Natural and sexual selection produce adaptations. It seems so obvious to us now that Darwin has explained it to us.
But it turns out to not be so simple. Natural and sexual selection may act directly upon certain traits, and indirectly upon others; the trait may have been a side-effect of the real story of natural selection. Stephen Jay Gould and Richard Lewontin realized this as they looked at the spaces between the arches of a cathedral, and the artwork contained therein. The artwork was constrained by the spaces. The design of the cathedral was focused on the arches; the spaces between them, or spandrels, were a side effect. Later, Gould and Elizabeth Vrba expanded this concept. Some evolutionary changes occurred, like artwork in spandrels, within necessary constraints. Other evolutionary changes in traits resulted from natural selection acting on other traits; they called these changes exaptations rather than adaptations.
Some characteristics of organisms are, like spandrels, structurally inevitable. Consider the patterns of allometry. Large trees must have relatively thick trunks, and large animals must have relatively thick legs. An animal that is twice as large as another in all linear dimensions would weigh eight times as much but have legs only four times as strong; to make up for this, the legs must be more than twice as thick. They have to be the square root of eight times as thick. There is no choice in the matter. Natural selection may have made the animal larger, an adaptation; but the thicker legs are an exaptation.
One humorous example of an exaptation (three exaptations walked into a bar…) is the human chin. Why do we have chins, and chimps do not? Some scientists have really used their imagination on this one. They imagined that the chin served as a way of expressing threat gestures. No kidding; I read that. (Post a comment to tell us your favorite story about why humans have chins.) But it turns out that the human chin is not an adaptation at all; it is an exaptation. One of the major characteristics of human evolution has been neoteny, the retention of juvenile characteristics into adulthood. Neoteny affected the growth patterns of the bones of the skull; in addition, natural selection favored jaws and teeth suitable for an omnivorous diet in human ancestors. The chin was a side-effect of these different facial bone and jaw growth patterns. The chin was probably not, itself, selected for anything; natural selection acted upon neoteny, not directly upon the chin.
Sometimes a feature can evolve for one reason and then turn out to be useful for another. Here are a couple of examples. Today, birds use feathers primarily for flight. But the earliest birds (according to fossils discovered in 2009) did not use feathers for flight; they had down feathers, which held in body heat. Once they had feathers, natural selection could act upon the feathers to make them into flight feathers. Another example is one of the most famous evolution just-so stories, the neck of the giraffe. What a weird adaptation. Giraffes have to spread their legs awkwardly just to drink from a pond. (Three giraffes walk into a bar…) To most observers, it would appear obvious that long necks are an adaptation that allows giraffes to feed at the tops of trees. But if this is so, why do female giraffes have shorter necks than male giraffes, and why are giraffes so frequently observed actually bending down to eat leaves? Careful observations have shown that males with longer necks prevail in battles with one another, and that females choose the males with longer necks. Long necks apparently evolved by competition for mates. Now that giraffes have long necks, they can use them for eating leaves from treetops, but that is not the original reason that long necks evolved in these animals.
Creationists could come up with their own version of exaptation. They could say that God created sex so that humans could make babies. But once sex existed, it could be used just for fun. Sexual fun is an exaptation, or sexaptation, or ecstatic-taptation. But I am not aware that any creationists have made this argument.
Adapted from the entry “Adaptation” in Encyclopedia of Evolution (Facts on File, 2006).