The effect of scale dependent processes on kin selection: mating and density regulation.

Kin selection models describe fitness determining interactions that occur within small clusters of individuals often referred to as trait groups (Wilson, 1975). Because each individual influences not only its own fitness, but also the fitness of other members of the trait group, kin selection has both inter- and intra-group components (Hamilton, 1975; Wade, 1980). The importance of intergroup selection depends on the magnitude of inter-group differences in fertility and on the amount of genetic variation among trait groups. Migration and density regulation are likely to be important determinants of productivity and variation. Existing models integrate density regulation and migration structure into kin selection theory by applying carrying capacities or migration rates to trait groups. This requires that dispersal and density regulation both operate at the spatial scale of trait groups and excludes many cases of biological interest. I present a model that allows interaction, density regulation, and dispersion to operate over distinct spatial scales. The results of existing models can be retrieved as special cases of the general framework developed here. This model indicates that the appropriate scale for studying kin selection is determined by the spatial area over which the population is regulated. In effect, only a fraction of the total inter-trait group variance is available to intergroup selection. Processes that generate genetic variation over spatial scales larger than the "regulation scale" cannot aid the evolution of altruism. The analysis of a specific mating/migration model indicates how the various components of variation are determined by biological parameters such as migration rate. These results have important implications for the study of relatedness in natural populations.