Experimental evidence that sexual conflict influences the opportunity, form and intensity of sexual selection

Sexual interactions are often rife with conflict. Conflict between members of the same sex over opportunities to mate has long been understood to effect evolution via sexual selection. Although conflict between males and females is now understood to be widespread, such conflict is seldom considered in the same light as a general agent of sexual selection. Any interaction between males or females that generates variation in fitness, whether due to conflict, competition or mate choice, can potentially influence sexual selection acting on a range of male traits. Here we seek to address a lack of direct experimental evidence for how sexual conflict influences sexual selection more broadly. We manipulate a major source of sexual conflict in the black field cricket, Teleogryllus commodus, and quantify the resulting changes in the nature of sexual selection using formal selection analysis to statistically compare multivariate fitness surfaces. In T. commodus, sexual conflict occurs over the attachment time of an external spermatophore. By experimentally manipulating the ability of males and females to influence spermatophore attachment, we found that sexual conflict significantly influences the opportunity, form and intensity of sexual selection on male courtship call and body size. When males were able to harass females, the opportunity for selection was smaller, the form of selection changed, and sexual selection was weaker. We discuss the broader evolutionary implications of these findings, including the contributions of sexual conflict to fluctuating sexual selection and the maintenance of additive genetic variation.

Authors: Matthew D. Hall, Luc F. Bussière, John Hunt and Robert C. Brooks

Direct selection on male attractiveness and female preference fails to produce a response

Background: Theoretical studies suggest that direct and indirect selection have the potential to cause substantial evolutionary change in female mate choice. Similarly, sexual selection is considered a strong force in the evolution of male attractiveness and the exaggeration of secondary sexual traits. Few studies have, however, directly tested how female mate choice and male attractiveness respond to selection. Here we report the results of a selection experiment in which we selected directly on female mating preference for attractive males and, independently, on male attractiveness in the guppy, Poecilia reticulata. We measured the direct and correlated responses of female mate choice and male attractiveness to selection and the correlated responses of male ornamental traits, female fecundity and adult male and female survival. Results: Surprisingly, neither female mate choice nor male attractiveness responded significantly to direct or to indirect selection. Fecundity did differ significantly among lines in a way that suggests a possible sexually-antagonistic cost to male attractiveness. Conclusions: The opportunity for evolutionary change in female mate choice and male attractiveness may be much smaller than predicted by current theory, and may thus have important consequences for how we understand the evolution of female mate choice and male attractiveness. We discuss a number of factors that may have constrained the response of female choice and male attractiveness to selection, including low heritabilities, low levels of genetic (co)variation in the multivariate direction of selection, sexually-antagonistic constraint on sexual selection and the “environmental covariance hypothesis”.

Authors: Matthew D. Hall, Anna K. Lindholm & Robert C. Brooks.