Phylogeographic variability in traditional societies

Our perception of the amount, and distribution, of human genetic diversity is becoming radically altered by the introduction of sophisticated molecular techniques into the field of evolutionary biology. Along with the exponential increase in the number of informative DNA markers, has come an increased precision in estimating the evolutionary relationships between populations. Evaluating DNA variability in terms of the phylogenetic analysis of sequence variability at the population level has been especially informative in illuminating the long-term ancestry of our species. An extension of this strategy, phylogeography, aims to evaluate the evolutionary ancestry of specific genomic regions in terms of the geographic distribution of phylogenetic lineages within and among populations. We have started to apply this approach by studying the distribution of mitochondrial DNA sequence diversity within and among a variety of Amerindian tribes. These data provide an illuminating contrast to regional data on sequence variability, especially when analysed within the theoretical framework of the coalescent. To interpret these results, we have analysed a simple model in which the rate of coalescence between subpopulations varies with respect to the rate of coalescence of individual ancestry. The model indicates that extensively isolated subpopulations will have distinct distributions of ancestry, and hence of sequence variability, compared to subpopulations which arise due to a series of rapid fissioning events. Subpopulations within tribes, such as bands, appear to correspond to the latter category, whereas tribal populations appear intermediate between the two extremes.