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The Daily Wildcat

The Daily Wildcat

 

    Darwinian natural selection not always the best explanation

    In an age where most biology is motivated by the theory of evolution, it’s easy to assume that Darwinian natural selection explains everything from why people eat too much to why women continue to date popped-collared frat boys.

    But as evolution continues to come under assault from the far right, its foundation in good science is also being attacked, unwittingly, by unknowing people who have a poor understanding of evolution and who continue to make faulty claims regarding it. This makes evolution look bad, and it makes scientists look stupid.

    A great deal of popular discourse regarding evolution seems to be based on three faulty lines of thought.

    First, assume that whenever you see a trait in an organism, such as a behavior or a morphological feature, it has a strong genetic basis. This assumption is not always borne out by reality; many traits are determined strongly by environmental or (in humans) cultural factors, so genetics is not always at the root of the problem.

    Next, assume that, since it has a genetic basis, it must be an adaptation – that is, it must have evolved because it was useful. This doctrine is known as adaptationism or the “”Panglossian”” paradigm, after the eternal optimist from Voltaire’s “”Candide”” who asserts that we live in the best of all possible worlds. Actually, many traits that evolve aren’t adaptations at all; they’re neutral or even bad, and they become common due to a random process known as genetic drift. At the DNA sequence level, most evolution appears to be neutral.

    Finally, since the trait evolved, come up with some cockamamie explanation for why it’s adaptive. These explanations often bear a striking resemblance to Rudyard Kipling’s humorous “”just-so”” stories, and there is sometimes little to no evidence backing them up. In practice, it is often difficult to tell which of several evolutionary explanations for a trait is correct; in some cases, it may be that there is no good explanation at all.

    One example of an evolutionary “”just-so”” story is the giraffe’s neck. The cartoon picture that continues to be presented to high school students is that giraffes with longer necks were favored by natural selection, since they could avoid competing with smaller animals like kudus for food; they’d have access to higher leaves and branches.

    But it isn’t clear that giraffes really eat this way frequently. One alternative hypothesis is sexual selection; male giraffes spar for potential mates by swinging their necks like baseball bats, trying to hit the opposing male’s head and knock him out. A longer neck corresponds to a greater moment of inertia, leading to greater success in sparring and more mating opportunities.

    Which hypothesis is correct? It isn’t easy to tell. Both may be correct, to a certain extent – but the important thing is that scientists should (and some do) devise ways to test these hypotheses rather than claiming that one or the other is right without evidence.

    A lot of bad evolutionary science comes from fields like Darwinian medicine and evolutionary psychology, which seek to explain medical and behavioral anomalies in humans in terms of natural selection.

    One particularly egregious Darwinian medicine paper argued that human diabetes-related genes may have been favored for their ability to protect cave men during ice ages. Seriously, I am not making this up.

    Evolutionary psychologists are notorious for claiming that everything from social stratification in human societies to oddities in female mate choice can be explained in terms of natural selection during the Paleolithic era.

    Some simple math calls a lot of this into question. Humans appear to have about 25,000 genes, most of which probably do not influence behavior directly, so it seems unlikely that natural selection could have acted finely on as many different behaviors as evolutionary psychologists claim. The genes that appear to have undergone the most positive selection relate to the immune system and competition between sperm.

    Moreover, humans have had a fairly small population size for most of our evolutionary history, and natural selection tends to be less of a factor when populations are small.

    By far the worst example of this kind of pseudoscience I’ve read came from a handful of psychologists at Northumbria University who performed a study on a number of British women. They found that women tend to prefer men with stubble over men who were clean-shaven or who had full beards. One proposed evolutionary explanation is that stubble accentuates the jaw and teeth, making men appear more aggressive, so female preference for light stubble is an adaptation.

    You could fly a mothership through the holes in that argument.

    One serious problem is probably poor science writing; journalists who do not understand science frequently write bad summaries of good research. In one article I read about the above study, a researcher expressed some skepticism about his results. He acknowledged that culture plays a big role in female perceptions of stubble and that he’d like to control for that in the future. In two other versions, this skepticism was omitted, so the researcher was made to look like an idiot.

    Journalism and liberal arts majors who are wondering why they have to take science classes, take note.

    Moreover, summaries of evolutionary psychology and Darwinian medicine papers often look like they were written by apologists for some unfortunate human behavior, like gluttony or the oppression of women; they frequently have an undertone of, “”See? Evolution favors it, so it can’t be that bad.”” This, too, is never a conclusion of real science.

    The bottom line is that, any time you hear an evolutionary account for something, you shouldn’t blindly accept it. Lack of skepticism really does cause the line between science and ideology to become blurry. Use good judgment and remember that evolutionary questions aren’t always clear-cut.

    -ÿTaylor Kessinger is a senior majoring in ecology and evolutionary biology, math and physics. He can be reached at letters@wildcat.arizona.edu.

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