Thursday, October 29, 2009

Does Natural Selection Account for All Biophenomena?

No. But too many people who like to dabble in evolutionary explanations assume that it does.


The book, How Women Got Their Curves and Other Just So Stories is a good example of this. Many hypotheses are put forward for this or that characteristic or behavior, all explained in terms of selective pressure acting upon our forbears. We are as we are precisely because our clever genes have tried out various reproductive strategies, and the cleverest genes have won out.

What the authors fail to consider is the missing evidence.

Hair color can illustrate what I mean. Has hair color been strongly selected for? Are brunettes more common than redheads because brunettes possess an evolutionary advantage? Probably not. After all, there are many different common shades and hues, and no obvious advantage of one color over another presents itself. One might point out that light-colored hair is correlated with light-colored skin, which does possess an evolutionary advantage for people in northern lands. But one needs only to travel to the Caucasus to find that light-skinned people can have dark-colored hair.

So, maybe hair color has not been strongly selected for. Perhaps it is merely the result of chance, isolated populations, and group identity that Swedes tend to be blonde while Han Chinese are nearly uniformly dark-haired. Indeed, a fundamental mechanism at work within evolution is chance mutation. Only after a feature has appeared, as the result of a genetic accident, can the feature become the subject of selective pressure.

Imagine, for a moment, a world in which a Great Calamity early in human history has by chance killed off the ancestors of all modern humans except for a small group who were to become the forbears the Chinese. In such a world, nearly everyone would have dark hair since we all would have descended from the dark-haired survivors. Scientists studying evolution in that world would perhaps, upon considering themselves and their fellows, conclude that it must be the case that there had been some strong advantage to their ancestors in having dark hair, since human chemistry could permit other hair colors but no living humans were in fact so colored. The missing evidence that could have revealed the truth, died with the proto-Europeans in the Great Calamity.

Such scenarios have in fact happened repeatedly throughout the history of life. Why are the creatures that have built universities, governments, and shopping malls descended from the ancestors of marmots instead of from the ancestors of falcons? Because of a chance extinction and climate change that gave mammals an opening against dinosaurs.

Indeed, assuming that a trait has been selected for merely because it exists, or even because it is common or exclusive, is bad science. Science demands evidence to connect premises with conclusions (reasoning alone is not sufficient). Evolution is powerful science, and may be invoked in the work of others who are seeking to describe the world, but there is more to Evolution than selective pressure.

12 comments:

  1. Excellent post, and it is worth pointing out that a lot of evolved traits can be thought of as mistakes. For example, men are susceptible to prostate cancer precisely because the urethra goes through the prostate whereas it would have been evolutionarily beneficial for the urethra to go around the prostate. That is just one example.

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  2. I think you've hit on something that most scientists overlook entirely, but how can you account for missing evidence? I agree that it's bad science to use assumptions to replace evidence -- even in cases where the equation makes perfect rational sense, that does not mean it is correct.

    Not everything was selected for, and Darwin is even clear on this point, that nature does not care at all about many things that people are fixated upon, such as appearance. A great deal of characteristics have nothing whatever to do with survival and aren't relevant to evolution.

    It makes sense that one can't use natural selection as a blanket explanation, though I wonder how science can possibly live up to such a high standard of proof. There is a fine distinction between theory and presumption.

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