Tuesday, March 20, 2007

Group selection, the parameters   posted by Razib @ 3/20/2007 10:10:00 AM

I wanted to a wait a bit before I posted this, but the essential part of Elisabeth Lloyd's essay Units and Levels of Selection is this:
It is widely held, for instance, that the conditions under which group selection can effect evolutionary change are quite stringent and rare. Typically, group selection is seen to require small group size, low migration rate, and extinction of entire demes.[3] Some modelers, however, disagree that these stringent conditions are necessary. Matesi and Jayakar, for example, show that in the evolution of altruism by group selection, very small groups may not be necessary (1976, p. 384; contra Maynard Smith 1964). Wade and McCauley also argue that small effective deme size is not a necessary prerequisite to the operation of group selection (1980, p. 811). Similarly, Boorman shows that strong extinction pressure on demes is not necessary (1978, p. 1909). And finally, Uyenoyama develops a group selection model that violates all three of the "necessary" condition usually cited (1979).

In short, if there is too much migration between demes (breeding groups) then selection upon group heritable traits can not occur. Between group variance is diminished too quickly. The emphasis on the extinction of demes is due to the fact that that this makes the selective punch strong (i.e., the selection coefficient upon the group level trait). As for group size, David S. Wilson often uses an "organismic" analogy. Increased size means increased complexity, so a model which already suffers from a lack of parsimony benefits from simplicity. A large group is presumably less cohesive, and at a certain point it is pointless to speak of a "group" in any real sense (e.g., the "deme" of North Americans?). The main issue I have with group selection is that between group movement seems to be a major parameter in human societies, "exterminating" another group often involves killing them men and taking their women. In a marginal society this may not be viable as "big men" might not be able to support extra wifes, but with societies with some stratification and excess surplus, this is the norm (more realistically, surplus allocated in an equal fashion). In the comments below Jason points to this paper:
My empirical estimates show that genetic differences between early human groups are likely to have been great enough so that lethal intergroup competition could account for the evolution of altruism. Crucial to this process were distinctive human practices such as sharing food beyond the immediate family, monogamy, and other forms of reproductive leveling.

Monogamy generally implies some sort of resource constraint. Though I think that pre-Neolithic group selection is more plausible than post-Neolithic group selection (see the parameters above!), the work looks at modern hunter-gatherers as his models. A primary objection to this back extrapolation is that modern groups, like the Bushmen, have been marginalized and driven to the least desirable locales by agricultural groups. They might not be good representatives of pre-modern hunter-gatherer groups, some of whom might have been rather complex. I will agree though that the non-group selective explanations for "food sharing" have struck me as unsatisfactory (e.g., enhancement of reputation).

Anyway, here is a verbal description how a group selective mechanism could. Let's assume a trivial amount of migration/mutation, so demes have a mix of strategies in various proportions. Take two demes, Mostly Cooperator (MC) and Mostly Defector (MD). Cooperators are selfness, and so they increase their group fitness. Defectors don't increase group fitness. In the MC average fitness is higher than in the MD group because the former has more group contributors. On the other hand, within MC and MD the defectors are both fitter than the cooperators, they're free riders. Over time, MC would marginalize MD, but, MC itself would be effected by a turn over from cooperation to defection since the latter strategy is fitter within group. How can cooperation persist? A solution would be for the initial MD to go extinction, and MC to them redistribution itself into smaller groups. At this point, these smaller groups would exhibit imbalances in the number of cooperators, and so the process would recapitulate itself.

Addendum: From this explanation, and an internalization of the parameters above, I think you can intuite while cultural group selection is more plausible than biological group selection to most workers.

Labels: ,