The short answer
Yes. That some vestigial structures have some function or another is irrelevant, as is the fact that they are the products of degeneration. What matters is that they are clear signs of evolutionary descent from organisms for whom the structures were fully developed.
The longer answer
I. Do vestigial structures exist?
Some creationists deny that vestigial structures exist. To understand why, it is important to note how creationists define vestigial. Here is a sample definition from creationist Scott Huse: “Vestigial organs are those structures which are presumed by evolutionists to be the useless remains of an organ which was once fully developed and operational in ancestral types” (Huse 1982: 107, emphasis mine). I have emphasized the word “useless” in Huse’s definition, because that is critical.
Huse goes on to assert (incorrectly) that “all organs formerly classified as vestigial are known to have some function during the life of the organism” (Huse 1982: 107). Most creationists are not quite so cavalier. They are satisfied to point out (correctly) that many of the structures scientists classify as vestigial continue to have some function or another, which leaves open at least the possibility that functions eventually will be discovered for the remainder.
However, the above is beside the point, because Huse’s definition is mistaken: although vestigial structures can be useless, they are not by definition useless. Biologist Jerry Coyne explains:
Evolutionary theory doesn’t say that vestigial characteristics have no function. A trait can be vestigial and functional at the same time. It is vestigial not because it is functionless but because it no longer performs the function for which it evolved. The wings of an ostrich are useful, but that doesn’t mean that they tell us nothing about evolution. Wouldn’t it be odd if a creator helped an ostrich balance itself by giving it appendages that just happen to look exactly like reduced wings, and which are constructed in exactly the same way as wings used for flying? (Coyne 2009: 58)
Coyne’s definition may at first seem to make the evidential relationship between vestigial structures and evolution circular, since creationists like Huse naturally will deny that there ever was an original function for which the structure initially evolved. However, as Coyne’s last sentence makes clear, the evidence for a trait being vestigial is in fact accessible to both creationists and evolutionists. We see many structures in the biological world that look exactly the way we would expect them to if they were the evolutionary remnants of structures with different functions: that much is perfectly clear to both sides.
Given this, creationists have a few options. One is to suppose that the creator is a liar and deceiver, though this would completely undercut any reason to trust the religious texts that motivate creationism in the first place. Another is to suppose that the creator is guided by some inscrutable aesthetic sense which leads him to imitate evolution, mindless of the fact that this would lead his subjects to actually come to believe in evolution: like a confused artist he staggers around the cosmos, perhaps with a dog-eared copy of L’Etre et le Néant in the back pocket of his vaguely hemp-scented jeans, bewildered that his work is so misunderstood.
Finally, one can concede that vestigial structures do in fact exist, but contend that they provide evidence for only a degenerative form of evolution, since creationists generally don’t have a problem with the idea that things can become worse through evolution. How does this strategy fare?
II. Do vestigial structures provide evidence for degenerative evolution only?
Huse actually says something along the above lines as well:
Even if the concept of vestigial organs were valid, it still would not lend support to evolution since it implies structures on the way out, not in. Nascent organs, those under construction into a functional unit, are completely nonexistent. (Huse 1984: 107)
Now, it certainly is true that the degeneration of some structure into a vestigial version cannot provide direct evidence for the evolutionary origin of the original structure. For instance, the fact that cave fish have evolved to lose their sight does not, of course, demonstrate that a lineage of initially eyeless organisms can evolve eyes. However, the wider evidential importance of vestigial traits for evolution is that they provide evidence for common descent, and evidence for common descent is indirect evidence for precisely the kind of the evolution in which Huse is interested.
Take the human coccyx (the tailbone) as an example. Evolutionists will grant that the transition from tail to coccyx primarily is degenerative, involving mostly the loss of structure and function. But the implication of our having a coccyx is striking, because it provides strong evidence that we share common ancestry with creatures that have tails; indeed, it provides strong evidence that somewhere in the ancestral lineage leading to humanity, there was a species of creatures that had tails. Since this meshes so well with the evolutionary account of the origin of humanity, it provides some evidence for that whole account; in doing so, it therefore provides some evidence for the other parts of that picture, including the evolution of novel positive characteristics in our lineage. To be sure, vestigial traits do not show us how such characteristics evolved, but they provide some evidence that they evolved.
Coyne JA. 2009. Why Evolution is True. New York: Viking.
Huse SM. 1983. The Collapse of Evolution. Grand Rapids, MI: Baker.