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:

• Some pythons have pelvic bones, which are apparently useless for any organism without legs.
• The human appendix serves no digestive function and its surgical removal causes no known health problems. It looks like an atrophied version of a similarly placed structure that serves a vital digestive function in other animals.
• In some species of cave-dwelling fish and subterranean mammals, the eyes are covered with opaque membranes, precluding any possibility of sight. Since they live in constant darkness, this is not a problem for them. They have no need for eyes in the first place. But they have them anyway, and for no apparent reason.
• Some legless reptiles that look superficially like snakes are for various anatomical reasons classified as lizards. Some of them actually have legs, but only underneath the skin where they can serve no locomotive purpose.
• Dandelions reproduce asexually. They also have flowers and produce pollen. Flowers are a sexual organ in all other plants that have them, and pollen is the plant analogue of animal sperm.
• Beetles that fly have wing covers that fold over their wings to protect them when they're not in use for flight. There are many flightless species of beetles. Some have no wings. Many do have fully developed wings, but the wing covers are fused shut over them.

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

• No reptile or any other non-mammal has ever been found with even rudimentary nipples.
• No arthropod has ever been found with anything like a backbone.
• No amphibian or mammal has ever been found with anything resembling feathers.

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.)