Competition for mates and the improvement of nonsexual fitness

Abstract

Competition for mates can be a major source of selection, not just on secondary sexual traits but across the genome. Mate competition strengthens selection on males via sexual selection, which typically favors healthy, vigorous individuals and, thus, all genetic variants that increase overall quality. However, recent studies suggest another major effect of mate competition that could influence genome-wide selection: Sexual harassment by males can drastically weaken selection on quality in females. Because of these conflicting effects, the net effect of mate competition is uncertain, although perhaps not entirely unpredictable. We propose that the environment in which mate competition occurs mediates the importance of sexual selection relative to sexual conflict and, hence, the net effect of mate competition on nonsexual fitness. To test this, we performed experimental evolution with 63 fruit fly populations adapting to novel larval conditions where each population was maintained with or without mate competition. In half the populations with mate competition, adults interacted in simple, high-density environments. In the remainder, adults interacted in more spatially complex environments in which male-induced harm is reduced. Populations evolving with mate competition in the complex environment adapted faster to novel larval environments than did populations evolving without mate competition or with mate competition in the simple environment. Moreover, mate competition in the complex environment caused a substantial reduction in inbreeding depression for egg-to-adult viability relative to the other two mating treatments. These results demonstrate that the mating environment has a substantial and predictable effect on nonsexual fitness through adaptation and purging.

Sexual selection and inbreeding: Two efficient ways to limit the accumulation of deleterious mutations

“…In agreement with this idea, advantages of polygamy were observed in environments reducing opportunities for male harassment (Yun et al. ). Other studies tracked the fate of a limited number of purely deleterious mutations introduced, induced or accumulated de novo in populations (Radwan ; Hollis et al.…”

Dietary stress increases the total opportunity for sexual selection and modifies selection on condition‐dependent traits

“…Considering that condition (i.e. the total resources available to fitness-related traits) is likely to be influenced by the load of deleterious mutations (Rowe & Houle 1996;Yun et al 2018), our results indirectly support the hypothesis that sexual selection can improve populations fitness under environmental stress by reducing mutation load (Cally et al 2019;Dugand et al 2019). Environmental perturbations, other than those related to food availability or diet, for example, based on temperature and climatic anomalies, may have different effects on the strength and direction of sexual selection (Berger et al 2014;De Lisle et al 2018).…”

Testing for local adaptation in adult male and female fitness among populations evolved under different mate competition regimes

“…These are the same populations in which Yun et al. () quantified egg‐to‐adult survivorship and inbreeding depression. In the current experiment, we are not examining local adaptation with respect to these different larval rearing conditions.…”