Male competition and speciation: expanding our framework for speciation by sexual selection.

Abstract

Sexual selection is a powerful source of rapid evolutionary change, and there is a long-standing hypothesis that it can cause reproductive isolation. However, our understanding of speciation by sexual selection is largely limited to mechanisms by which sexual selection via female mate choice can drive divergence (i.e., when male mating signals and female preferences for those signals diversify; Panhuis et al. 2001; Maan and Seehausen 2011). Male competition for mates—Darwin’s second mechanism of sexual selection—can also favor rapid and dramatic phenotypic and genotypic changes, yet it has been all but overlooked in speciation research (Darwin 1859, 1871; Seehausen and Schluter 2004; Qvarnstrom et al. 2012; Tinghitella et al. forthcoming). Evidence suggests that male competition is capable of driving divergence and potentially contributing to the speciation process. First, male competition can generate strong selection that favors divergent phenotypes within and between populations. In some mating systems, male competition primarily determines mating success within populations (e.g., resource or harem defense polygyny; West-Eberhard 1983; Andersson 1994). In other mating systems, male competition acts as a filter, determining which males have access to females and, thus, the phenotypes available for female mate choice (Wong and Candolin 2005; Hunt et al. 2009). Further, the remarkable diversity in competitive phenotypes (i.e., weapons, agonistic signals, and competitive strategies; Seehausen and Schluter 2004; Grether et al. 2013; McCullough et al. 2014) likely results from population differences in selection generated by competition for mates. Second, it is well established that competition for resources can drive speciation via natural selection (Schluter 2001; Pfennig and Pfennig 2010). Competition for mating resources could have similar potential to shape the speciation process. This special column addresses how and when competition for mates can generate and maintain divergent phenotypes and facilitate reproductive isolation. Moreover, the contributed papers consider how competition for mates might hinder divergence and speciation. Our aims are to expand our current speciation framework to include the contribution of male competition, explore the diversity of mechanisms by which male competition drives divergence, and motivate future work by identifying key questions and gaps in our current understanding. Whereas most work in this emerging field has focused on male–male competition, female–female competition may be similarly capable of driving divergence and speciation. Female–female competition can arise when males are a limiting resource, including, but not limited to, sex-role–reversed mating systems. Within-sex competition for mates among males or females can generate disruptive, frequency-dependent selection of mate preferences and facilitate divergence and speciation (van Doorn et al. 2004). Thus, the ideas discussed throughout this column apply broadly to within-sex competition for mates.