The evolution of new beak structures to exploit new food sources. Galápagos finches that feed on different, specialized food sources have different beak structures, as shown for the cactus finch and the large ground finch. (a) The beak structures can be varied along three dimensions. (b) The differences observed in the two finch species were produced largely through natural selection acting on a few mutations that altered the timing and level of expression of just two genes: those encoding Bmp4 and calmodulin (CaM).