The coalescent in an island model of population subdivision with variationamong demes

A simple genealogical structure is found for a general finite island model of population subdivision. The model allows for variation in the sizes of d emes, in contributions to the migrant pool, and in the fraction of each dem e that is replaced by migrants every generation. The ancestry of a sample o f non-recombining DNA sequences has a simple structure when the sample size is much smaller than the total number of demes in the population. This all ows an expression for the probability distribution of the number of segrega ting sites in the sample to be derived under the infinite-sites mutation mo del. It also yields easily computed estimators of the migration parameter f or each deme in a multi-deme sample. The genealogical process is such that the lineages ancestral to the sample tend to accumulate in demes with low m igration rates and/or which contribute disproportionately to the migrant po ol. In addition, common ancestor or coalescent events tend to occur in deme s of small size. This provides a framework for understanding the determinan ts of the effective size of the population, and leads to an expression for the probability that the root of a genealogy occurs in a particular geograp hic region, or among a particular set of demes. (C) 2001 Academic Press.