The Genetic Demography of the Gainj of Papua New Guinea. 2. Determinants of Effective Population Size

The Hill-Emigh-Pollak model of effective population size for dioecious species with overlapping generations is subjected to a sensitivity analysis designed to measure the relative importance of the model's parameters. According to this analysis, effective size appears to be most sensitive to changes in the parameters related to reproductive heterogeneity (the variances and covariances in the net lifetime production of male and female offspring) and less sensitive to changes in parameters related to demographic recruitment (the mean generation length and the number of males and females reaching reproductive age each year). The recruitment parameters, however, are more likely to vary among populations and, on balance, are probably more important as causes of interpopulation variation in effective size. Since the model's covariance terms are new to population genetics, analytical expressions are derived for them under the Poisson and negative binomial birth processes. The model is applied to demographic data on the Gainj, a tribal population of highland New Guinea. The reproductive output of Gainj females follows a Poisson distribution, whereas that of males follows a negative binomial distribution with a variance three times that of females. This increased reproductive heterogeneity in males is attributable to polygyny. Demographic recruitment is slow, largely because of slow sexual maturation and late marriage. In consequence, the mean generation time is long and the number of individuals initiating reproduction each year is small. The net effect of these factors is to reduce the effective size of the Gainj population to about one-half of the total population size.