Human population expansion and microsatellite variation

Polymorphisms at di-, tri-, and tetranucleotide microsatellite loci have be en analyzed in 14 worldwide populations. A statistical index of population expansion, denoted S-k, is introduced to detect historical changes in popul ation size using the variation at the microsatellites. The index takes the value 0 at equilibrium with constant population size and is positive or neg ative according to whether the population is expanding or contracting, resp ectively. The use of Sn requires estimation of properties of the mutation d istribution for which we use both family data of Dib ct al. for dinucleotid e loci and our population data on tri- and tetranuclcotide loci. Statistica l estimates of the expansion index, as well as their confidence intervals f rom bootstrap resampling, are provided. In addition, a dynamical analysis o f S-k is presented under various assumptions on population growth or declin e. The studied populations are classified as having high, intermediate, or low values of S-k and genetic variation, and we use these to interpret the data in terms of possible population dynamics. Observed values of S-k for s amples of di-, tri-: and tetranucleotide data are compatible with populatio n expansion earlier than 60,000 years ago in Africa, Asia, and Europe if th e initial population size before the expansion was on the order of 500. Lar ger initial population sizes force the lower bound for the time since expan sion to be much earlier. We find it unlikely that bottlenecks occurred in C entral African, East Asian, or European populations, and the estimated expa nsion times are rather similar for all of these populations. This analysis presented here suggests that modern human populations departed from Africa long before they began to expand ill size. Subsequently, the major groups ( the African, East Asian, and European groups) started to grow at approximat ely same time. Populations of South America and Oceania show almost no grow th. The Mbuti population from Zaire appears to have experienced a bottlenec k during its expansion.