Abstract
Sexual dimorphism in life history traits and their trade‐offs is widespread among sexually reproducing animals and is strongly influenced by the differences in reproductive strategies between the sexes. We investigated how intrasexual competition influenced specific life history traits, important to fitness and their trade‐offs in the outcrossing nematode Caenorhabditis remanei. Here, we altered the strength of sex‐specific selection through experimental evolution with increased potential for intrasexual competition by skewing the adult sex ratio towards either females or males (1:10 or 10:1) over 30 generations and subsequently measured the phenotypic response to selection in three traits related to fitness: body size, fecundity and tolerance to heat stress. We observed a greater evolutionary change in females than males for body size and peak fitness, suggesting that females may experience stronger net selection and potentially harbour higher amounts of standing genetic variance compared to males. Our study highlights the importance of investigating direct and indirect effects of intrasexual competition in both sexes in order to capture sex‐specific responses and understand the evolution of sexual dimorphism in traits expressed by both sexes.
Highlights
Sexual dimorphism (SD) refers to phenotypic sex differences in traits other than sexual organs and can be manifested both as sex-limitation of traits, and differential phenotypic expression of shared traits in the two sexes
We investigated how intrasexual competition acts on different life history traits important to fitness and affect their trade-offs, using an outcrossing nematode, C. remanei, as the model system
Our analysis found no overall effect of treatment on body size (F2,1 = 0.79, p = .56); there was a significant interaction between sex and treatment (F2,2 = 8.12, p < .001), suggesting that the two sexes have responded differently to the experimental evolution treatments when compared to the ancestral state (Figure 2.)
Summary
Sexual dimorphism (SD) refers to phenotypic sex differences in traits other than sexual organs and can be manifested both as sex-limitation of traits, and differential phenotypic expression of shared traits in the two sexes. The most obvious sexually dimorphic traits include conspicuous ornaments, common but not limited to males (such as the peacock's tail) (Amundsen, 2000; Nordeide et al, 2013), which has led to a historic focus on sexual selection on males (Andersson, 1994). The overall trend is that sexual selection is stronger on males, there is tremendous variation between taxa which we still need to understand better (Janicke et al, 2016), and how intense competition among females influences the evolution of sexual dimorphism is still poorly understood (Clutton-Brock, 2007, 2009; Hare & Simmons, 2019). Many sexually dimorphic traits are correlated and involved in life history trade-offs, and currently our knowledge of how sex-specific selection operates on these tradeoffs through male–male and female–female competition is limited (but see Dale et al, 2015)
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