Maintenance of polymorphism, notably by net deleterious mutations, under the joint fluctuation of demography and selection in age-structured populations

Abstract

How non-neutral genetic variation is maintained in natural populations is a fundamental question in evolutionary biology. With environmental fluctuations, the effect of a mutant allele on fitness may also fluctuate between beneficial and deleterious relative to the wild-type allele. If this fitness fluctuation is specific to individuals of a certain stage or age in the life cycle, negative frequency-dependent selection arises, leading to high levels of polymorphism. This effect of within-population heterogeneity in fluctuating selection is an example of a diversity-promoting mechanism known as the storage effect. To obtain further insights on this effect and explore the conditions of polymorphism, models of age-structured populations with increasing complexity were investigated. How the geometric mean fitness of a rare allele is modified under the joint fluctuation of demography and selection was analysed, yielding the prediction that mutations whose average effects on fitness are deleterious can be maintained in polymorphism. It was also found that adding minimum iteroparity to the conventional model of discrete non-overlapping generations fully exerts the storage effect. These results suggest that the storage effect is expected under general within-population heterogeneity and they reinforce the conclusion that significant non-neutral genetic variation is highly plausible in natural populations under fluctuating environments.This article is part of the theme issue 'Exploring negative frequency dependent selection across levels: from genetics to ecology and back again'.