Homology associated with an organism that exists seemingly only because this aspect also existed in that organism's ancestors.

Vestigial structure are strong evidence for the existence of since it is difficult to imagine why an organism would possess structures that it does not need unless it simply is relatively difficult for species to lose those structures .

Thus, why do some have the of , or blind animals , or ? The answer is that the ancestors of these organisms possessed legs, eyes, or pelvises because in fact these used these structures.

In a more subtle sense, vestigial structures can represent evidence for natural selection in natural populations. In particular, one reason that it can be difficult to lose vestigial structures is that mutations conferring that loss can sufficiently impact other, still needed structures, that those mutations are selected against (which is an example of a pleiotropy).

In other words, vestigial structures can persist not just because it takes time for genetic drift to remove alleles from populations, but also because replacement alleles that completely remove structures without cost – and therefore which able to persist in the face of natural selection – may be relatively rare.