Genetic structure of a population with social structure and migration

Long-tailed macaques (Macaca fascicularis) live in social groups consisting of resident adult females and their offspring, and immigrant males. Subadult males leave their birth group, and might establish themselves as reproducing males in another group. Females do not leave their birth group. Such a social pattern might have consequences for the genetic differentiation between groups and the genetic relationships within groups. In a field study of long-tailed macaques (Macaca fascicularis) in Ketambe, Sumatra, Indonesia, blood samples were taken from individuals in seven adjacent social groups. Electrophoretic analysis showed 17 blood proteins and enzymes to be polymorphic, allowing the computation of heterozygosities and of the F-statistics. Of the F-statistics, F(IS) indicates the deviation from Hardy-Weinberg equilibrium averaged over local populations, FST indicates the differentiation in allele frequency between local populations, and F(IT) indicates the deviation from Hardy-Weinberg equilibrium over the total population. In a computer simulation of the population of long-tailed macaques using many loci with many neutral alleles, F(IS) and FST values proved to be characteristic for a certain demography and life history of the population, and proved not to depend upon the number of alleles or level of heterozygosity. FST values found in the simulation were compatible to those found in the field; in the simulation, values for F(IS) and F(IT) were consistently negative. The explanation for the negative F(IS) appears to be that genetic drift causes differentiation in allele frequencies between groups, and that due to this differentiation, allele frequencies differ between resident females and immigrant males, leading to offspring with an excess of heterozygotes (negative F(IS)) relative to the expectation based upon the overall allele frequency. The excess of heterozygotes might imply that slightly deleterious alleles are protected from selection. A population with a social structure and differential migration of the sexes is liable to accumulate deleterious recessives and, as a consequence, to be very sensitive to inbreeding on disruption of the social structure, as for instance in zoos.