The facts in evidence: Vestigial structures

We define vestigial structure as follows.

We observe some organism -- an ostrich, for example -- and we also observe some organism to which it looks related. For our ostrich example, the related organism would be almost any other bird.

We then observe two things about the ostrich's wings. (1) They look incompletely developed compared with other birds' wings. (2) Because of that apparent developmental deficiency, they cannot serve the function that the wings of other birds serve.

We call any such structure vestigial. It looks like an underdeveloped version of the same structure found in related organisms, and it is useless for the function that the structure serves in those other organisms.

We also observe something else in most cases like this. Most of these structures seem to serve no function at all, and in the few cases where some function is apparent, it is not a vital function. The organism could easily survive without it.

It is sometimes said that vestigial structures are invariably useless. For many of them that does seem be the case, but a complete absence of function is not definitive of vestigial structures. Ostriches may well put their wings to some very good uses, but they cannot use them for flight, and that suffices to justify classifying their wings as vestigial.

Other examples:

Sometimes negative evidence can have considerable relevance to an interpretation of positive evidence. There are some vestigial structures that have never been observed.

Vestigial structures are easily explicable in terms of descent with modification. Descent with modification also explains very nicely why vestigial nipples never show up on reptiles, birds, or amphibians, why clams and lobsters never have any bones, and why no creature but a bird ever grows feathers.

Other explanations may be possible. My purpose in this essay is not to rule out alternatives to evolution. It is to answer the question: What evidence exists for evolution? Vestigial organisms are evidence for evolution, and they do exist.

Next: The facts: Atavisms

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(This page last updated on August 6, 2010.)