March 5  Alan Fix, UC Riverside Anthropology
Evolution of Altruism in Kin-Structured and Random Subdivided Populations
A population structure favorable to the evolution of an altruistic trait is studied by Monte Carlo simulation.  The model is based on as small-scale nonindustrial human society but seems generalizable to other highly social mammals.  Three hierarchical levels are recognized: 1) the ecologically isolated local group (hamlet) which may be composed of kin and/or unrelated individuals; 2) the deme (settlement) comprising several such groups which interbreed; and 3) the set of demes (metapopulation) among which gene flow occurs.  The first two levels of the model are based on D.S. Wilson’s structured deme concept; the third allows for gene flow among demes in the metapopulation and for the structured diffusion of alleles across a wider area than might be included within the scope of a single deme.
The simulation models genetic drift by a process of hamlet formation which may be random, or variously kin-structured.  Hamlets may then become extinct based on a probability function of their gene frequencies.  Individual selection within settlements is modeled deterministically, and gene flow among settlements is modeled as two-dimensional stepping-stone migration of random or kin-structured groups.
Results of the simulations show that, with realistic values for group sizes, moderate extinction rate, and high rates of migration (m > 27%), disadvantageous alleles (s = 10% and 25%) may increase markedly due to differential hamlet extinction over the course of 50 generations.  The greater the degree of kin-structuring of founder groups, the higher the variance among hamlets and the faster the rate of increase of the allele for altruism.  Nonetheless, even in some randomly founded groups, a clear increase in the altruism gene frequency occurred.  It is also notable that kin-structured group selection by hamlet extinction may be effective when the initial frequency of altruism genes is very low (average of one per deme) and among a relatively small number of demes (25).  Thus the process of group extinction in a hierarchically structured population allows rapid increase of an allele for altruism under plausible demographic conditions.
For the full article, see Evolution 1985 39:928-39.