The effective size of a hierarchically structured population

A knowledge of the effective size of a population (N-e) is important in und erstanding its current and future evolutionary potential. Unfortunately, th e effective size of a hierarchically structured population is not, in gener al, equal to the sum of its parts. In particular, the inbreeding structure has a major influence on N-e. Here I link N-e to Wright's hierarchical meas ures of inbreeding, F-IS and F-ST, for an island-structured population (or metapopulation) of size NT The influence of Fs, depends strongly on the deg ree to which island productivity is regulated. In the absence of local regu lation (the interdemic model), interdemic genetic drift reduces N-e. When s uch drift is combined with local inbreeding under otherwise ideal condition s, the effects of F-IS and F-ST are identical: increasing inbreeding either within or between islands reduces N-e, with N-e = N-T/[(1 + F-IS)(1 + F-ST ) - 2F(IS)F(ST)]. However, if islands are all equally productive because of local density regulation (the traditional island model), then N-e = N-T/[( 1 + F-IS)(1 - F-ST)] and the effect of F-ST is reversed. Under the interdem ic model, random variation in the habitat quality land hence productivity) of islands act to markedly decrease N-e. This variation has no effect under the island model because, by definition, all islands are equally productiv e. Even when no permanent island structure exists, spatial differences in h abitat quality can significantly increase the overall variance in reproduct ive success of both males and females and hence lower N-e. Each of these ba sic results holds when other nonideal factors are added to the model. These factors, deviations from a 1:1 sex ratio, greater than Poisson variance in female reproductive success, and variation in male mating success due to p olygynous mating systems, all act to lower N-e. The effects of male and fem ale variance on N-e have important differences because only females affect island productivity. Finally, it is noted that to use these relationships, F-IS and F-ST must be estimated according to Wright's definition land corre cted to have a zero expectation under the null model). A commonly used part itioning (theta, theta(g)) can be biased if either island size or the numbe r of islands is small.