Current Zoology, 2018, 64(1), 69–73 doi: 10.1093/cz/zoy009 Advance Access Publication Date: 23 January 2018 Editorial Editorial Male competition and speciation: expanding our framework for speciation by sexual selection a, b c Alycia C. R. LACKEY *, Michael D. MARTIN , and Robin M. TINGHITELLA a b Department of Biological Sciences, Binghamton University, Binghamton, NY 13902, USA, Department of Biology, Oxford College of Emory University, Oxford, GA 30054, USA, and Department of Biological Sciences, University of Denver, Denver, CO 80208, USA *Address correspondence to Alycia C. R. Lackey. E-mail: firstname.lastname@example.org. the contribution of male competition, explore the diversity of mech- Introduction anisms by which male competition drives divergence, and motivate Sexual selection is a powerful source of rapid evolutionary change, future work by identifying key questions and gaps in our current and there is a long-standing hypothesis that it can cause reproduc- understanding. Whereas most work in this emerging field has tive isolation. However, our understanding of speciation by sexual focused on male–male competition, female–female competition may selection is largely limited to mechanisms by which sexual selection be similarly capable of driving divergence and speciation. Female– via female mate choice can drive divergence (i.e., when male mating female competition can arise when males are a limiting resource, signals and female preferences for those signals diversify; Panhuis including, but not limited to, sex-role–reversed mating systems. et al. 2001; Maan and Seehausen 2011). Male competition for Within-sex competition for mates among males or females can gen- mates—Darwin’s second mechanism of sexual selection—can also erate disruptive, frequency-dependent selection of mate preferences favor rapid and dramatic phenotypic and genotypic changes, yet it and facilitate divergence and speciation (van Doorn et al. 2004). has been all but overlooked in speciation research (Darwin 1859, Thus, the ideas discussed throughout this column apply broadly to 1871; Seehausen and Schluter 2004; Qvarnstro ¨ m et al. 2012; within-sex competition for mates. Tinghitella et al. forthcoming). Evidence suggests that male competition is capable of driving divergence and potentially contributing to the speciation process. Key Questions in the Study of Competition for First, male competition can generate strong selection that favors Mates and Speciation divergent phenotypes within and between populations. In some mat- Recent work sheds light on how and when competition for mates ing systems, male competition primarily determines mating success likely affects speciation (Seehausen and Schluter 2004; Dijkstra and within populations (e.g., resource or harem defense polygyny; West- Groothuis 2011; Qvarnstro ¨ m et al. 2012; Tinghitella et al. forth- Eberhard 1983; Andersson 1994). In other mating systems, male coming). One critical challenge has been to determine whether and competition acts as a filter, determining which males have access to how divergence favored by competition for mates could contribute females and, thus, the phenotypes available for female mate choice to reproductive isolation. We briefly review recent findings organ- (Wong and Candolin 2005; Hunt et al. 2009). Further, the remark- ized by 3 key questions. We then describe the contributions in the able diversity in competitive phenotypes (i.e., weapons, agonistic special column that address each of these questions. signals, and competitive strategies; Seehausen and Schluter 2004; Grether et al. 2013; McCullough et al. 2014) likely results from How does competition for mates contribute to population differences in selection generated by competition for divergence and speciation in different geographic mates. Second, it is well established that competition for resources can drive speciation via natural selection (Schluter 2001; Pfennig contexts? In sympatry, the best-studied mechanism of divergence and specia- and Pfennig 2010). Competition for mating resources could have tion via competition for mates is negative frequency-dependent similar potential to shape the speciation process. This special column addresses how and when competition for selection, which could allow a novel competitive phenotype to mates can generate and maintain divergent phenotypes and facilitate invade a population and stabilize the presence of multiple morphs (Mikami et al. 2004; Seehausen and Schluter 2004; van Doorn et al. reproductive isolation. Moreover, the contributed papers consider how competition for mates might hinder divergence and speciation. 2004). In this scenario, males bias aggression to similar phenotypes Our aims are to expand our current speciation framework to include because, for example, they compete for shared resources. Thus, V C The Author(s) (2018). Published by Oxford University Press. 69 This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact email@example.com Downloaded from https://academic.oup.com/cz/article-abstract/64/1/69/4822073 by Ed 'DeepDyve' Gillespie user on 16 March 2018 70 Current Zoology, 2018, Vol. 64, No. 1 males with a rare phenotype experience a fitness advantage because How do aggression biases and competitive they receive relatively little aggression (Seehausen and Schluter asymmetries arise and affect divergence? 2004; van Doorn et al. 2004). Furthermore, selection could favor Competitors may bias aggression toward particular phenotypes females who choose males with a rare phenotype because, for exam- (e.g., homotypic, heterotypic, or a certain trait value regardless of ple, these males may have more energy to invest in mate provision- species or population; Pauers et al. 2008; Dijkstra and Groothuis ing or paternal care. Competition for mates may also favor 2011; Lehtonen 2014; Lehtonen et al. 2015). In sympatry, an divergent competitive phenotypes independent of their frequency aggression bias for homotypics could evolve early in divergence if because alternative fitness optima exist (Lackey and Boughman there is pleiotropy or tight linkage between the competitive pheno- 2013; Keagy et al. 2016). In this case, males with phenotypes inter- type and aggression, when aggression is learned and targeted toward mediate to existing fitness optima would be selected against in com- the most common phenotype encountered, or via imprinting petition. Moreover, selection against mating between types could (Seehausen and Schluter 2004; Dijkstra and Groothuis 2011). In sec- favor the evolution of prezygotic isolation via reinforcement ondary contact, selection against competition with heterospecifics (Servedio and Noor 2003). In allopatry, competitive phenotypes that do not share mating resources could favor the evolution of con- could diverge due to environmental differences (e.g., Scordato 2017) specific aggression bias (Grether et al. 2009; Anderson and Grether or via the accumulation of different alleles in populations adapting 2010). Aggression biases toward homotypics generate negative to similar environments (i.e., mutation order divergence; Martin and frequency-dependent selection that can allow a novel competitive Mendelson 2012; Mendelson et al. 2014). Ecological conditions trait to invade the population and facilitate initial divergence in sym- that likely shape divergence in competitive phenotypes include dif- patry. Aggression biases for homotypics can also stabilize coexis- ferences in signaling environments, presence of predators or para- tence of closely related species with different competitive sites, availability of prey, and habitat structure (e.g., Maan and phenotypes (Seehausen and Schluter 2004; Dijkstra et al. 2007; Cummings 2012; Qvarnstro ¨ m et al. 2012, McCullough et al. 2016; Pauers et al. 2008; Lehtonen 2014). Scordato 2017; Tinghitella et al. forthcoming). If competitive phe- Aggression biases and competitive ability may differ between notypes are locally adapted, then divergent natural selection morphs with important consequences for maintaining divergent between populations from different environments could facilitate competitive phenotypes. An asymmetric aggression bias occurs habitat isolation, and divergent sexual selection from mate preferen- when 1 morph biases aggression to homotypics, for example, while ces for locally adapted traits could result in sexual isolation. In sec- the other morph does not bias aggression and competes equally with ondary contact, the likelihood of divergence between previously both morphs. Competitive asymmetry can occur when 1 morph has isolated populations depends largely on the extent to which compet- an advantage at winning contests over the other morph, and com- itors share mating resources. When competitors from different pop- petitive asymmetries appear to be very common (Martin et al. ulations do not compete for access to the same mates, competitive 2017). Asymmetries in aggression bias and competitive ability could phenotypes are expected to diverge to reduce costly agonistic inter- arise, for example, in allopatry due to differences in selective envi- actions with noncompetitors (i.e., agonistic character displacement, ronments (e.g., signaling contexts, resource density, predators) or Grether et al. 2009; Grether et al. 2013). In contrast, shared resour- via mutation order divergence (Qvarnstro ¨ m et al. 2012; Tinghitella ces among interspecific competitors favors convergence of agonistic et al. forthcoming). An asymmetry between morphs in aggression traits (Grether et al. 2009, 2013; Drury et al. 2015). bias could lead to competitive exclusion of the morph that receives more aggression (i.e., receives both homo- and heterotypic aggres- How do natural selection, mate choice, and competition sion), although habitat or resource partitioning between morphs could facilitate coexistence (Dijkstra et al. 2007; Dijkstra and for mates interact to affect the potential for speciation? Groothuis 2011; Lehtonen et al. 2015). A competitive asymmetry in Work on the contribution of female mate choice to speciation suggests which a novel morph has a competitive advantage would allow the that sexual selection is most likely to cause speciation when acting in novel morph to invade a population, which could facilitate early concert with natural selection, rather than when acting alone (van divergence between morphs in sympatry (Dijkstra et al. 2005). Doorn et al. 2004; Ritchie 2007; Servedio and Boughman 2017). Competitive asymmetries between species in secondary contact Competition for mates may also facilitate divergence and speciation, could result in competitive exclusion and potentially facilitate habi- most often through interactions with natural selection and/or mate tat isolation (Owen-Ashley and Butler 2004; Duckworth 2006; choice. Numerous examples demonstrate that environmental depend- Vallin et al. 2012; Lipshutz 2017). Alternatively, competitive asym- ence of competitive phenotypes could affect divergence (e.g., Vallin metries could homogenize populations through asymmetric intro- and Qvarnstro ¨ m 2011; Lackey and Boughman 2013; Heathcote et al. gression (While et al. 2015). The latter scenario may be more likely 2016; McCullough et al. 2016). Divergence and speciation in 2 well- with strong competitive asymmetries and weak divergent selection studied examples of rapid speciation (i.e., cichlid fish, limnetic– from other sources. benthic stickleback fish) involve interactions between divergent ecol- ogy, female mate choice, and male competition (Schluter 1993, 1995; Boughman 2001; Seehausen and Schluter 2004; Seehausen et al. Contributions to This Issue 2008; Lackey and Boughman 2013). Yet, recent work in European wall lizards reveals that male competition and ecology can shape In this column, the 2 review papers and 4 empirical papers each divergence and reproductive isolation in the absence of female mate address the key questions described above. Importantly, these choice (Heathcote et al. 2016). Interactions among sources of selec- articles expand our framework for the role of speciation by sexual tion could also hinder speciation. For instance, sexual selection from selection by examining a variety of mechanisms through which com- competition for mates should limit trait divergence when selection on petition for mates could contribute to divergence and speciation. competitive phenotypes conflicts with divergent natural selection The review by Lipshutz (2018) broadens our understanding of (see Servedio and Boughman (2017) for parallel ideas for female how within-sex competition might drive divergence and contribute preferences). to speciation by reviewing a growing body of literature on species Downloaded from https://academic.oup.com/cz/article-abstract/64/1/69/4822073 by Ed 'DeepDyve' Gillespie user on 16 March 2018 Editorial 71 interactions in secondary contact and hybridization. The review males of multiple orangethroat darter species from populations sym- offers a fresh perspective on sexual selection and hybridization, pin- patric with the rainbow darter biased aggression toward conspe- pointing how the outdated dichotomy between choosy females and cifics, whereas males from populations allopatric to the rainbow competitive, but non-choosy, males oversimplifies sex roles and lim- darter showed no aggression bias. Similarly, males only biased their its our perspective on mechanisms that might contribute to specia- courting efforts toward conspecific females when males were from a tion. Lipshutz (2018) considers how female–female competition, population sympatric with the rainbow darter. Thus, Moran and interactions between female choice and male competition, and char- Fuller (2018) find evidence consistent with male-driven agonistic acter shifts in competitive traits and recognition can facilitate or and reproductive character displacement. Female mate choice impede reproductive isolation. Female–female competition might be appears unimportant as no species exhibited a significant conspecific particularly relevant when the fitness costs of mating with hetero- bias. In addition, the authors find evidence consistent with cascade specifics are low for females. Several mechanisms by which interspe- agonistic and reproductive character displacement. In staged cific within-sex competition might promote reproductive isolation encounters between 2 allopatric species of orangethroat darter, only are outlined, including some that involve interactions with mate males from populations sympatric with rainbow darters bias aggres- choice (e.g., reproductive and agonistic character displacement) and sion to conspecifics and choose conspecific mates. These findings some that do not (e.g., competitive asymmetry and reproductive suggest that sympatric interactions between orangethroat darter spe- exclusion). Finally, the flip-side of interactions in secondary contact cies and the rainbow darter may strengthen reproductive isolation is considered: when do interspecific interactions promote, rather between closely related, allopatric orangethroat darter species as a than reduce, hybridization? Lipshutz (2018) reviews evidence that byproduct. The patterns described by Moran and Fuller (2018) competitive asymmetry can promote directional hybridization motivate future work to test the roles of both inter- and intraspecific between species that still share mating resources, generating some- competition for mates in speciation. times asymmetric introgression of loci from the competitively supe- Tinghitella et al. (2018) advocate for placing our understanding rior parental type to the competitively inferior type. Our of heterotypic aggression biases and competitive asymmetries in a sexual signaling context because competitive outcomes may depend understanding of how within-sex competition facilitates hybridiza- tion is in its infancy, and female–female competition is understudied on how receivers perceive and respond to signals. The authors relative to male–male competition, making this area ripe for future staged competition trials over a single nesting site within and work. between color morphs (red or black) of threespine sticklebacks Dijkstra and Border (2018) review multiple mechanisms through Gasterosteus aculeatus from Washington state (United States) and which male competition can cause divergence of competitive pheno- assessed the relationships between competitive outcomes and puta- types and contribute to reproductive isolation. Although negative tively sexually selected colors and behaviors. Tinghitella et al. frequency-dependent selection has been the primary explanation of (2018) find a complex interaction between the traits that predict speciation in sympatry, Dijkstra and Border (2018) extend this competitive success within and between male color types, demon- hypothesis to the contexts of allopatry and secondary contact. The strating that males of the 2 color types use different competition authors describe how aggression biases toward homotypic males as “currencies.” Regardless of competitor type, red and black males well as a competitive advantage of the novel phenotype could yield who successfully established territories performed more aggressive negative frequency-dependent selection and disruptive selection, behaviors than losing males. However, color predicted competitive which can both facilitate the invasion of a novel phenotype and pro- outcomes differently for red and black males. Red males with more mote coexistence of 2 morphs (Seehausen and Schluter 2004; extensive red color were more likely to win territorial disputes with Dijkstra and Groothuis 2011). Furthermore, the authors discuss homotypic males, but less likely to win against heterotypic males. how competitive traits likely result in trade-offs with other traits, Thus, whether the extent of color serves as a signal of aggressive such as physiological or life history traits. Given these trade-offs, dif- behavior depends on the receiver. In black males, however, the ferent competitive phenotypes may be adaptive alternatives, with extent of black color does not predict competitive outcomes; only each phenotype maximizing a different end of the trait distribution. aggressive behaviors were associated with winning in territorial dis- For instance, if investing in color limits energy available for growth, putes. In summary, divergent competitive strategies may explain the then alternative competitive phenotypes may be colorful and small aggression biases and asymmetries that are frequently observed versus dull and large. Moreover, the authors draw parallels between upon secondary contact. Tinghitella et al. (2018) describe how how ecological competition and male competition contribute to spe- asymmetries in competitive abilities could facilitate spatial segrega- ciation. Considering these 2 mechanisms simultaneously provides a tion, reinforcing reproductive isolation. framework for understanding how natural and sexual selection Bierbach et al. (2018) investigate how abiotic environments interact during divergence. shape fighting ability and, thus, the outcomes of competition Moran and Fuller (2018) step outside the traditional dyadic between ecotypes at the interface between habitats. The authors model of comparative speciation studies to ask whether sympatric investigate competitive outcomes between populations of poeciliid interactions among congeners can facilitate divergence in competi- fish that are locally adapted to toxic, hydrogen sulfide-rich environ- tive phenotypes across an allopatric radiation of closely related fish ments and those from non-sulfidic aquatic environments. The species. Darters are a diverse group of fish that often exhibit strik- authors suggest a priori that physiological and metabolic adaptation ing, species-specific male nuptial color patterns and drably colored to extreme environments could incur costs that limit energy invest- females. The rainbow darter Etheostoma caeruleum is widely dis- ment in male competition and in reproductive interactions more tributed in rivers and streams of the Eastern United States and co- generally. Two major results consistent with this hypothesis occurs with several members of the orangethroat darter species emerged: 1) extremophile males from 1 drainage, but not all drain- group throughout the entirety of their ranges. Other species in the ages investigated, exhibit lower aggression levels than males from orangethroat darter group are either partially or completely allopa- non-sulfidic environments, and 2) in pairings between ecotypes, tric with respect to the rainbow darter. In staged contests, only non–sulfide-adapted ecotypes were more likely to win contests when Downloaded from https://academic.oup.com/cz/article-abstract/64/1/69/4822073 by Ed 'DeepDyve' Gillespie user on 16 March 2018 72 Current Zoology, 2018, Vol. 64, No. 1 work to this special column and to the reviewers who critically evaluated the fights were staged in non-sulfidic environments. Importantly, not all work. Thanks to Tamra Mendelson and Maria Servedio for the thoughtful extremophile males suffer the proposed costs of adaptation to feedback that improved this manuscript. extreme environments. Rather, the authors argue that adapting to hydrogen sulfide-rich environments limits resource holding potential when competing with males from non-sulfidic environments. This Funding competitive asymmetry could contribute to reproductive isolation The invitation to guest edit this special column resulted from a symposium by reducing gene flow when extremophile males migrate into more co-organized by ACRL, MDM, and RMT at the 2016 congress of the benign habitats because they lose fights against males who are International Society for Behavioral Ecology, and travel expenses for all locally adapted to those habitats. authors were supported in part by the National Science Foundation (IOS- Becher and Gumm (2018) investigate whether male competition 1637252). While guest editing this special column, ACRL was supported by and female mate choice are likely to facilitate coexistence upon sec- NSF (DEB-1638997) and the Watershed Studies Institute at Murray State ondary contact between endemic and recently introduced popula- University. RMT was supported by NSF (IOS-1601531) and the University of tions in the same genus: the endemic Red River pupfish Cyprinodon Denver’s Ofﬁce of the Associate Provost of Research. rubrofluviatilis and the introduced sheepshead minnow Cyprinodon variegatus. In interspecific dominance trials, males of each species References won an equal number of fights. However, males of the endemic spe- cies used a greater number of aggressive behaviors to gain domi- Andersson M, 1994. Sexual Selection. Princeton (NJ): Princeton University nance than males of the introduced species. Males that invest energy Press. Anderson CN, Grether GF, 2010. 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The contributed papers highlight sev- tive frequency-dependent selection and disruptive selection during specia- eral ways that competition for mates can affect divergence and the tion? Curr Zool 64:89–99. potential for reproductive isolation, many of which emphasize the Dijkstra PD, Groothuis TGG, 2011. Male-male competition as a force in evo- role of divergent natural selection and some of which involve little lutionary diversiﬁcation: evidence in haplochromine cichlid ﬁsh. Int J Evol Biol 2011:1–9. to no role for female mate choice. Future theoretical work will be Dijkstra PD, Seehausen O, Pierotti MER, Groothuis TGG, 2007. 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