Vestigial Structures
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[edit] Introduction
Some examples will make the point clear.
- The vestigial eyes of fish such as the cave-dwelling varieties of Astyanax mexicanus[1] are homologous to the eyes of the non-cavedwelling, sighted varieties of the same species, but have less function than their homologues, because they are sightless, and, indeed, grown over by the skin of the fish.
- The wings of an ostrich are homologous to the wings of birds that can fly, but have, obviously, less function than their homologues.
- The vestigal hair on humpbacked whales[2] is homologous to the hair on other mammals, but has less function than their homologues because it no longer serves as insulation.
It is sometimes the case that these structures will disappear during embyological development or maturation: for more information, see our article on Development and Evolution.
[edit] Vestigial structures and evolution
The theory of evolution predicts which vestigial structures we can and can't find in nature. The reasoning goes like this. The wing of the ostrich (for example) cannot have acquired its similarity to the wing of a hummingbird by convergent evolution. If the theory of evolution holds good, then this must be the result of common descent from an ancestor that could fly.
Hence, the theory of evolution predicts that we can only find vestigial structures which are consistent with the history of evolution as revealed by other means (for example, molecular phylogeny). So, for example, the theory predicts that we can find vestigial fur on whales, but not vestigial feathers; and vestigial legs, but not vestigial wings; snakes can have vestigial hipbones, but not vestigial antlers; blind cave fish can have vestigial eyes, but not vestigial legs. These features carry particular importance because unlike some other homologies that might be proffered as evidence for evolution, these cannot be explained away by creationists as "common design for a common purpose". Clearly, the wing of an ostrich and a humming bird do not have the same purpose.
We should note that this method of putting evolution to the test, like every valid method, does not require that one should first believe in evolution. In principle, any skeptical creationist could verify the homology, the reduction in function, and the agreement (or, as he would hope, disagreement), with a phylogenetic tree, without having to believe in the slightest that these structures were produced by evolution or that the phylogenetic tree is anything more than moonshine.
[edit] Creationist misconceptions
The standard creationist tactic for causing confusion over this issue is to pretend that vestigial structures are claimed to be completely without function. They can then either point out that the vestigial structure has some function (if it does) or that you can't prove that it has no function (if it doesn't). Indeed, some creationist sources explicitly define vestigial structures as "features that apparently serve no function in an organism"[3]. This is not how most dictionaries or biology textbooks define them[4].
When it is pointed out to creationists that vestigial structures need not be completely useless, they make accusations of Moving the Goalposts, as in this quotation from Jonathan Sarfatti.
- Some evolutionists, like Dr Meiss, now want to re-define 'vestigial' to mean simply 'reduced or altered in function'. ... AiG isn't going to let evolutionists change the rules at their whim when they are losing the argument.[5]
As we can see from the following quotation from Darwin, the goalposts remain exactly where they always were.
- An organ serving for two purposes, may become rudimentary or utterly aborted for one, even the more important purpose, and remain perfectly efficient for the other [...] Rudimentary organs [...] are either quite useless, such as teeth which never cut through the gums, or almost useless, such as the wings of an ostrich. (Darwin, Origin of Species, chapter 14[6], emphasis added.)
Moreover, it is clear from our discussion, above, of the evolutionary significance of vestigial structures, that nothing in the argument depends on the structure having no function. The wings of an ostrich may perfectly well have some function, as Darwin asserts, but so long as it is not flight, then the argument is unimpaired.
