THE EFFECTIVE SIZE OF A SUBDIVIDED POPULATION

This paper derives the long-term effective size, N-e, for a general mo del of population subdivision, allowing for differential deme fitness, variable emigration and immigration rates, extinction, colonization, and correlations across generations in these processes. We show that v arious long-term measures of N-e are equivalent. The effective size of a metapopulation can be expressed in a variety of ways. At a demograp hic equilibrium, N-e can be derived from the demography by combining i nformation about the ultimate contribution of each deme to the future genetic make-up of the population and Wright's F-ST's. The effective s ize is given by N-e = 1/(1 + var (theta))[[1 - f(STi))/N(i)n], where n is the number of demes, theta(i) is the eventual contribution of indi viduals in deme i to the whole population (scaled such that Sigma(i) t heta(i) = n), and [ ] denotes an average weighted by theta(i)(2). This formula is applied to a catastrophic extinction model (where sites ar e either empty or at carrying capacity) and to a metapopulation model with explicit dynamics, where extinction is caused by demographic stoc hasticity and by chaos. Contrary to the expectation from the standard island model, the usual effect of population subdivision is to decreas e the effective size relative to a panmictic population living on the same resource.