THE NUMBER OF SEGREGATING SITES IN EXPANDING HUMAN-POPULATIONS, WITH IMPLICATIONS FOR ESTIMATES OF DEMOGRAPHIC PARAMETERS

The frequency distribution of pairwise differences between sequences o f mtDNA has recently been used to estimate the size of human populatio ns before and after a hypothetical episode of rapid population growth and the time at which the population grew. To test the internal consis tency of this method, we used three different sets of human mtDNA data and the corresponding demographic parameters estimated from the distr ibution of pairwise differences to determine by simulation the expecte d number of segregating sites, S, and its empirical distribution. The results indicate that the observed values of S are significantly lower than expected in two of three cases under the assumption of the infin ite-sites model. Further simulations in which mutations were allowed t o occur more than once at the same site and in which there was variati on in mutation rate among sites show that the expected number of segre gating sites can be much lower than under the infinite-sites assumptio n. Nevertheless, the observed value of S is still. significantly diffe rent from the value expected under the expansion hypothesis in two of three cases.