Table 2.

Examples of Definitions of Sexual Selection from Darwin Onwards

Darwin 1859, p. 88: “Sexual selection […] depends, not on a struggle for existence, but on a struggle between the males for possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring.”

Darwin 1871, p. 256: “We are, however, here concerned only with that kind of selection, which I have called sexual selection. This depends on the advantage which certain individuals have over other individuals of the same sex and species, in exclusive relation to reproduction.”

Huxley 1938, p. 416: “Darwin’s theory of sexual selection was of the compound deductive-inductive type. Deductively he postulated: (1) that under certain circumstances there would occur a struggle between males for mates, and that the characters giving success in such a struggle would have sexually-selective value and would be perpetuated irrespective of their natural-selective value in the general struggle for existence; (2) that these characters would be of two main types, (a) those subserving male display, (b) those subserving combat between rival males, and that such characters could not be evolved except under the operation of sexual selection as defined by him. With regard to display characters, he further deduced a rudimentary esthetic sense in females, and also a process of female choice as between rival males.”

Ehrman 1972 (Chapter 6 in Campbell), p. 106: “At present it seems best to simply define sexual selection as all mechanisms which cause deviations from panmixia.”

Crook 1972 (Chapter 9 in Campbell), p. 264: Social selection “is primarily in relation to direct competition.”… “Social selection results from (a) effects of competition between the subject and others of either sex with respect of commodities essential to survival in a situation that will allow an attempt at reproduction, (b) competition for access to preferred members of the opposite sex for mating and (c) effects of competition between subjects for access to commodities in the environment essential for the rearing of their young to reproductive age. Of these b is the process most commonly referred to as sexual selection.” [In other words, sexual selection becomes a subset under social selection, but because social selection only relates to direct competition, sexual selection due to scramble or endurance competition is not included].

Maynard Smith 1978 (Chapter 10 Sexual selection, in The Evolution of Sex), p. 168: “As soon as aniosogamy has evolved, different selective forces may act on males and females; it is these differential forces with which I am concerned in this chapter.” [This can thus be read as if Maynard Smith defines sexual selection as selection acting differently on the two sexes].

West-Eberhard 1979, p222 and subsequently: West-Eberhard follows Darwin in viewing sexual selection as competition for mates, but also considers sexual selection a subset of social selection, with the latter characterized by competition within a social group for one or more resources (which might include mates). For example: “I agree with Mayr (1972, p.88) that “something rather important was lost” in the process of redefining fitness and erasing Darwin’s distinction between these two kinds of selection [natural and sexual selection] — just as something is lost by stretching the concept of sexual selection to make it suit new purposes which, however interesting in their own right, tend to obscure what Darwin was trying to say (for example, Ehrman’s1972, p.106, redefinition of sexual selection as “all mechanisms which cause deviations from panmixia,” or Maynard Smith’s, 1978, inclusion of all selection acting differently on the two sexes). When Darwin wrote about sexual selection he focused primarily on social competition for mates.”

Partridge and Halliday 1984 (Chapter 9 in Krebs & Davies, 2nd edition), p. 222: “It has long been obvious that the gametes produced in natural populations do not pair up at random. Leaving aside the obvious restrictions imposed by species and gender, some individuals may obtain more fertilizations than others, and particular types of parings may be more common than others. Such nonrandom mating is of fundamental evolutionary importance because different matings may have different fitness consequences.” Continued on p. 225: “As Darwin was first to recognize, variance in the number of successful matings is the raw material for sexual selection, defined as selection on characters giving certain individuals an advantage over others of the same sex in obtaining successful matings.”

Andersson 1994, p. 3: “According to Darwin (1871), sexual selection arises from differences in reproductive success caused by competition over mates.” Continued on p. 8: “Sexual selection of a trait can therefore be viewed as a shorthand phrase for differences in reproductive success, caused by competition over mates, and related to the expression of the traits”; and p. 9: “In spite of many suggestions to the contrary by leading biologists […] the term sexual selection in here restricted to competition over mates.”

Roughgarden et al. 2006, p. 965: “Since 1871, sexual selection theory has often been restated (4), yet contemporary definitions share Darwin’s central narrative: “We now understand… Males, who can produce many offspring with only minimal investment, spread their genes most effectively by mating promiscuously. Female reproductive output is far more constrained by the metabolic costs of producing eggs or offspring, and thus a female’s interests are served more by mate quality than by mate quantity” (5). … The reproductive social behavior of most species has not been studied, but a great many of those that have been do not conform to Darwinian sexual-selection templates. We suggest that sexual selection is always mistaken, even where gender roles superficially match the Darwinian templates.”

Kokko et al. 2006, p. 44: “Sexual selection: selection generated by differential access to opposite-sex gametes (or mates).” [This definition is by far the closest to our definition].

Ritchie 2007, p. 80: “Sexual selection: the component of natural selection arising owing to variation in mating or fertilization success”

Carranza 2009, p. 750: “In 1994, […] I proposed a definition for sexual selection as (page 380; translated from Spanish): ‘those natural selection forces that operate differently in males and females because of the strategies of the sexes’. This is simply to adopt the concept of sex-dependent selection as a modern use of the term sexual selection to investigate the evolution of differences between the sexes.”

Clutton-Brock 2009, p. 8: Contrasts in the operation of sexual selection in the two sexes raise the question of whether adaptations to intrasexual competition in females should be regarded as products of sexual selection or natural selection. In The Descent of Man Darwin sometimes described ‘sexual’ selection as selection operating through intrasexual competition to reproduce […] and sometimes as selection operating through competition for mates, although the term is now most commonly restricted to selection operating through intrasexual competition for mating opportunities (Andersson 1994). Because females more commonly need to compete for breeding opportunities than mating opportunities, defining sexual selection in terms of competition for mates has the effect of restricting its operation to males, creating unfortunate dichotomies where functionally similar traits are attributed to sexual selection if they occur in males but to natural selection if they occur in females. […] The most satisfactory solution might be to abandon the distinction between sexual and natural selection altogether and emphasize, instead, the contrasting ways in which selection operates in males and females (Clutton-Brock 2007). However, the distinction between sexual and natural selection is so heavily entrenched that this is unlikely to occur and the most feasible alternative is probably to broaden the concept of sexual selection to include all selection processes operating through intrasexual competition for breeding opportunities in either sex (Clutton-Brock 2007).”

Jones and Ratterman 2009: “Darwin makes it clear that not all selection related to reproduction constitutes sexual selection, as primary sexual traits—like ovaries and testes—can evolve as a consequence of natural selection. Even though he never spells it out in so many words, Darwin’s working definition of sexual selection is essentially identical to the one used by Andersson [1994] and most other scientists studying sexual selection. In particular, ‘‘sexual selection arises from differences in reproductive success caused by competition for access to mates’’ [Andersson 1994, p 3]. This definition admittedly focuses primarily on precopulatory sexual selection, so a more complete definition should also include postcopulatory processes, which can be accomplished by tagging the phrase ‘‘or fertilization opportunities’’ onto the end of Andersson’s definition.”

Kuijper et al. 2012: “Sexual selection is the process by which individuals compete for access to mates and fertilization opportunities.”

Safran et al. 2013, p. 644: …”we define sexual selection as the result of the differential reproductive success that arises from competition for mates and access to fertilizations.”

Rosenthal 2017, p. 503: “Sexual selection. A special case of natural selection: differential reproductive success due to the ability to secure matings and/or fertilization.”

Alonzo and Servedio 2019, Table 1: Their table offers a similar sample of definitions of sexual selection, which (together with our examples above) highlights the challenge for the field of sexual selection.