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.