Effects of metapopulation processes on measures of genetic diversity

Many species persist as a metapopulation under a balance between the local extinction of subpopulations or demes and their recolonization through disp ersal from occupied patches. Here we review the growing body of literature dealing with the genetic consequences of such population turnover. We focus our attention principally on theoretical studies of a classical metapopula tion with a 'finite-island' model of population structure, rather than on ' continent-island' models or 'source-sink' models. In particular, we concern ourselves with the subset of geographically subdivided population models i n which it is assumed that all demes are liable to extinction from time to time and that all demes receive immigrants. Early studies of the genetic ef fects of population turnover focused on population differentiation, such as measured by F-ST A key advantage of F-ST over absolute measures of diversi ty is its relative independence of the mutation process, so that different genes in the same species may be compared. Another advantage is that F-ST W ill usually equilibrate more quickly following perturbations than will abso lute levels of diversity. However, because F-ST is a ratio of between-popul ation differentiation to total diversity, the genetic effects of metapopula tion processes may be difficult to interpret in terms of F-ST-on its own, s o that the analysis of absolute measures of diversity in addition is likely to be informative. While population turnover may either increase or decrea se F-ST, depending on the mode of colonization, recurrent extinction and re colonization is expected always to reduce levels of both within-population and species-wide diversity (pi (S) and pi (T), respectively). One corollary of this is that pi (S) cannot be used as an unbiased estimate of the scale d mutation rate, theta, as it can, with some assumptions about the migratio n process, in species whose demes do not fluctuate in size. The reduction o f pi (T) in response to population turnover reflects shortened mean coalesc ent times, although the distribution of coalescence times under extinction- colonization equilibrium is not yet known. Finally we review current unders tanding of the effect of metapopulation dynamics on the effective populatio n size.