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Size matters, or it doesn't....

Gene ExpressionBy Razib KhanSeptember 8, 2007 12:02 PM


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Over at my other blog I reaffirm Richard Dawkins' criticism of Freeman Dyson's off the cuff opinions about evolutionary genetics. Dyson is basically asserting that the rate of evolution is inversely proportional to the square root of population size. In short, small populations evolve fast in his mind because of stochastic fluctuations, clearly drift. I've posted a fair amount about stochastic dynamics...and it's complicated. Science is complicated. That's just life. Now, Dyson is pretty much wrong. But his intuition is conventional; I've met many people who believe that somehow evolution can operate more effectively, faster, on small populations. I can't even count up how many people seem to believe that population bottlenecks are always the critical events in changing the character of a species. In a very primitive way I think people have a general sense that Sewall Wright's Shifting Balance model captures the essence of evolutionary dynamics; though like most interpreters of Wright's views they incorrectly place greater emphasis on random drift as opposed to genetic interactions. But why are people struck by the idea that small population sizes can change faster and more easily than large ones? I believe I held this intuition as well before engaging in a serious study of evolutionary process. Here are some thoughts. Consider that we normally talk of "gene pools." In other words, we conceptualize genes as floating in a mass, physically embedded within the "population." I suspect in our mind's eye we're conceiving of evolution as physically operating upon the mass of the gene pool somehow. Assuming that evolution is a constant pressure or force, a small mass would be more malleable and easier to shift because of the ratio between the force and the object which it operates upon increases. This fits pretty well with the impact of stochasticity, it isn't like deviation from generation to generation of allele frequencies isn't an issue, it is simply that the deviation is proportionately smaller because of increased sample size. The stochasticity is there, but it becomes less effective in "moving" the gene pool over time as it increases in size. Similarly with selection we're imagining that it takes more time to makes its force felt in a large population. Humans don't think in terms of population genetic parameters, rather, unless straight-jacketed into a precise formalism they'll make recourse to physical analogs. Now, you say "No shit Sherlock!" But remember that this post was triggered by the musings of one Freeman Dyson, no tard is he. A cautionary tale on the unnaturalness of science....

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