Animal Behaviour 77 (2009) 749–751
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Deﬁning sexual selection as sex-dependent selection
Biology & Ethology Research Group, University of Extremadura
a r t i c l e i n f o
Article history: Received 8 August 2008 Initial acceptance 29 September2008 Final acceptance 2 November 2008 Published online 3 December 2008 MS. number: 08-00516 Keyword: sexual selection
After almost 150 years of research, the very deﬁnition of sexual selection remains unclear. The term sexual selection was coined in 1859 by Darwin (1859), who described it in more detail in his later book (Darwin 1871), as an explanation for the evolution of those characters ofmales of some species that did not seem to contribute to survival in the struggle for existence, those features previously referred to by Hunter (1837) as ‘secondary sexual characters’. From that date, sexual selection has become a sexy topic that has attracted huge interest among behavioural ecologists and evolutionary biologists. However, as Clutton-Brock (2004, page 26) stated, ‘one of theproblems in writing about sexual selection today is that the term is used in so many different ways’, and recent debate on the basic structure of sexual selection theory (Roughgarden et al. 2006; Kavanagh 2007; Clutton-Brock 2007) indicates that its deﬁnition has probably never been clear enough. Darwin himself was ambiguous in the deﬁnition of the term, and was imprecise about what type of elements inthe behaviour and life history of individuals should or should not be included in the sexual selection process, and how to differentiate between natural and sexual selection. Darwin initially deﬁned sexual selection on the basis of intrasexual advantages in relation to reproduction. However, in describing the process he always referred to the advantages in relation to access to matingopportunities, either by competition with members of the same sex (in most cases males) or from the preferences of members of the other sex (mostly females; Darwin 1871). From the discussions shortly after the publication of Darwin’s book (e.g. Wallace 1889) to more recent reviews (e.g. Andersson 1994), sexual selection has
´ ´ * Correspondence: J. Carranza, Unidad de Biologıa y Etologıa, Facultad de Veter´inaria, Universidad de Extremadura, 10071 Caceres, Spain. E-mail address: email@example.com (J. Carranza)
generally been interpreted solely in terms of competition for mates. Only in some cases have authors extended sexual selection to other reproductive events (Møller 1994; Reynolds & Harvey 1994) or referred to the ‘narrow sense’ when using sexual selection only for competition for mates(Partridge & Endler 1987, page 272). Recently, Clutton-Brock (2007) has reviewed sexual selection, focusing especially on the operation of sexual selection in females in the context of the current debate (Roughgarden et al. 2006; Kavanagh 2007), and has proposed a return to Darwin’s initial words on sexual selection as the intrasexual competition to breed, rather than the most commonly acknowledgedcompetition for mates. When Darwin discussed whether certain reproductive structures were the products of natural or sexual selection, he explained that those directly connected to reproduction such as the mammary glands of mammals and abdominal pouches of marsupials in the case of females, or the receptacles for the ova in certain male ﬁshes, were selected by natural selection. However, he alsostated ‘in many cases it is scarcely possible to distinguish the effects of natural and sexual selection’ (Darwin 1871, page 257). Darwin adopted the term ‘secondary sexual characters’ (Hunter 1837) for those traits not ‘directly’ linked to reproduction, and used the qualiﬁers primary and secondary to assign to a given character the natural or sexual selection processes, respectively. However, the...
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