MICROSATELLITE DATA SUPPORT AN EARLY POPULATION EXPANSION IN AFRICA

We have developed a method for the analysis of microsatellite data tha t is useful in the elucidation of the demographic history of populatio ns. This method, the P-K distribution method of pairwise comparisons, is analogous to the mismatch distribution of sequence comparisons deve loped for the analysis of mitochondrial sequence data by Rodgers and H arpending and is defined as the distribution of the number of repeat u nit differences between alleles when each allele in a sample is compar ed with every other allele in the sample. Using computer simulations o f microsatellite loci, we show that the shape of the distribution of P -K changes in a distinctive manner as a function either of time since population expansion or effective population size. Increases in both o f these affect the P-K distribution in a similar fashion leading to a change from a steep distribution with a P-0 peak to one with a nonzero peak. Analysis of three data sets from surveys of microsatellite loci in ethnographically defined populations reveals that most (9/12) of t he African populations analyzed, but none of the 30 non-African popula tions showed P-K distributions with nonzero peaks. These P-K distribut ions indicate either an earlier expansion or a larger effective popula tion size for African populations. This observation is consistent with the hypothesized African origin of modern human.