Atavisms, like vestigial structures, look like underdeveloped and relatively unfunctional versions of structures that appear fully formed and fully functional in related organisms. The main difference is in their occurrence. A vestigial structure is a characteristic feature of the species in which it occurs, i.e. all normal members of the population have it. An atavism is exceptional: It is not normal for a member of that species to have it. Some atavisms are extremely rare, but if they occur at all, an explanation is called for, and common descent provides one.
Some examples:
As with vestigial structures, common descent easily explains these occurrences. It also explains why there are certain similar anomalies that never happen. No shark or trout has ever been found with anything like a leg. Why not? If whales sometimes can grow them, why not sharks? No modern animal ever grows feathers unless it is a bird. Why not? If a snake can grow legs once in a great while, why not a few feathers once in a great while?
Some species of cave-dwelling salamanders normally have no eyes, not even vestigial ones. But once in a while one of them is born with eyes. No worm or snail, however, is ever born with eyes.
No species ever develops any structure unless it looks very closely related to a species that normally has the structure. Whales have no more use for legs than a shark does, but whales sometimes grow them anyway while sharks never do.
This particular difference between sharks and whales is is highly suggestive of a connection with numerous other differences between sharks and whales. Despite superficial similarities between whales and fish in general, a whale's overall anatomy makes it look much more like a modified land animal than like a modified fish. This adds a great deal of potential significance to the fact that whales sometimes grow legs while sharks never do. It makes it reasonable to suspect at least the possibility that whales had ancestors with legs but sharks did not.
Next: The facts in evidence: Embryology
(This page last updated on August 6, 2010.)