Determinants of effective population size for loci with different modes of inheritance

Here we report an assessment of the determinants of effective population size (N_e) in species with overlapping generations. Specifically, we used a stochastic demographic model to investigate the influence of different life-history variables on N_eI N (where N = population census number) and the influence of sex differences in life-history variables on N_e for loci with different modes of inheritance. We applied an individual-based modeling approach to two datasets: one from a natural population of savannah baboons (Papio cynocephalus) in the Amboseli basin of southern Kenya and one from a human tribal population (the Gainj of Papua New Guinea). Simulation-based estimates of N_eIN averaged 0.329 for the Amboseli baboon population (SD = 0.116, 95% Cl = 0.172 - 0.537) and 0.786 for the Gainj (SD = 0.184, 95% Cl = 0.498 - 1.115). Although variance in male fitness had a substantial impact on N_eIN in each of the two primate populations, ratios of N_e values for autosomal and sex-linked loci exhibited no significant departures from Poisson-expected values. In each case, similarities in sex-specific N_e values were attributable to the unexpectedly high variance in female fitness. Variance in male fitness resulted primarily from age-dependent variance in reproductive success, whereas variance in female fitness resulted primarily from stochastic variance in survival during the reproductive phase.