Darwin said a lot of good stuff in The Origin. One of his less-recognized achievements was to explain not just the presence of rudimentary (“vestigial”) features as evidence for evolution, but also to recognize that such features, insofar as they’re not essential to the individual, would be more free to vary than would features critical for survival and reproduction. Here’s his discussion from Chapter 13: “Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Organs.” (I’ve put the sentence most important for this post in bold.)

On my view of descent with modification, the origin of rudimentary organs is simple. We have plenty of cases of rudimentary organs in our domestic productions, as the stump of a tail in tailless breeds, the vestige of an ear in earless breeds, — the reappearance of minute dangling horns in hornless breeds of cattle, more especially, according to Youatt, in young animals, and the state of the whole flower in the cauliflower. We often see rudiments of various parts in monsters. But I doubt whether any of these cases throw light on the origin of rudimentary organs in a state of nature, further than by showing that rudiments can be produced; for I doubt whether species under nature ever undergo abrupt changes. I believe that disuse has been the main agency; that it has led in successive generations to the gradual reduction of various organs, until they have become rudimentary, as in the case of the eyes of animals inhabiting dark caverns, and of the wings of birds inhabiting oceanic islands, which have seldom been forced to take flight, and have ultimately lost the power of flying. Again, an organ useful under certain conditions, might become injurious under others, as with the wings of beetles living on small and exposed islands; and in this case natural selection would continue slowly to reduce the organ, until it was rendered harmless and rudimentary. Any change in function, which can be effected by insensibly small steps, is within the power of natural selection; so that an organ rendered, during changed habits of life, useless or injurious for one purpose, might easily be modified and used for another purpose. Or an organ might be retained for one alone of its former functions. An organ, when rendered useless, may well be variable, for its variations cannot be checked by natural selection.

Note the important point—often neglected by creationists—that vestigial organs can be functional (“Any change in function, which can be effected by insensibly small steps, is within the power of natural selection; so that an organ rendered, during changed habits of life, useless or injurious for one purpose, might easily be modified and used for another purpose.”). Thus the wings of penguins, no longer used for flying, are useful in helping them swim, and at the same time testify to penguins’ origin from flying birds. They are useful vestigial traits.

There are are least three reasons why a now-useless feature can be eliminated. First, mutations affecting it are no longer deleterious, so it would degenerate over time. Second, the feature could be injured and, if no longer useful, would be eliminated by natural selection because its injury would reduce fitness (this may be the reason why the eyes of cave animals gradually disappear). Finally, the metabolic energy that goes into producing a useless feature could be diverted to other features that are still useful (producing a useless organ like an eye has physiological “costs,” and maybe you could divert the metabolic resources involved in making it into other features, like the brain).

From all of these, but particularly the first, you can derive Darwin’s conclusion why useless features should be more variable than essential ones. In the technical jargon of evolutionists, those features are no longer under “stabilizing selection”, the kind of natural selection that eliminates deviations from features that are at a local optimum for being fit. Usually mutations causing deviations from such a feature are selected against, but may no longer be deleterious if the feature is no longer useful. Mutations can thus accumulate willy-nilly, and the feature becomes more variable.

The variability of vestigial organs is well known. The classic example is that of wisdom teeth, which are highly variable in both presence (Tasmanian aboriginals don’t have them; indigenous Mexicans all have them), and degree of expression, as many of you know. Some people can’t wiggle their ears because their ear muscles aren’t functional (we no longer need them since we can localize sound by moving our heads), while others, like me, can wiggle them freely. The palmaris longus muscle has been reduced to a small tendon in our forearm; it’s absent in 14% of people and, being pretty useless, is often used for tendon grafts in the wrist.

Further support for Darwin’s notion of increased variation in less-useful characters comes from a new study of variation in the inner ear of the three-toed sloth (Bradypus variegatus). A paper in the Proceedings of the Royal Society by Billet et al. (download free; reference below) shows that several features of the inner ear, a part of the body crucial for maintaining balance, are much more variable in the sloth than in its relatives. This is expected if sloths, being notoriously slow, don’t have a critical need to maintain balance. The authors were particularly concerned with the semicircular canals (SC) of the animals, which are really crucial for keeping balance. There are three semicircular canals, each of which contains fluid and small hairs that detect the motion of the fluid. In that way the animal is able to sense its orientation.

As Billet et al. note:

Accordingly, one would expect to find among sloths a substantial degree of morphological variation in their SC, because significant travelling distances, speed and agility are not part of their locomotor repertoire. In this context, sloths constitute an ideal case to address Darwin’s hypothesis.

Well, sloths still have to hang in trees and maintain balance when they defecate (they do so once a week, climbing all the way down from their trees to do their business at the base of the tree before climbing back up, a behavior that’s not really understood), but maybe they don’t need a perfectly-formed inner ear to do that, so variations caused by mutation or developmental accidents might not be terribly injurious.

To test “Darwin’s hypothesis,” the authors used high-resolution computer tomography to examine and make 3-D reconstructions of the inner ears of individual three-toed sloths as well as of red squirrels (Sciurus vulgaris), European moles (Talpa europaea), the collared anteater (Tamandua tetradactyla), and the nine-banded armadillo (Dasypus novemcinctus). The anteater and armadillo are members of the Xenarthra, a suborder of New World mammals that include the sloths, and thus these species serve as “controls” for degree of variation in related species.

Billet et. al measured 12 features of each inner ear, and, since variation might be correlated with size, expressed variation as the variation among individuals in the ratios of one feature to another. Here’s the inner ear in situ of a three-toed sloth skull, and its reconstruction from the computer scan (“ASC,” “PSC” and “LSC” are the three semicircular canals, also called “horizontal,” “superior,” and “posterior”):

I’ll show the numerical values of variation in a second, but this figure from the paper shows the higher variation in sloths just by inspection of the degree of variation among individuals of four species. The sloth is clearly more variable than the anteater, armadillo, and squirrel. As you might expect, the squirrels show almost no variation:

Here are the data, with the height of the bar showing the degree of variability of ratios of inner-ear features. The red bars are the sloths, green bars are anteaters, purple bars are armadillos, yellow bars are squirrels, and blue bars are moles. In all cases the variation is highest in sloths, and in many cases it’s much higher than the other species (in nearly all cases the difference between sloths and other beasts is statistically significant):



The authors conclude that they’ve verified Darwin’s hypothesis and advance this reason for their observation:

Therefore, we hypothesize that relaxed selective pressure on the SC has permitted the persistence of the observed variants in three-toed sloths. The level of genetic and/or developmental constraints on the production of these variants might also depart from that of other placentals. . . The most plausible reason for such a released selective pressure on their SC morphology lies in their reduced activity pattern. Such high variation of the SC has not been detected in faster-moving xenarthrans, nor has it been documented in other fast-moving mammals.

It’s known that animals that are agile and move fast have relatively larger semicircular canals, so I find this conclusion plausible. What remains is a more definitive test of whether variation in the inner ears of sloths really does matter less than such variation in other species. I’m not sure how one would test that, for you can’t just go in and just manipulate the inner ear. Perhaps you could put sloths in jet planes, subject them to various sorts of vertigo, and see if they do better than other animals!

Another possibility is that sloths are simply more variable than related mammals in many features, and so the ear variation isn’t that special. Maybe, for instance, the sloths measured came from genetically isolated subgroups, and the variation we see is just variation among subspecies or ecotypes. The authors argue pretty convincingly that this isn’t true, but at least two other studies show substantial variation in other skeletal features of sloths that aren’t related to balance, and in the timing of when their cranial sutures (the fissures in the skull) close. So we need to look at variation in many morphological traits of sloths. Given the funding difficulties of this type of science, I doubt we’ll see such work in the near future.

One thing I found amusing was the blurb for the short HuffPo piece on this research:

It sort of suggests that Darwin’s theory in general was under question, rather than just this one small prediction that Darwin made. But even that prediction has already been verified in characters like wisdom teeth and human muscles, so the present study doesn’t really add much to substantiating that idea. What the work does suggest, though, is that it’s less important for sloths than for other animals to maintain their balance, and that’s cool in itself.

Finally, here’s a video of a three-toed sloth (and its faster two-toed relative, which isn’t all that closely related):

“Sloth” of course, is an adjective that applies to lazy humans. That reminds me of a joke: A city boy goes to the country and sees a bunch of hogs slopping noisily and messily at their trough. “No wonder they call them pigs!”, he exclaims.

I’ll be here all week, folks.

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Billet, G., L. Hautier, R. J. Asher, C. Schwarz, N. Crumpton, T. Martin, and I. Ruf. 2012. High morphological variation of vestibular system accompanies slow and infrequent locomotion in three-toed sloths. 10.1098/rspb.2012.1212. Proceedings of the Royal Society B: Biological Sciences