Effects of metapopulation processes on measures of genetic diversity

Abstract

Many species persist as a metapopulation under a balance between the local extinction of subpopulations or demes and their recolonization through dispersal 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 metapopulation 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 in 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 effects of population turnover focused on population differentiation, such as measured by F(ST). A key advantage of F(ST) over absolute measures of diversity 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) will usually equilibrate more quickly following perturbations than will absolute levels of diversity. However, because F(ST) is a ratio of between-population differentiation to total diversity, the genetic effects of metapopulation processes may be difficult to interpret in terms of F(ST) on its own, so that the analysis of absolute measures of diversity in addition is likely to be informative. While population turnover may either increase or decrease F(ST), depending on the mode of colonization, recurrent extinction and recolonization is expected always to reduce levels of both within-population and species-wide diversity (piS and piT, respectively). One corollary of this is that piS cannot be used as an unbiased estimate of the scaled mutation rate, theta, as it can, with some assumptions about the migration process, in species whose demes do not fluctuate in size. The reduction of piT in response to population turnover reflects shortened mean coalescent times, although the distribution of coalescence times under extinction colonization equilibrium is not yet known. Finally, we review current understanding of the effect of metapopulation dynamics on the effective population size.

Limiting inbreeding in disjunct and isolated populations of a woody shrub

“…The mating system is considered to be a key factor influencing the genetic diversity of plant populations (Pannell and Charlesworth 2000; Charlesworth 2006; Duminil et al. 2009).…”

Does genetic variation and gene flow vary with rarity in obligate seeding Persoonia species (Proteaceae)?

“…The predicted relationship between genetic variation and drift assumes that the system is at equilibrium (Wright 1969). In reality while the general distribution of these species may have been relatively constant for thousands of years, metapopulation processes may reduce genetic variation (Pannell and Charlesworth 2000) in these fire prone landscapes through continual colonization and extinction events in these obligate-seeding Persoonia species. Whatever the mechanism, the question of how this variation relates to population persistence still remains.…”

Genetic analysis of metapopulation processes in Silene latifolia

“…In a metapopulation in which the extinction-recolonization rate exceeds the migration rate, a lower effective population size is expected, and we should see reduced diversity both within demes and in the species as a whole (PANNELL and CHARLESWORTH 2000b;WHITLOCK and BARTON 1997). These predictions are nicely illustrated by comparisons of the genetic structure for maternally versus bi-parentally inherited genes: the former may retain the signature of colonisation if seed dispersal is limited, while genetic structure for the latter is eroded by pollen dispersal.…”

“…Even the simplest models that assume demes of similar size and extinction probability include several parameters that all have an important effect on the effective size and genetic diversity, including the migration and extinction rates, the number of demes and their sizes, the number of founding individuals, and the extent to which they come from the same source deme or a mix of different demes PANNELL and CHARLESWORTH 2000a;WADE and MCCAULEY 1988;. Because different processes can affect particular summary statistics in similar ways, our challenge is to devise sampling and analysis that allow us to distinguish them, e.g., to move beyond the use of single statistics that only summarise part of the pattern.…”

“…1 has led to a number of useful inequalities that are useful for predicting the effect of particular metapopulation processes, such as colonization, in increasing or decreasing total population differentiation . Each parameter has been shown to have a substantive effect on the genetic diversity distributed within and among demes, as well as on the potential outcome of evolution PANNELL and CHARLESWORTH 2000).…”