Genes are Under Constant Attack

Every animal and plant genome is subject to a constant bombardment of deleterious mutations: a hailstorm of attrition. It is a bit like the moon's surface, which becomes increasingly pitted with craters due to the steady bombardment of meteorites. With rare exceptions, every time a gene concerned with an eye, for example, is hit by a marauding mutation, the eye becomes a little less functional, a little less capable of seeing, a little less worthy of the name of eye. In an animal that lives i...