The genetic consequences of primate social organization: a review of macaques, baboons and vervet monkeys.

Primates, as long-lived, iteroparous, socially complex mammals, offer the opportunity to assess the effects of behavior and demography on genetic structure. Because it is difficult to obtain tissue samples from wild primate populations, research in this area has largely been confined to terrestrial and semi-terrestrial old world monkeys (e.g., rhesus and Japanese macaques, vervets and several subspecies of baboons). However, these species display a multi-male, multi-female social structure commonly found in many other primate and non-primate mammals. Electrophoretic analyses of blood proteins from individually recognized and/or marked wild Himalayan rhesus monkeys, themselves the subject of long-term behavioral and demographic research, have begun to reveal the genetic consequences of such phenomena as social group fission, male-limited dispersion, non-consanguineous mating patterns, and agonistically defined male dominance. Specifically, rhesus social groups, consisting primarily of clusters of maternal relatives, appear to be non-random samples of a population's genotypes and genes. The genetic effects of social group fission are highly dependent on each group's size, demographic structure, and average degree of relatedness. In all cases fission contributes to the degree of intergroup genetic differentiation. Male-limited dispersion appears both to retard genetic differentiation between social groups and to lead to mating patterns that result in an avoidance of consanguinity. Groups, therefore, appear to be genetically outbred. Comparing these results with studies of other free-ranging or wild cercopithecines allows several generalizations: (a) genetic variation seems to be evenly distributed throughout each local population of multi-male social groups; (b) social groups, however, because they contain clusters of relatives, are distinctive in their specific frequencies of genes; (c) the degree of genetic differentiation between a population's social groups, because of the effects of social group fission and non-deterministic forms of male dispersal, is somewhat greater than expected on the basis of migration rates alone; and (d) the asymmetrical pattern of dispersion with respect to sex effectively precludes inbreeding in any one social group or the population as a whole. These observations have important implications for understanding the unusually rapid rates of evolution among the primates.