Abstract
Sex ratios can differ from an expected equal proportion of males and females, carrying substantial implications for our understanding of how mating systems evolve. Typically, macro-evolutionary studies have been conducted without assessing how deviations from an equal sex ratio could be explained by sex-biased mortality or dispersal. Our understanding of sex ratio evolution independent of these confounds, in addition to any putative links between skewed sex ratios and other factors (e.g. life history), therefore remains largely unexplored. Here, we conducted an exploratory study investigating differences in sex ratios across closely related species while controlling for extrinsic mortality. We also tested two factors, non-overlapping/overlapping generations and the social environment, which have both been hypothesised to affect sex ratios. Specifically, we raised 15 species of killifish, which have either overlapping or discrete generations, under both solitary and social treatments. We found substantial divergences in sex ratios across closely related species, which exhibited both male and female biases. In conjunction with a low phylogenetic signal, our results suggest that sex ratios can evolve rapidly in this group. However, we found no evidence that overlapping generations or the social environment affected sex biases, suggesting that other factors drive the rapid evolution of sex ratios in killifishes.
Highlights
Classic evolutionary theory predicts an equal sex ratio, with negative frequency dependent selection acting on the excess production of any one sex (Düsing 1884; Fisher 1930)
Sex ratios differed across species, with the percentage of males varying between 23% in Fundulopanchax cinnamomeus to 76% in the closely related F. scheeli
Our main model showed no significant effects of life history or social treatment on sex ratio, either in the model output, or when testing absolute sex-bias with a folded normal distribution (Table 1; Fig. 2)
Summary
Classic evolutionary theory predicts an equal sex ratio, with negative frequency dependent selection acting on the excess production of any one sex (Düsing 1884; Fisher 1930). Any variation in sex ratios across taxa carries substantial evolutionary relevance, influencing sexual selection (Janicke and Morrow 2018), and by playing a ubiquitous role in shaping population demographics and mating systems. Empirical studies on the evolution of sex ratios, at macro-evolutionary scales, have typically been biased towards the adult (ASR) or operational (i.e. ratio of sexual receptive adults) sex ratio Pipoly et al 2015) While such studies are informative for the evolution of mating systems, our understanding of the key mechanisms driving evolutionary distortions in the initial sex ratio remains limited. Adult sex ratios are not necessarily a reliable measure of the initial sex ratio when estimated in natural systems, even if the ASR evolved as a function of the initial sex ratio, which in turn, is determined by zygote formation or sex-allocation (see Clutton-Brock and Parker 1992)
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