The definition of sexual selection and its implications for dinosaurian biology

Abstract

We are delighted by the constructive and thoughtful comments of Knell & Sampson (2010) on our original article (Padian H whereas, to Knell and Sampson, sexual selection does not require sexual dimorphism, and mate recognition is a more closely related concept to sexual selection. In our view, Charles Darwin understood organismal biology better than anyone of his time, partly because he thought through problems so thoroughly. In devising his theory of natural selection, he realized that certain living animals bore some salient phenotypic characteristics, such as horns and antlers, that could not be readily explained through the agency of natural selection. He knew that these sorts of features (and their associated behaviors) would pose a threat to the acceptance of his theory of natural selection (because they would be seen as fatal exceptions), and he also understood that these features were not, in most cases, directly relevant to an individual’s survival (i.e. ecologically adaptive). Rather, they helped an individual attract mates or repel rivals for mates. The opposite sex lacked these features (or did not use them in mating). To this phenomenon he gave the name sexual selection, and he explicitly defined it (1859, pp. 89–90 and nearly the same words in 1871, p. 243) by stating that these features were present in one sex and not the other, and that they either attracted mates or repelled rivals. This was the genesis of the term sexual selection. Darwin needed this concept as a contrast to natural selection, in order to defuse a false argument of his detractors; he knew exactly what he was doing. Darwin cannot be ‘wrong’ about the definition of this concept, despite the protests or confusion of later authors, because he invented it, and his empirical basis for it is entirely valid; he was not ‘imprecise’ (pace Carranza, 2009). Myriad examples prove the presence of distinct, monosexual characters in species that are used to attract mates and repel rivals (Darwin, 1871; Andersson, 1994). Thus, the only possible definition of sexual selection requires sexual dimorphism (and not simply allometric sexual differences: Padian & Horner, 2010). To say this does not deny that many factors are involved in the attraction of mates, the repulsion of rivals, success in mating and the differential production of offspring. But sexual selection is only a small part of this, and Darwin was not trying to explain all aspects of mate recognition, attraction, competition and reproductive success. The notion that sexual selection involves more or different aspects than those defined by Darwin is an historical error of misinterpretation in the scholarly literature that has sadly become entrenched. But one cannot change the definition of a term at will. This only creates confusion and fosters misinterpretation [consider the later misuses of Van Valen’s (1973) original concept of the ‘Red Queen,’ which denoted the control of energy in an ecosystem by individual species]. We acknowledge that Darwin’s term is widely misused in the recent literature, and unfortunately, this has brought confusion to an extremely interesting and productive field. Futuyma (2009), whose textbooks have been the ne plus ultra of the field for many years, views sexual selection as a subtype of natural selection, and many biologists agree. There is an historical context for this misunderstanding. In the early 20th century, mathematical modelers of the Modern Synthesis needed to examine whether natural selection could be a viable force in populations (Mayr & Provine, Journal of Zoology