Academic journal article Genetics

Evolution of Fitnesses in Structured Populations with Correlated Environments

Academic journal article Genetics

Evolution of Fitnesses in Structured Populations with Correlated Environments

Article excerpt

ABSTRACT

The outcome of selection in structured populations with spatially varying selection pressures depends on the interaction of two factors: the level of gene flow and the amount of heterogeneity among the demes. Here we investigate the effect of three different levels of spatial heterogeneity on the levels of genetic polymorphisms for different levels of gene flow, using a construction approach in which a population is constantly bombarded with new mutations. We further compare the relative importance of two kinds of balancing selection (heterozygote advantage and selection arising from spatial heterogeneity), the level of adaptation and the stability of the resulting polymorphic equilibria. The different levels of environmental heterogeneity and gene flow have a large influence on the final level of polymorphism. Both factors also influence the relative importance of the two kinds of balancing selection in the maintenance of variation. In particular, selection arising from spatial heterogeneity does not appear to be an important form of balancing selection for the most homogeneous scenario. The level of adaptation is highest for low levels of gene flow and, at those levels, remarkably similar for the different levels of spatial heterogeneity, whereas for higher levels of gene flow the level of adaptation is substantially reduced.

(ProQuest: ... denotes formulae omitted.)

THE possibility of the maintenance of genetic variation by some form of balancing selection has been a long-standing issue in theoretical population genetics (Lewontin 1974). One form of balancing selection, heterozygote advantage, can maintain genetic variation, but standard population-genetic theory shows that the conditions for stable maintenance are highly restrictive: The proportion of random fitness arrays leading to stable, fully polymorphic equilibria becomes vanishingly small for even a moderate number of alleles (Lewontin et al. 1978). Another potential solution, balancing selection arising from spatial heterogeneity, what we call here local selection, has also often been suggested as a reason for the high levels of genetic variation found in natural populations (Kassen 2002). Theoretically, by combining both forms of balancing selection, the potential for viability selection to maintain variation increases, reducing the restrictions that exist for heterozygote advantage alone (Levene 1953; Karlin 1982; Nagylaki and Lou 2006b, 2007). Nevertheless, investigation of the proportion of random fitness arrays leading to stable, fully polymorphic equilibria in a spatial context shows that the potential for both forms of balancing selection to maintain variation is still very restrictive for higher numbers of alleles (Star et al. 2007a).

While such a historic fitness-space investigation is useful for characterizing the restrictive parts of fitness space that maintain variation, by using a differentmethodological approach, it has been shown that such parts are quite easily reached by a construction approach incorporating a dynamic process of mutation and selection (Spencer and Marks 1988; Marks and Spencer 1991). Moreover, using such a construction approach in a spatial context shows that both forms of balancing selection, heterozygote advantage and local selection, emerge from such a model and the relative importance of each depends on the level of gene flow. For high levels of gene flow, heterozygote advantage is more important, whereas for lower gene-flow rates, local selection predominates in maintaining variation (Star et al. 2007b). Thus the restricted parts of fitness space that maintain variation easily evolve out of a simple evolutionary process of mutation and selection. A critical assumption of this study, however, was that the fitness values for the mutants were uncorrelated between the demes (Star et al. 2007b).

More realistically, fitness values for the same genotypes in different environments are likely to be correlated in some way, depending on the degree of heterogeneity of the environment (Kassen 2002). …

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