Weakly deleterious mutations and low rates of recombination limit the impact of natural selection on bacterial genomes

Morgan N. Price and Adam P. Arkin, Lawrence Berkeley National Lab


Free-living bacteria are usually thought to have high effective population sizes, so that tiny selective differences can drive their evolution. However, because recombination is infrequent, "background selection" against slightly-deleterious alleles should reduce the effective population size (Ne) by orders of magnitude. For example, for a well-mixed population with 1012 individuals and a typical level of homologous recombination (r/m=3, or nucleotides change due to recombination at three times the mutation rate), we predict that Ne < 107. An argument for high Ne in bacteria has been the high genetic diversity within many bacterial "species," but this diversity may be due to population structure: diversity across subpopulations can be far higher than diversity within a subpopulation, which makes it difficult to estimate Ne correctly. Given an estimate of Ne, standard population genetics models imply that selection should be sufficient to drive evolution if Ne * s > 1, where s is the selection coefficient. We show that this remains approximately correct if background selection is occuring or when population structure is present. Overall, we predict that even for free-living bacteria with enormous populations, natural selection is only a significant force if s is above 10-7 or so.


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Contact: Morgan Price