Understanding intrasexual competition and sexual selection requires an evolutionary ecology framework

Abstract

Across most species, sexually selected traits have a dual role in both gaining mates and in intrasexual competition (Hunt et al. 2009). As suggested by Rosvall in this review, most definitions of sexual selection restrict it to the evolutionary dynamics of competition for mates, yet in females, results point to the fact that intrasexual competitions between females often occur for access to high-quality mates and resources rather than for the number of mates. As the review highlights, both males and females may compete for resources necessary to raise offspring, and in many cases where it appears that females are competing for access to males, they are probably competing for resources in the form of direct fitness benefits. Therefore, distinguishing between components of selection is seldom easy in either sex, particularly when natural and sexual selection operate in the same direction. I realize that there is debate to be had and this review makes suggestions as to how to move forwards, but in my opinion instead of formally distinguishing components of sexual selection from natural selection and arguing about the true nature of sexual selection, we will gain greater insight into the evolutionary dynamics of male and female phenotypes if we instead concentrate on quantifying the important components of selection within an evolutionary ecology framework. As Rosvall suggests, examining contrasting fitness components and quantifying the intensity and targets of selection in males and females can reveal important insights. Studies that have examined both the factors that influence the fitness component of gaining mates (i.e., the number of matings), and the fitness outcomes of those matings (i.e. the number/ quality of the offspring produced), have revealed that selection components may not always act in the same direction, if competitively superior individuals do not always gain increased fitness or convey increased fitness to their mates. Allocating resources to competitive intrasexual interactions for mates may trade-off against survival (Robinson et al. 2008), sperm competition (Preston et al. 2001), and parental care in males (Duckworth 2006), and there is no reason why females should not also face resource allocation trade-offs under certain conditions. Furthermore, if either sex face trade-offs then the fitness benefits gained from intrasexual competition or intersexual mate choice will be dependent on these, although at present, there are few estimates of selection acting on these components. Therefore, rather than formally defining which components are under sexual versus natural selection, we need to quantify individual differences in breeding success and determine its relationship with various underlying components in both sexes. Only in this way, will we understand the potential factors that maintain variation that we often see in natural populations or drive the differences that we observe between the sexes. Although research over the past decade has greatly extended our knowledge of the operation of sexual selection in females (LeBas 2006; and this review), quantifying the selection is only one side of the coin. As pointed out in this review, understanding the evolutionary dynamics of a trait will also require estimates of the genetic basis in males and females, the cross-sex genetic correlation, of the traits under selection. Furthermore, the dynamics of sexual selection will vary across species as mating systems vary but will also vary within species or within populations as social and environmental factors change. As described in this review, density, resource availability, and many other social and environmental factors will drive competitive interactions, and the strength of selection acting on phenotype. As the genetic basis of traits under selection is also likely to change across similar abiotic and biotic environments as a result of genotype–environment interactions (for a review see Ingleby et al. 2010), we really require simultaneous estimation of the genetic basis of characters in males and females, and the selection pressures acting on each sex. In my opinion, this should also be a priority for future research. Applying an evolutionary ecology framework to the study of sexual selection in both sexes will enable us to better understand the evolutionary dynamics of sexual selection within the natural world.