Sunday, August 23, 2009

Darwin's Appendix

...Or maybe his bibliography?

This piece is from The Panda's Thumb, under the category of "blogging on peer-reviewed research":

Last night, I asked for a copy of an article (I have plenty now, thanks!) that was getting a lot of press. The reason I was looking for it is two-fold: the PR looked awful, expressing some annoying cliches about evolution, but the data looked interesting, good stuff that I was glad to see done. Awful and interesting — I’m a sucker for those jarring combinations. My favorite pizza is jalapeno and pineapple, too.

The paper is about the appendix, that tiny little organ in your gut that doesn’t have a whole lot of obvious function. The point of the work is to try and show that yes, it does something — which is fine and interesting, although I will quibble a bit with their interpretation. Where they go awry, though, is in trying to pick a fight with a dead man, and making that the focus of their public relations.
Now, some of those same researchers are back, reporting on the first-ever study of the appendix through the ages. Writing in the Journal of Evolutionary Biology, Duke scientists and collaborators from the University of Arizona and Arizona State University conclude that Charles Darwin was wrong: The appendix is a whole lot more than an evolutionary remnant. Not only does it appear in nature much more frequently than previously acknowledged, but it has been around much longer than anyone had suspected.

“Maybe it’s time to correct the textbooks,” says William Parker, Ph.D., assistant professor of surgical sciences at Duke and the senior author of the study. “Many biology texts today still refer to the appendix as a ‘vestigial organ.’”

Charles Darwin is dead. Your research can’t be very cogent if your approach to drum up interest is to dig up a 120-year-old corpse and kick it around; is there anyone alive who disagrees with you who can put up a more informative and entertaining struggle? What this does is pick this one fellow as a symbol of the whole edifice of evolutionary theory, which has the advantage of making one’s work seem very, very important (even if one is stacking the deck to do it), but has the disadvantage of giving every creationist on the planet something to masturbate over, and they’re icky enough without your help.

It’s also annoying. Charles Darwin was wrong about many things — I’ll even give an example at the end of this article — and it’s part of the nature of science that everyone’s work will be revised and refined over time, and some of us will even be shown to be completely wrong. It’s rather unseemly to collect a lot of data that Darwin did not have, run it through PAUP 4.0 on a fast computer, map the data onto a molecular consensus phylogeny, and cackle gleefully over discovering something Darwin did not know. Really, it doesn’t make you a better scientist than Darwin.

To make it even worse, people who do this can’t even make the corpse-fight a fair fight — they have to stuff the pathetic dead body with straw. In this case, they’re padding Darwin’s investment in the appendix a fair amount. They cite one work by Darwin, The Descent of Man, which mentions this issue. He wrote one whole paragraph on the topic, and here it is, in its entirety; it was presented briefly as part of a long list of human rudimentary structures, such as wisdom teeth, muscles of the ear, and the semilunar fold of the eye.

Note why Darwin classed this appendage as vestigial: because it is greatly reduced compared to the homologous organs in non-human relatives, and because it currently exhibits a great range of variation, which is apparently non-functional. These are criteria which the paper in question does not refute at all. Darwin does say that the appendix is “useless”, and the paper will show some evidence that that is wrong. It’s also irrelevant.

The reason why it is irrelevant is that the presence of some function is not part of the definition of a vestigial or rudimentary organ — Darwin obligingly concedes that evolution will salvage some utility out of organs with little retention of their original function, but which are present as a consequence of contingency. He discusses this at greater length in On the Origin of Species, and here is a significant chunk of the relevant writing.
Organs or parts in this strange condition, bearing the plain stamp of inutility, are extremely common, or even general, throughout nature. It would be impossible to name one of the higher animals in which some part or other is not in a rudimentary condition. In the mammalia, for instance, the males possess rudimentary mammae; in snakes one lobe of the lungs is rudimentary; in birds the “bastardwing” may safely be considered as a rudimentary digit, and in some species the whole wing is so far rudimentary that it cannot be used for flight. What can be more curious than the presence of teeth in foetal whales, which when grown up have not a tooth in their heads; or the teeth, which never cut through the gums, in the upper jaws of unborn calves?

Rudimentary organs plainly declare their origin and meaning in various ways. There are beetles belonging to closely allied species, or even to the same identical species, which have either full-sized and perfect wings, or mere rudiments of membrane, which not rarely lie under wing-covers firmly soldered together; and in these cases it is impossible to doubt, that the rudiments represent wings....

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. Thus in plants, the office of the pistil is to allow the pollen-tubes to reach the ovules within the ovarium. The pistil consists of a stigma supported on a style; but in some Compositae, the male florets, which of course cannot be fecundated, have a rudimentary pistil, for it is not crowned with a stigma; but the style remains well developed and is clothed in the usual manner with hairs, which serve to brush the pollen out of the surrounding and conjoined anthers. Again, an organ may become rudimentary for its proper purpose, and be used for a distinct one: in certain fishes the swimbladder seems to be rudimentary for its proper function of giving buoyancy, but has become converted into a nascent breathing organ or lung. Many similar instances could be given.
I’ve highlighted the part most important for this discussion. Darwin did not discuss the appendix or caecum at all in the Origin, but this description does apply. If a portion of the gut, a digestive organ, is diminished in size such that it no longer contributes to the primary function of the organ, but does retain a secondary function, such as assisting in immunity, or as the authors of the recent paper will argue, in acting as a reservoir of bacteria for recolonizing the gut, then it is still a vestigial organ. It has lost much of its ancestral function.

I do not understand why this is so hard for so many people to comprehend. Biology is plastic and opportunistic. Accidents of history will always still be incorporated into the whole of the organism; we make do, or we die. Just because something is does not mean that the entirety of its nature is the product of selection.
I mentioned that I’d point out errors in Darwin’s understanding. They’re there, but note that seeing them now 150 years after he wrote his big book does not make me smarter than Darwin, nor does it invalidate the overall picture of his theory. You can see one ‘error’ in the quote above: we are now pretty certain that the original function of the swimbladder in fish was respiratory. It evolved first as a supplement to the gills, providing access to the rich oxygen content of the atmosphere, and was secondarily adapted to function for bouyancy. Hah, silly Darwin, that he did not know a detail of paleontology and phylogeny that would be worked out a century after his death!

He also made a more substantial error. He wondered how organs became smaller over time, and his answer was, unfortunately, a bit Lamarckian and also a bit muddled.
“Disuse” is the magic word there: if a cavefish lived in the dark and never used its eyes, the idea was that its progeny would then have smaller eyes. This is not correct, but it was a central part of Darwin’s invalid theory of heredity. This is a much more substantial failing of Darwin’s work, but again, I can’t claim credit for figuring this out; it took the work of Mendel to get the core of genetics puzzled out, and then it took a whole generation of scientists to work out how genetics and evolution fit together. We can say “DARWIN WAS WRONG!” about that, but we can’t really say that about his treatment of vestigial organs in general, which seems to hold up fairly well…perhaps because Darwin himself was not so fervently committed to the absolute adaptedness of every single feature of every single organism as some of his later followers.

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