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Health

Sociogenetics, or levels of selection?

Gene ExpressionBy Razib KhanJune 18, 2007 8:58 PM

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Nature Reviews Genetics has a short article titled Sociogenetics: Cheating gets you nowhere (free registration). I was intrigued by the title, "Sociogenetics." The Chomskysists always talk about how infinitely flexible language is, and in science the proliferation of fields like "biophysical chemistry" are examples of that range. But a close reading this article shows that "sociogenetics" is just a new sticker on an old field within genetics & evolution:

When food is scarce, solitary D. discoideum cells aggregate into a fruiting body that distributes spores. However, only 75% of the aggregating cells become spores -- the other 25% form the body's stalk and altruistically die. Mutants that are less likely to contribute to the stalk are at a selective advantage, and one such mutant is fbxA.

The rest of the piece covers the "altruism problem" which W.D. Hamilton and later thinkers have been tackling for the better part of 2 generations. In short, the selfish mutant is more fit within the demes, the demes here being fruiting bodies. But, the demes with a critical mass of cheaters are less fit on average than demes without cheaters. The point when cheaters cross the 1/4 threshold in proportion within the population of cells is when these groups begin to exhibit marked overall reduced fitness. Now, we do know that D. discoideum generate fruiting bodies, so the question is how the cheaters are kept in check.

The answer is Hamiltonian kin selection

, the levels of relatedness between cells within a fruiting body seem to be about 0.86. For comparison the level of relatedness between fulls siblings or between parent and child are 0.5. Cheaters which emerge in these populations which are verging on clones are simply cheating themselves, as the cell populations are near clonal. Non-cheaters who replicate their genes aid themselves at the expense of the fruiting bodies where cheaters are common (and organismal analogy is apt here). The article concludes:

It remains unclear why relatedness is so high in D. discoideum populations, higher even than in eusocial insects, but it does explain how cooperative behaviour can be maintained. This has been difficult to show in other multicellular species, for which relatedness is easily measured but in which no cheater mutations are available for study.

Eusocial insects don't seem as closely related within colony as W.D. Hamilton assumed when he originally introduced them as biological illustrations of his concept of inclusive fitness. This is why E.O. Wilson is pushing for a resurrection of group selective ideas. Human groups beyond the stage of the hunter-gatherer also tend to have low levels of relatedness. For D. discoideum kin selective dynamics are manifestly the answer. For the broad expanse of multicellular life we'll probably need to branch out a bit.

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