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September 28, 2004

Heterosis revised

I'm about 1/3 of the way through Narrow Roads of Gene Land and I must report that I misled (inadvertently) when I suggested that suggested that William Hamilton believed that interpopulation marriage would perpetuate heterozygote advantage immunologically, something I've discussed before. Well, from what I gather, Hamilton didn't buy heterozygote advantage. Partially this is because it is rather rare to detect it empirically (this I knew) in relation to how much polymorphism exists within populations (allelic diversity at a locus), and, because he worries that a overdominant line could mutate toward asexuality and so make sexual individuals less fit over the long run.

So what did Hamilton believe? Well, I'm only 1/3 through, but it seems he thought that variance in fitness as a function of time of any given homozygote genotype (this is a model that restricts the number of locii for simplicity's sake) resulted in prevention of fixation (that is, one type becoming universal). Where do the heterozygotes come in? Well, in a random mating population you will have the production of heterozygote individuals when two differing homozygotes mate. In short, where before I was asserting that homozygotes are byproducts of the maximization of heterozygosity in a random mating population, Hamilton seems to be suggesting that heterozygotes are transitionary forms that are byproducts of fact that the fitness of various homozygote types oscillates as a function of time within a population (they are repositories of genetic diversity that sexual reproduction utilizes to stay ahead of parasitic infection).

I think this can be related to my post on the personality types of "Great Men." I noted that the Mongols tended to decimate local elites, so that the nobility of the Tatar people was gutted and its male lineages exterminated, but they often treated the common people with magnanimity. Where 10 years previous a humble smith or other artisan would be part of the servile class, and perhaps have few children if any in comparison to the Tatar nobility, with the coming of the Mongols their fitness in comparison with the Tatar leadership would have increased greatly. I also recall reading once that the Spartans often killed Helot males who seemed to display evidence of future leadership capacities. The implication is that the fitness of dominant personalities is a function of time, the perpetuation of less dominant lineages might be the result of the constant churn and elite turnover which this personality type can persevere through. In a period of peace the elite of a given population reproduces as a higher rate, but during times of chaos and war it might be better to keep a low profile and be beneath notice.

Addendum: The variation in fitness of homozygotes as a function of time might explain why in many species there is so little evidence of a reproductive barrier between variant forms: limiting the range of one's potentional mates is diminishing the recombinant power of sex. Imagine a "ideal" form A, with a variance within the population of the capacity to be fecund witn non-A forms. If at some point in the future A's fitness drops in relation to non-A forms, A individuals who became reproductively isolated would be at a sharp disadvantage.

Posted by razib at 02:59 PM