Frequency-Dependent Selection in Light of Phage Exposure

Abstract

A special form of natural selection involves the evolutionary fitness of different alleles or genotypes varying with their frequencies within populations. Thus, fitness may be higher when an allele is rarer or instead fitness may be lower when an allele rarer. The resulting frequency-dependent selection can be stabilizing versus disruptive, respectively, in either case of polymorphisms: Either rarer alleles are protected by natural selection from being driven to extinction or instead rarer alleles are driven to extinction by natural selection (not just genetic drift) particularly as they become rarer. With frequency-dependent selection as driven by phages, I suggest that it often is not so much the frequency of bacterial alleles or genotypes that are most relevant but instead actual densities, that is, absolute bacterial prevalence rather than relative bacterial allele or genotype prevalence. Thus, either high concentrations of one genotype drives the extinction of a rarer genotype (disruptive) or instead a genotype by existing at a low concentration is protected from extinction (stabilizing). In this chapter, I also consider how thinking about these sorts of things evolutionarily versus ecologically can differ. I then describe what can be a purely ecological process that is similar to stabilizing frequency-dependent selection, i.e., killing the winner, and also how stabilizing frequency-dependent selection could result in Muller’s ratchet.