In their paper on Evolvability, Marc Kirschner and John Gerhart discuss the separation of concerns within our genetic code. They pay special attention to limbs: The limb is a complex structure with precisely placed bone, cartilage, muscle, nerves, and vascular elements, and one might think it is difficult for such a structure to change in evolution.1 Every vertebrate uses the same genes to encode their limbs. This mechanism can describe wings, flippers, arms, and everything in between. But ho...
