only sang more variably than usual but also inaccurately imitated the songs of adults—in much the same way as children with mutant FOXP2 genes produce variable and inaccurately copied speech.
If natural selection is survival of the fittest (a phrase coined by the philosopher Herbert Spencer, not by Darwin), then sexual selection is reproduction of the sexiest. It has the delightful effect of generating weapons, ornaments, songs, and colors, especially on male animals. Darwin believed that some such ornaments, such as stags' antlers, helped males fight each other for females; others, such as peacocks' tails, helped males "charm" (his word) females into mating. It was, in truth, an idea born of desperation, because useless beauty worried him as an apparent exception to the ruthlessly practical workings of natural selection. He wrote to the American botanist Asa Gray in April 1860 that "The sight of a feather in a peacock's tail, whenever I gaze at it, makes me sick!"