Origin Ch.5: Drift

This post is part of a series on The Origin Of Species.  It was originally posted on the old blog in feb 2009, during the Darwin 200 celebrations.

It has been noted enough times now that Darwin knew nothing of genetics, and I already described Darwin’s bizarre Lamarckian hypothesis for why some domestic animals have droopy ears (they don’t need to prick them up to listen out for predators or prey, so they are lazy and let the muscles waste). But in chapter five, Darwin goes into a little more detail about the loss of previously evolved traits. “Natural selection is continually trying to economise every part of the organisation,” he says. And, “natural selection will tend in the long run to reduce any part of the organisation, as soon as it becomes, through changed habits, superfluous.”

This might be — indeed, almost certainly is, to some extent — true. But we don’t need to posit active selection to explain dereliction of traits. If you take out the Lamarckian tones of the ear-disuse hypothesis, there might actually be something in the idea. Indeed, from our modern understanding of biology, the most parsimonious explanation for the droopy ears of some domesticated animals may well be that strong and structured upright ears became superfluous for survival. The mechanism would not be laziness causing heritable muscle wastage, though. Rather, it is random mutation, coupled with that oft neglected evolutionary process, genetic drift.

Random mutations are appearing all the time, and, silent mutations aside, probability is more on the side of such mutations destroying complex traits than developing them. In the absence of any stabilising selection to maintain that complex trait, the new variant alleles will be randomly buffeted around the gene pool, perhaps dying out, or perhaps spreading. This is one of the many reasons why we can be confident that humans are still evolving: if it true that advances in agriculture, technology, medicine, and culture, have lifted selection pressures on some traits, it follows that genetic drift will have been unleashed in their place. We are rightly celebrating Darwin’s great discovery of natural selection, but don’t forget that there are other important evolutionary processes.

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