The genetic consequences of social group fission in a wild population of rhesus monkeys (Macaca mulatta)

Summary

Micro-and macroevolutionary effects of social group fission will be enhanced if genetic differentiation between fission products is greater than expected by randomly dividing a parent group. There is evidence that division along lines of maternal relatedness produces such an enhanced effect in the provisioned colony of rhesus monkeys on Cayo Santiago Island. In contrast, the genetic analysis presented here of group fission in a wild population of rheusus monkeys in the Himalyan foothills of northern Pakistan shows no ‘matrilineal effect’. There is a greater than 70% chance of obtaining the observed differences between fission products by random fissioning alone (Fig. 1).

The differing consequences of fission between these two populations are most likely the results of differences in their demographic structure and patterns of paternity. Under conditions of rapid population growth, diffuse paternity and clear genetic differences between matrilines, the division of social groups along lines of maternal relatedness should have the greatest genetic effects. When populations are growing slowly and groups are composed of many small matrilines or when restricted paternity prevails the genetic consequences of matrilineal fission should be no different from those resulting from random group division.

Fission with and without a matrilineal effect probably occurs at different points in the evolutionary history of a primate population. In either case the fission process usually accelerates subpopulational differentiation beyond the rate expected by drift alone and may yet prove most important in understanding the genetic structure of many mammalian populations.