THE POPULATION GENETICS OF SOCIAL INTERACTIONS

Abstract

The concept of inclusive fitness plays a key role in much of sociobiology. Yet most theoretical studies concerning the evolution of social behavior circumvent inclusive fitness by mobilizing the concept of frequency dependent individual fitness. Given certain assumptions, it is shown that models based on these two different concepts are dynamically equivalent. The models do differ, however, in bookkeeping methods which are advantageous under different circumstances. A knowledge of these circumstances should prove of value to students of social behavior. It is then shown that evolution acts according to an adaptive landscape based on Hamilton's inclusive fitness in the absence of strong selection and inbreeding. This yields an inclusive fitness analogue to much of traditional population genetics. For example, heterozygote superiority in inclusive fitness yields stable polymorphisms, while intermediate dominance results in fixation of one of the alleles. When individuals do not affect one another's fitnesses, the inclusive fitness topography collapses to one based on individual fitness. A general rule for the evolution of social behavior under intermediate dominance is shown to yield Hamilton's Rule as a special case. Next, a general model for examining the evolution of social behavior is developed which, unlike inclusive fitness models, does not require that benefits received be linear functions of the number of social donors encountered. The subsocial route for the evolution of eusociality in haplodiploid organisms is then examined within the context of this model. Nonlinearities render conditions for frequency independent fixation or loss of sister-helping alleles more stringent than expected from models based on the assumption of linear benefits. In particular, both stable polymorphisms and frequency dependent selective thresholds for sister-helping behavior may commonly obtain.