Saturday, August 18, 2007

Evolution, a story told by the winners   posted by David Boxenhorn @ 8/18/2007 09:54:00 PM

I had what I seemed to me like an interesting thought when I read this, and I wanted to explore it further. But I have been very busy these days, and I just don't have the spare cycles, so I'm just going to throw it out there. My biggest question is: "What am I missing?" R. A. Fisher didn't think that epistasis was an important evolutionary force. I can't believe he would miss this, so the only alternative is that he considered it...

From the link:

Finally, that we failed to find a significant grandfather effect in our monogamous society in which we restricted our data to those men who married only once in their lifetimes (and hence could only gain fitness by grandfathering after the menopause of their wife) strongly suggests that the evolution of prolonged life in men cannot be explained by the selective benefits of grandfathering.

My thought, as I expressed in the comments of that post, was that average fitness is not particularly meaningful, since a relatively small number of males at the top of the social pyramid probably had a disproportionate evolutionary impact - what really counts is the grandfather effect among them. I can easily imagine a scenario where grandfathers decrease fertility of ordinary families (another mouth to feed...), but increase it among the rich. The long-term fitness impact of grandfathers could well be positive, even though the average impact is negative, since the rich have the biggest long-term evolutionary impact.

I can tell this same story on the gene level. Imagine a population which is 99% "aabb" and 1% "aabB", each of which have equal fitness. Now, imagine that there's a mutation "A" that reduces fitness by 10% in "bb" individuals, but raises fitness by 10% in "bB" individuals. Let's say by chance we get a "aAbB" individual before the "A" allele dies out. That "aAbB" individual will have the same fitness as a normal "aabb" individual, since its offspring will be 25% "aabb" (average fitness), 25% "aAbb" (10% lowered fitness), 25% "aAbB" (10% higher fitness), and 25% "aabB" (average fitness). Nevertheless, over time, the "A" allele will increase and eventually fix (together with the "B" allele). (Those of you who want to quibble about the percentages can adjust them accordingly.)

Now that seems like an interesting result to me! We talk a lot about average fitness here, but if I am not mistaken, average fitness can tell a story that's very different from what's really going on. Increasing the fitness of winners seems to count a lot more than decreasing the fitness of losers - and in evolution it's the winner's story that will eventually be told.

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