The definition of sexual selection

Abstract

Sexual selection is a key component of evolutionary biology. However, from the very formulation of sexual selection by Darwin, the nature and extent of sexual selection have been controversial. Recently, such controversy has led back to the fundamental question of just what sexual selection is. This has included how we incorporate female-female reproductive competition into sexual or natural selection. In this review, we do four things. First, we examine what we want a definition to do. Second, we define sexual selection: sexual selection is any selection that arises from fitness differences associated with nonrandom success in the competition for access to gametes for fertilization. An important outcome of this is that as mates often also offer access to resources, when those resources are the targets of the competition, rather than their gametes, the process should be considered natural rather than sexual selection. We believe this definition encapsulates both much of Darwin's original thinking about sexual selection, and much of how contemporary biologists use the concept of sexual selection. Third, we address alternative definitions, focusing in some detail on the role of female reproductive competition. Fourth, we challenge our definition with a number of scenarios, for instance where natural and sexual selection may align (as in some forms of endurance rivalry), or where differential allocation means teasing apart how fecundity and access to gametes influence fitness. In conclusion, we emphasize that whilst the ecological realities of sexual selection are likely to be complex, the definition of sexual selection is rather simple.

Keywords: natural selection; sexual selection.

Figure 1

Female competition for (a–c) gametes in the common glow worm, and (d–e) resources required for reproduction in the parasitoid wasp Goniozus legneri. (a) A female Lampyris noctiluca glowing to attract mates (photo: Jouni Valkeeniemi). (b) Experimental evidence that male L. noctiluca are attracted to brighter green LED lights, used here to mimic female displays (High brightness and Low brightness are 12.6 × 10¹² and 7.0 × 10¹¹ photons cm⁻² s⁻¹ respectively), whilst females with larger lanterns are also more fecund (c). (d) Female G. legneri fight for possession of a host on which to lay eggs, which can lead to vigorous struggles (inset; photos: Sonia Dourlot). (e) These contests are determined by differences in aggression between the owner and intruder, with the more aggressive individual, that initiates more interactions, winning the resource. The fitted line is from a logistic regression. (b-c) Figures redrawn from Open Access Data provided by Hopkins et al. (2015) under a CC0 1.0 Universal Public Domain Dedication license. (d–e) Figures reproduced from Goubault et al. (2006) with permission of The Royal Society.

Figure 2

Routes to fitness for female and male gamete producers. (a) Routes to fitness for your sex, and (b) routes to fitness for the opposite sex. The two sexes are completely interchangeable. In (a), individuals and their gametes of the focal sex are the subject of competition, and in (b) they are the object of competition. As such, in (a) selection arises within the focal sex, and in (b) selection arises within the opposite sex, as a result of gametic traits and other phenotypes in the focal sex. Both (a) and (b) will occur concurrently. Yellow lines denote routes to fitness via narrow-sense natural selection, blue lines denote routes to fitness via sexual selection, and green lines denotes routes where both natural and sexual selection may yet occur. Solid lines denote fitness routes influenced by gamete quality or quantity in the focal sex, and dotted lines denote fitness routes influenced by other traits.