How the elephant got its trunk, the deer its antlers and the rattlesnake its rattles may seem like disparate questions of developmental biology, but the origins of these novelties, according to the genes of butterflies, may have much in common. By tracing the genetic circuits that govern the development of wing shape and coloration, the Wisconsin scientists discovered that butterflies, instead of inventing new genes for color, simply reuse a part of the wing-building genetic circuit to turn on enzymes that produce the pigments responsible for wing decoration. You're using a circuit over and over again. Moreover, according to Carroll, since these luxuries are genetically wired to a specific developmental output, they can be further tinkered with at the genetic level to create new variants. Think, for example, of the enormous diversity of pattern and color in butterflies, or the array of antlers sported by different members of the deer family such as elk and moose.
Evolution can muck with that by changing things in the circuit.