Field of Science

George Williams on pleiotropy

Another great evolutionary biologist, George Williams, died recently. Carl Zimmer quotes himself from a 2004 interview:
One reason that the book was so effective was that Williams demonstrated how natural selection could influence the full course of a species’ life history. It wasn’t necessary to think of growing old as being for the good of the species, for example. Instead, Williams argued that the decline of old age could be caused by pleiotropy–in other words, the harmful side effects of genes selected for advantages they offered during youth. Just as long as the advantages of these genes outweighed the disadvantages, they would become widespread. Ironically, cancer, declining stamina, deteriorating vision, and various diseases of old age could all be the result of natural selection, says Williams: “Pleiotropy is the ultimate reason for all these things.”
Pleiotropy is indeed possibly the reason why people get cancer, etc. I think of pleiotropy as this crazy wild card with which all sorts of weird and unexpected traits can evolve. A disadvantageous trait, which natural selection under normal circumstances would purge, may exist in a population (go to fixation) because an allele (i.e., a version of a gene) wholly or partly responsible for the disadvantageous trait also affects another trait that is advantageous. If the combined effect of the two (or more) traits is that the organism has higher fitness, then the disadvantageous trait will evolve. Many organisms have traits that some people propose evolved because they initially were adaptations, like big brains or feathers. And many of these may well be, but it isn't out of the question that they initially evolved via a pleiotropic effect.

Pleiotropy may aid in adaptation when epistatic interactions create a rugged fitness landscape. In a rugged fitness landscape, it will sometimes be the case that in order to adapt, a lineage/population must cross through a valley in the fitness landscape before it can reach higher ground. When several mutations are necessary to change a gene (or a set of genes) enough that the organism becomes fitter than the original, and the intermediate genotypes are of lower fitness than both beginning and end product, then pleiotropy may rescue the situation by negating (or attenuating) that negative effect, such that the lineage can manage crossing the valley (which then really isn't a valley, when the second function of the gene (or set of genes) is taken into account).

After having to explain the meaning of epistasis and fitness landscapes to some people who have zero background in biology and evolution, I have become keenly aware just how strange the above may sound when one is unfamiliar. Apologies. Questions are welcome.

I am hosting the next edition of Carnival of Evolution myself over on Carnival of Evolution on October 1st (submit your posts here). Perhaps in memory of Williams?


  1. Re: Laymen understanding Fitness landscapes -- that animation you posted a while back of a model of a fitness landscape with peaks and valleys, showing the populations eventually leaping to the other peaks, that was awesome. It really helped me to visualize the idea.

  2. There's a growing body of evidence suggesting a neurological foundation for fundamentalism. I keep saying we need to breed it out of the genome (which, of course, we won't do) for the sake of human continuity, and someone told me the other day that it's a bad idea, because bad mutations are sometimes good for the species, and that removing them can create a genetic bottleneck, and so on.

    I have to say that I find it difficult to accept that there is an evolutionary advantage to fanatical stupidity.

  3. You mean 'difficult to accept that there is asocietal advantage to fanatical stupidity'. It is easy to imagine an evolutionary advantage: e.g., if fundamentalism directs how to breed. Like the quiverfull.

  4. I suppose, if we evaluate success in terms of the dissemination of genetic material.

    But I actually did mean "evolutionary". I think we might have talked about this before (I say it all the time, in any case) - in my view, these people are driving us toward extinction. They've spent the past thirty years voting into office the criminals and lunatics chiefly responsible for our current socioeconomic crisis, from which we aren't going to recover - and, due to the interconnected nature of the global economy, as we continue to go down, well be taking everyone else down with us.

    We're facing, within the next few years, the end of our global civilization, possibly the end of our species - and I'm not even taking into consideration the possibility of a fundamentalist lunatic setting off a nuclear holocaust because he can't wait to meet Jesus in the sky, or he think he's going to get 72 virgins, or whatever.

  5. Gotcha. I understand your fear and frustration, and agree to some extent, but do find it highly unlikely that fundamentalism will mean the end of our species, in any of the scenarios you present, or any other I can think of. Not impossible, just unlikely. The chance that someone, somewhere will somehow survive out of a population of more than 6 billion seems pretty good, no matter what anyone can throw at us all.

    But let me ask you, how likely do you think it is that the consequences of fundamentalism will doom us all?

  6. Well, as I say, Bjorn - I think it's pretty likely.


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