SEXUAL SELECTION, OR ADAPTATION TO MATING DEMANDS
In The Descent of Man, and Selection in Relation to Sex, Darwin defined sexual selection as the “advantage which certain individuals have over other individuals of the same sex and species, in exclusive relation to reproduction.” Darwin appreciated that sexual selection could be mediated by intrasexual combat (e.g., between males) or by intersexual preferences (e.g., female choice of attractive mates). He also appreciated that sexual selection could be in opposition to natural selection with respect to particular phenotypic traits (such as a peacock’s tail), but he generally viewed sexual selection as less effective than natural selection.
After discussing Darwin’s original ideas about sexual selection (especially as presented in The Descent of Man), Adam Jones and Nicholas Ratterman in Chapter 9 identify three modern triumphs in sexual selection research: the introduction and widespread use of molecular markers to assess genetic parentage (the key to describing mating systems in nature) unequivocally; a better conceptual understanding (at least in formal models) of the mechanisms and consequences of mate choice by females (or by males in role-reversed taxa); and a better appreciation of why differences exist among lineages in the intensity of various forms of sexual selection. For the latter two topics, the histories of ideas on sexual selection—beginning with Darwin—are interwoven with how those notions laid the foundation for categorizing various forms of sexual selection, and for extending and expanding modern research into various aspects of sexual selection both in theory and empirically. The authors close by suggesting several lines of future research on sexual selection.
In Chapter 10, Stephen Shuster provides a comprehensive overview (and contrast) of how sexual selection has been measured and studied in plant systems versus animal systems. One general theme that he emphasizes is the need to reconcile Darwin’s idea that sexual selection tends to be less rigorous than natural selection with the observation that sexual selection would seem to be responsible for many if not most differences between the two genders (in features other than the primary sex organs). In The Origin, Darwin wrote that sexual selection “depends, not on the struggle for existence, but on a struggle between males for possession of the females; the result is not death of the unsuccessful competitor, but few or no offspring. Sexual selection is, therefore, less rigorous than natural selection.” Shuster, by contrast, views sexual selection as being “among the most powerful of evolutionary forces.” The author proposes to reconcile these two stances in evolutionary models that combine quantitative differences in the fitness variances between the sexes (an approach traditionally applied to animal systems) with phenotypic and genotypic correlations underlying reproductive traits among breeding pairs (an approach often taken in plant systems). The net result, he claims, will be the ability to predict the magnitudes of sexual dimorphism and classify mating systems using existing genetic and life history data.
In Chapter 11, Patricia Gowaty and Stephen Hubbell offer a novel perspective on what underlies the individual decision-making process that in turn underlies patterns of mate choice and sexual selection in various species. Their central thesis is that even stochastic variation in various parameters that predict the time available for mating might promote considerable flexibility in individual decision making with regard to mate choice; and that even consistent sex differences in these mating proclivities might therefore, in at least some cases, reflect ecological constraints (habits-of-life considerations) rather than behavioral differences that might otherwise be genetically hardwired between the sexes. The parameters that might impact available time for mating include the probabilities of encountering mates, individual survivorships, mating latencies (times-out between mating events), and fitness distributions, all of which are likely to vary as functions of the natural histories as well as the evolutionary histories of species. Such considerations lead the authors to their switch point theorem, which in principle can quantitatively evaluate what proportion of potential mates in a population a focal individual should find acceptable as mating partners if it is to maximize its relative lifetime fitness.
In an uncharacteristic conceptual lapse, Darwin failed to appreciate that sexual selection (including both male-male competition and female choice) can continue even after copulation has begun. The intrasexual (male-male) component can happen via sperm competition for the fertilization of ova, and the intersexual component can occur via cryptic female
choice of alternative sperm, all within the female’s reproductive tract. William Eberhard reviews the history of ideas for these underappreciated but nevertheless intense forms of postcopulatory sexual selection in Chapter 12. The miniature worlds of gametic competition and gametic choice have proven to be every bit as fascinating and compelling as the macroscopic worlds of mating competition and mate choice that have been the traditional foci of sexual selection studies. Eberhard brings this miniature Kama Sutra realm to light by detailing fascinating examples of sexual selection in the time interval (which is often but not invariably brief) between the onset of copulation and the completion of fertilization.