Sexual selection definition

Sexual selection definition DEFAULT

Bateman, A. J. Inter-sexual selection in Drosophila. Heredity2, 349-368 (1948).

Birkhead, T. R. & Moller, A. P. Sperm Competition and Sexual Selection. San Diego, CA: Academic Press, 1998.

Calhim, S. & Birkhead, T. R. Testes size in birds: quality versus quantity — assumptions, errors and estimates. Behavioral Ecology18, 271-275 (2007).

Chapman, T., Arnqvist, G. et al. Sexual conflict. Trends in Ecology and Evolution3, 41-47 (2003).

Clutton-Brock, T. H. & Parker, G. A. Sexual coercion in animal societies. AnimalBehavior49, 1345-1365 (1995).

Cronin, H. The Ant and the Peacock. Cambridge, UK: Cambridge University Press, 1991.

Darwin, C. The Descent of Man, and Selection in Relation to Sex. London, UK: Murray, 1871.

Eberhard, W. Female Control: Sexual Selection by Cryptic Female Choice. Princeton, NJ: Princeton University Press, 1996.

Emlen, D. J. The Evolution of Animal Weapons. Annual Review of Ecology, Systematics, and Evolution39, 387-413 (2008).

Fisher, R. A. The Genetical Theory of Natural Selection. Oxford, UK: Clarendon Press, 1930.

Hamilton, W. D. & Zuk, M. Heritable true fitness and bright birds: a role for parasites? Science218, 384-387 (1982).

Keller, L. & Reeve, H. K. Why do females mate with multiple males? The sexually selected sperm hypothesis. Advanced Studies in Behavior, 24, 291-315 (1997).

Kirkpatrick, M. Sexual selection and the evolution of female choice. Evolution82, 1-12 (1982).

Lande, R. Models of speciation by sexual selection on polygenic traits. Proceedings of the National Academy of Sciences, USA78, 3721-3725 (1981).

LeBoeuf, B. Male-male competition and reproductive success in Elephant seals. American Zoologist14, 163- 176 (1974)

Parker, G. Sperm competition and its evolutionary consequences in the insects. Biological Reviews45, 525-567 (1970).

Parker, G. Sexual selection and sexual conflict. In Sexual Selection and Reproductive Competition in Insects. eds. Blum, M. S. & Blum, N. A. (New York: Academic Press, 1979): 123-166.

Prum, R. O. Phylogenetic analysis of the evolution of display behavior in the neotropical manakins (Aves: Pipridae). Ethology84, 202-231 (1990).

Stutt, A. D., Siva-Jothy, M. T. Traumatic insemination and sexual conflict in the bed bug Cimex lectularius. Proceedings of the National Academy of Sciences, U.S.A.,98, 5683-5687 (2001)

Trivers, R. L. Parental investment and sexual selection. In Sexual Selection and the Descent of Man1871-1971. ed. Campbell, B. (London: Heinemann 1972): 136-179.

Zahavi, A. Mate selection: a selection for a handicap. Journal of Theoretical Biology53, 205-214 (1975).

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sexual selection

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nounBiology.

a special type of natural selection in which the sexes acquire distinct forms either because the members of one sex choose mates with particular features or because in the competition for mates among the members of one sex only those with certain traits succeed.

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Words nearby sexual selection

sexually transmitted diseases, sexual orientation, sexual relations, sexual reproduction, sexual revolution, sexual selection, sex up, Sexwale, sex work, sex worker, sexy

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2021

How to use sexual selection in a sentence

  • Mate choice and competition through sexual selection should improve mitonuclear matching in two ways.

    Sex Is Driven by the Impetus to Change - Issue 88: Love & Sex|Jill Neimark|August 12, 2020|Nautilus

  • The CDA was passed not in the name of censorship but in the name of protecting children from stumbling across sexual material.

    How the PC Police Threaten Free Speech|Nick Gillespie|January 9, 2015|DAILY BEAST

  • But he, like many people using dating apps whatever their sexual identity, remains stoutly positive.

    Grindr’s Trans Dating Problem|David Levesley|January 9, 2015|DAILY BEAST

  • The program has not made a final selection on which upgrades will actually be included in future versions of the F-35.

    Pentagon Misfires in Stealth Jet Scandal|Dave Majumdar|January 8, 2015|DAILY BEAST

  • Yeah, the “Giant man-puppy” that is Gronkowski won't hold a sexual candle to the blue-eyed dreamboat.

    ‘A Gronking to Remember’ Speed Read: 8 Naughtiest Bits|Emily Shire|January 7, 2015|DAILY BEAST

  • Early on, the sexual protagonist complains that her Molson-drinking husband is pretty much an incompetent Neanderthal.

    ‘A Gronking to Remember’ Speed Read: 8 Naughtiest Bits|Emily Shire|January 7, 2015|DAILY BEAST

  • The significance of time is determined by the movement of any selection, or, in other words, the rhythm.

    Expressive Voice Culture|Jessie Eldridge Southwick

  • At the close of the reception a band of Philippine musicians played a selection of graceful airs of their native isles.

    The Philippine Islands|John Foreman

  • The sexual cycle can take place only within the body of one genus of mosquito, anopheles.

    A Manual of Clinical Diagnosis|James Campbell Todd

  • This selection was unfortunate; good strategist and organiser, he was not the man the Emperor required.

    Napoleon's Marshals|R. P. Dunn-Pattison

  • But in doing this, the intention and power of selection guided by sound judgment at once asserted itself.

    Antonio Stradivari|Horace William Petherick

British Dictionary definitions for sexual selection


noun

an evolutionary process in animals, in which selection by females of males with certain characters, such as large antlers or bright plumage, results in the preservation of these characters in the species

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Medical definitions for sexual selection


n.

Selection that is driven by the competition for mates and that is considered an adjunct to natural selection.

The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

Scientific definitions for sexual selection


The process by which certain organisms produce more offspring by mating more frequently than other organisms of the same sex and thereby ensure the survival of more of their genetic traits. Sexual selection is a form of natural selection in which organisms are competing not for food or other resources in the environment but for mates. The development of size difference between males and females in mammals and birds, in which the greater strength (and often aggressiveness) of larger males allows them to have greater success mating, is seen as a consequence of sexual selection. The development of secondary sex characteristics, such as colored feathers in male birds or large antlers in male deer, which are attractive to the opposite sex as signs of fitness but are not directly involved in reproduction is also attributed to sexual selection. These features are often disadvantageous to the organism's survival-the colored feathers make the male bird more visible to predators, for instance-but can provide the organism with a competitive advantage over rivals in mating. The theory of sexual selection was first proposed by Charles Darwin in The Origin of Species.

The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

Cultural definitions for sexual selection


In evolution, the selection of a mate based on secondary sex characteristics. Sexual selection is thought to lead distinct differences in the appearance of the two sexes within a species. For example, the tail of the male peacock may be the result of sexual selection.

The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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Abstract

Sexual selection is a key component of evolutionary biology. However, from the very formulation of sexual selection by Darwin, the nature and extent of sexual selection have been controversial. Recently, such controversy has led back to the fundamental question of just what sexual selection is. This has included how we incorporate female-female reproductive competition into sexual or natural selection. In this review, we do four things. First, we examine what we want a definition to do. Second, we define sexual selection: sexual selection is any selection that arises from fitness differences associated with nonrandom success in the competition for access to gametes for fertilization. An important outcome of this is that as mates often also offer access to resources, when those resources are the targets of the competition, rather than their gametes, the process should be considered natural rather than sexual selection. We believe this definition encapsulates both much of Darwin’s original thinking about sexual selection, and much of how contemporary biologists use the concept of sexual selection. Third, we address alternative definitions, focusing in some detail on the role of female reproductive competition. Fourth, we challenge our definition with a number of scenarios, for instance where natural and sexual selection may align (as in some forms of endurance rivalry), or where differential allocation means teasing apart how fecundity and access to gametes influence fitness. In conclusion, we emphasize that whilst the ecological realities of sexual selection are likely to be complex, the definition of sexual selection is rather simple.

INTRODUCTION

Sexual selection has a contentious history. Originally, sexual selection was proposed by Darwin (1859, 1871) to explain the evolution of traits that do not appear to increase longevity or fecundity, and so would not be favored by natural selection. Whilst the rest of the Darwinian view of evolution has been widely accepted, sexual selection has remained a focus of debate and argument for more than a century and a half (Cronin 1991; Andersson 1994; Kokko et al. 2006; Alonzo and Servedio 2019). First, the very notion of sexual selection itself proved less acceptable to early evolutionary biologists than natural selection, no doubt in part due to social attitudes at the time. For instance, Darwin removed the very term “sexual selection” from the title of The Descent of Man at the behest of his publisher John Murray, leaving instead the rather mangled “selection in relation to sex” (Dawson 2007), which has perhaps led to some misinterpretations of Darwin’s ideas (see below).

Second, Darwin was clearly aware of the potentially scandalous nature of sexual selection, and especially the idea of mate preferences. For example, he left the discussion of some human male mate preferences for certain female morphological attributes to a footnote written in Latin (Darwin 1871, p. 345; Dawson 2007). Mate choice in particular failed to gain widespread acceptance with Darwin’s contemporaries and those who came after. This may have been because Darwin emphasized female mate choice, and the idea of female animals expressing an “aesthetic taste” may have been hard to swallow, although there were notable exceptions (for example Peckham and Peckham 1889, 1890; see also Cronin 1991; Hamlin 2014). Certainly, the idea of male–male competition was more widely accepted, not least because male combat for mates was self-evident in animals such as rutting ungulates or horned beetles (Emlen 2008, 2014; for various perspectives on the history of sexual selection see for instance Cronin 1991, Andersson 1994, and Milam 2010).

Third, the resurgence of interest in sexual selection in the 1970s and 1980s (for example Campbell 1972; Blum and Blum 1979; Bateson 1983; Thornhill and Alcock 1983; Smith 1984; Bradbury and Andersson 1987; reviewed by Andersson 1994) also brought forth controversy. This resurgence included the rehabilitation of mate choice as a potent evolutionary mechanism, on the back of ground-breaking theoretical and empirical papers (Lande 1981; Andersson 1982; Kirkpatrick 1982). However, why those mate preferences evolved has proved challenging for theorists to understand and for empiricists to test. Mate choice associated with direct benefits (such as access to parental care, resources, or territories) has always been relatively straightforward to comprehend (Andersson 1994). Mate choice when males only provide sperm to females, and thus genes to offspring, remains contentious though (for recent discussions see Ingleby et al. 2010; Prum 2010; Hunt and Hosken 2014; Rosenthal 2017; Achorn and Rosenthal 2020).

Fourth, the conceptual basis of sexual selection was more recently challenged in a highly controversial paper (Roughgarden et al. 2006 and resulting commentaries). Whilst much of that paper has had little bearing on what sexual selection researchers do and think, perhaps the most relevant fall-out was attempts to redefine or reinterpret what sexual selection actually is. This has included reconsidering the role of females in sexual selection (for example LeBas 2006; Clutton-Brock 2007, 2009, 2017; Rosvall 2011; Stockley and Bro-Jørgensen 2011; Tobias et al. 2012; Hare and Simmons 2019) and also the consideration of new definitions of sexual selection (for example Carranza 2009; see discussion in Shuker 2010, 2014; Alonzo and Servedio 2019).

Here we will explore definitions of sexual selection, provide what we think is the most suitable definition, and attempt to bring conceptual clarity to a concept about which there has perhaps been some intellectual complacency. We will begin by considering what a definition of sexual selection should and should not be able to achieve, then we will provide a definition, which will be familiar to many readers, before discussing alternative definitions and finally cases that push our definition to the limit. We will need to discuss how natural and sexual selection interact, and to do so we will follow Endler (1986) by considering sexual selection to be a component of “broad-sense” natural selection. “Narrow-sense” natural selection will be used to describe other components of fitness, such as longevity, fecundity, and parental investment (see below for a fuller discussion of this terminology).

WHAT TO EXPECT FROM A DEFINITION OF SEXUAL SELECTION

A definition of sexual selection should fulfill a number of criteria. First, it needs to be sex neutral (that is, not dependent on a given sex or sexual function). This is because both males and females may present traits that are under sexual selection, even in the same species. Moreover, in species in which there are no sexes, that is isogamous species such as Saccharomyces yeasts, individuals within a mating type may compete for mates in a way that recalls sexual selection in anisogamous species (for example Rogers et al. 2009; Reding et al. 2013). In other words, a definition of sexual selection should require neither sexes nor specific sex roles (Shuker 2010, 2014).

Second, a definition of sexual selection should identify – in general terms – what individuals are competing for. Competition is important here. DNA sequences are competing for representation in the next generation, competition that can occur even if a population is expanding rapidly in size. An attempt to divorce sexual selection from the notion of competition was part of the critique of sexual selection put forward by Roughgarden et al. (2006). Those authors wanted to view interactions between individuals in terms of reproduction as cooperative, with males and females working together as a “team.” However, cooperation – like selfishness – is just another strategy by which organisms may increase their genetic representation in the next generation, at the expense of others. After all, cooperation is in the eye of the beholder – your cooperation may be my cronyism and corruption. Putting forward cooperation in opposition to competition is therefore misguided and misinterprets levels of explanation. Darwin himself made it clear that selection is all about doing better than rivals, whether that be through teamwork or aggression.

On the other hand, there are some things that we should not expect a definition of sexual selection to be able to sort out for us. To begin with, sexual selection should be agnostic about (that is, unconcerned with) other components of fitness. In other words, it should not be defined in terms of other forms of selection. If it is, then sexual selection risks becoming contingent on other forms of selection that may or may not be present, which does not seem satisfactory to us. This might seem a reasonable proposition, but Darwin of course mentioned sexual selection in terms of traits that natural selection did not appear to favor (Darwin 1871, for example pp 278-279). This opposition to natural selection is still often emphasized, for instance when natural selection halts the exaggeration of sexually selected ornaments in models of mate choice (Lande 1981; Andersson 1994). However, when he introduced sexual selection in the first edition of The Origin, Darwin noted from the outset that males with the greatest “general vigour” (that is, most favored by natural selection) may also be the most successful at winning mates (Darwin 1859). Here, natural and sexual selection both favor “general vigour” and are not in opposition. What this means is that empirically we may not necessarily be able to discriminate between fitness associated with sexual selection, and fitness associated with other components of selection, if those fitness components partially or completely align (Darwin 1871, p. 257; Andersson 1994). However, it is an empirical problem that we should expect to face, given the countless ways in which components of fitness may instantiate themselves across the tangled bank of ecologies organisms inhabit in the wild.

Many mechanisms can result in sexual selection (Table 1). However, a definition of sexual selection should not be tied to one or more mechanisms of sexual selection, such as mate choice or male–male competition. This is important, as much of the Roughgarden et al (2006) critique focused on sexual selection through female mate choice, and indeed other recent reviews have tended to equate the two (for example Kuijper et al. 2012; a similar point was made by McCullough et al. 2016). Arguments over mate choice for instance – however prominent historically and contemporaneously – should not distract us from a more fundamental definition of sexual selection. Indeed, clarity over the definition of sexual selection may help the field from becoming preoccupied with one or other form of sexual selection.

Table 1

Mechanisms of Competition Over Mates and Gametes, Based on and Developed from Table 1.1.1 in Andersson (1994)

Mechanisms . Favoured traits in competing sex . 
Scramble competition sensory and locomotory organs to quickly locate mates (for example hearing, olfaction) 
Endurance rivalry ability to endure prolonged reproductive activity (for example condition, lifespan) 
Contest competition ability to outcompete rivals before mating through direct combat (for example body size, weapons), or ability to avoid such competition through alternative reproductive tactics 
Mate choice competition to be chosen through behavioral or morphological traits that the opposite sex finds attractive (for example ornaments, indicators of ‘good’ or compatible genes), or resources that the other sex needs (for example territory, nuptial gift, parental care), or ability to circumvent mate choice (for example forced copulations) 
Gamete competition after mating ability to outcompete rivals through gamete competition after mating (for example large numbers of sperm, large size of eggs), or ability to avoid that gamete competition (for example mate guarding, mating plugs) 
Cryptic mate choice competition to be chosen after mating through traits that the opposite sex prefers (for example ‘good’ or compatible genes) 
Mechanisms . Favoured traits in competing sex . 
Scramble competition sensory and locomotory organs to quickly locate mates (for example hearing, olfaction) 
Endurance rivalry ability to endure prolonged reproductive activity (for example condition, lifespan) 
Contest competition ability to outcompete rivals before mating through direct combat (for example body size, weapons), or ability to avoid such competition through alternative reproductive tactics 
Mate choice competition to be chosen through behavioral or morphological traits that the opposite sex finds attractive (for example ornaments, indicators of ‘good’ or compatible genes), or resources that the other sex needs (for example territory, nuptial gift, parental care), or ability to circumvent mate choice (for example forced copulations) 
Gamete competition after mating ability to outcompete rivals through gamete competition after mating (for example large numbers of sperm, large size of eggs), or ability to avoid that gamete competition (for example mate guarding, mating plugs) 
Cryptic mate choice competition to be chosen after mating through traits that the opposite sex prefers (for example ‘good’ or compatible genes) 

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Table 1

Mechanisms of Competition Over Mates and Gametes, Based on and Developed from Table 1.1.1 in Andersson (1994)

Mechanisms . Favoured traits in competing sex . 
Scramble competition sensory and locomotory organs to quickly locate mates (for example hearing, olfaction) 
Endurance rivalry ability to endure prolonged reproductive activity (for example condition, lifespan) 
Contest competition ability to outcompete rivals before mating through direct combat (for example body size, weapons), or ability to avoid such competition through alternative reproductive tactics 
Mate choice competition to be chosen through behavioral or morphological traits that the opposite sex finds attractive (for example ornaments, indicators of ‘good’ or compatible genes), or resources that the other sex needs (for example territory, nuptial gift, parental care), or ability to circumvent mate choice (for example forced copulations) 
Gamete competition after mating ability to outcompete rivals through gamete competition after mating (for example large numbers of sperm, large size of eggs), or ability to avoid that gamete competition (for example mate guarding, mating plugs) 
Cryptic mate choice competition to be chosen after mating through traits that the opposite sex prefers (for example ‘good’ or compatible genes) 
Mechanisms . Favoured traits in competing sex . 
Scramble competition sensory and locomotory organs to quickly locate mates (for example hearing, olfaction) 
Endurance rivalry ability to endure prolonged reproductive activity (for example condition, lifespan) 
Contest competition ability to outcompete rivals before mating through direct combat (for example body size, weapons), or ability to avoid such competition through alternative reproductive tactics 
Mate choice competition to be chosen through behavioral or morphological traits that the opposite sex finds attractive (for example ornaments, indicators of ‘good’ or compatible genes), or resources that the other sex needs (for example territory, nuptial gift, parental care), or ability to circumvent mate choice (for example forced copulations) 
Gamete competition after mating ability to outcompete rivals through gamete competition after mating (for example large numbers of sperm, large size of eggs), or ability to avoid that gamete competition (for example mate guarding, mating plugs) 
Cryptic mate choice competition to be chosen after mating through traits that the opposite sex prefers (for example ‘good’ or compatible genes) 

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This is not to say that we would not like a body of theory that makes specific predictions about how sexual selection arises, or indeed that predicts specific traits or trait values favored by sexual selection. Fortunately, we do have that body of theory though, both in terms of general frameworks like mating systems theory (for example Emlen and Oring 1977; Davies 1991; Reynolds 1996; Shuster and Wade 2003; Shuker 2010; ,Kokko et al. 2014) and in terms of specific optimality models predicting sexually selected traits, such as copulation duration in dung flies (for example Parker 1970a; Parker and Simmons 1994) or time of emergence (protandry) in butterflies (for example Wiklund and Fagerström 1977; Bulmer 1983; Iwasa et al. 1983; Morbey and Ydenberg 2001; Morbey et al. 2012). Here, the analogy with the rest of natural selection theory is clear. For instance, we have a wonderful body of evolutionary theory used to predict and explain many of the traits we see, such as sex allocation theory (West 2009), but we do not include those specifics in our definition of natural selection.

Finally, we suggest that a definition should be agnostic about the number of mates individuals have, and indeed the nature and extent of variation in the number of mates or resulting fertilizations that individuals within a sex exhibit. A straightforward reason for this is that sexual selection can still occur in fully monogamous species (Kirkpatrick et al. 1990; Andersson 1994; Dougherty et al. 2016; Kvarnemo 2018), for example, if the adult sex ratio is biased (generating strong mating competition within the sex in excess; for example Price 1984; Kvarnemo et al. 2007; Fromhage and Jennions 2016), or through assortative pair-formation, often driven by mutual mate choice (for example Parker 1983; Johnstone et al. 1996; Jones and Ratterman 2009). We will consider variation in the number of mates in more detail below when we address definitions of sexual selection based on the variance in mating success.

A DEFINITION OF SEXUAL SELECTION

After what we see as a continuity from the words and works of Darwin, as captured by Andersson (1994) and Shuker (2010, 2014), we define sexual selection as follows:

Sexual selection is any selection that arises from fitness differences associated with nonrandom success in the competition for access to gametes for fertilization.

Fertilization is important, hence, by “access to gametes” (as used as shorthand below) we emphasize that the route to fitness through competition for access to gametes requires fertilization as the outcome. For completeness, we assume that gametes are viable and able to take part in fertilization; this primary function of male and female gametes will be under natural selection. As such, a trait is under sexual selection when variation in that trait is nonrandomly associated with variation in access to gametes, when those gametes form a limiting resource. Evolution by sexual selection follows when there is a heritable component to the trait under sexual selection.

Indeed, in more genetical terms, the extent to which a given allele is under sexual selection is the extent to which there is a marginal change in allele frequency associated with nonrandom access to gametes for fertilization. Sexual selection is therefore one way of partitioning allele frequency change – that is into natural and sexual selection components of fitness – in an analogous way to partitioning allele frequency change into direct and indirect components of fitness in an inclusive fitness framework (for example Gardner et al. 2011). Henceforth we will assume additive genetic variation in relevant traits, to make the discussion more concise.

From the outset, we do not wish to suggest that our definition is new; indeed, our definition is very close to definitions of sexual selection given before (see, for instance, Kokko et al. 2006). We hope to make this abundantly clear when we review other definitions below. However, we consider that there are problems with alternative definitions of sexual selection and that the underlying logic of sexual selection is not always fully appreciated.

As such, several important points emerge from this definition. First, competition is for access to gametes. We recognize that access to mates is a typical first step towards obtaining access to gametes (in particular in internal fertilizers), and that we indeed expect two broad classes of sexual selection fitness components: those associated with precopulatory mating success (for example Bateson 1983; Hardy and Briffa 2013) and those associated with post-copulatory fertilization success (for example Parker 1970b; Smith 1984; Eberhard 1996; Birkhead and Møller 1998; Simmons 2001). As discussed by Birkhead (2010), it was this second component that was elusive to Darwin. However, the traditional focus on access to mates as the keystone to sexual selection is perhaps because humans are not broadcast spawners. We will return to the case when competition for mates may not yield sexual selection below.

Importantly, gametes as a resource can vary in two ways, and thus be limiting in two ways. They can vary in quantity, and they can vary in quality. Competition can therefore arise for access to gametes in terms of getting access to the most gametes, but also in terms of access to gametes of the highest quality. We will revisit the importance of competition in terms of quality below (as this is one of the areas where things can get hairy).

Second, we are agnostic as to the sexual identity of the competitors (it could be males, females, hermaphrodites, or individuals of mating type A or mating type B). This means that we are also agnostic as to whether or not sexual selection happens in both sexes/mating types at the same time; all options are possible. However, implicit in our definition is that individuals of the same sex, same mating type, or tissues with the same sexual function (for example male function in hermaphrodites), compete for the gametes of the opposite sex or complementary mating type. In other words, sexual selection arises from competition within a sex or mating type.

Whilst this again might seem straightforward, confusion can arise when sexual selection is influenced by interactions between the sexes. Such interactions certainly influence fertilization success, through mate choice (when one sex competes to be chosen by the other sex). But care is needed in noting which sex is competing for whose gametes.

The definition of mate choice has itself been recently reassessed (Edward 2014). We consider mate choice to arise from the nonrandom patterns of mating, and subsequent fertilization success, in the chosen sex due to one or more phenotypic traits in the choosy sex. These phenotypic traits may be behavioral (and approach what we might think of as a more active “choice”) or be morphological or physiological; what is important is that the chooser biases mating and/or fertilization as a result. Crucially, whilst there must be nonrandom success with respect to some aspect of the phenotype of the chosen sex for choice to occur, there need not be any variation amongst choosers (that is they may all choose the same individual or individuals). Mate choice thus mediates a form of within-sex competition for access to gametes, but it does not replace it.

Our definition is sex neutral, but it does identify what is being competed for (gametes for fertilization), albeit in general terms. Whilst we highlight competition for mates and gametes as two major classes of sexual selection fitness components, we have not identified specific mechanisms by which sexually selected fitness may be obtained. This also means that we have not specified whether or not sexual selection aligns with other components of fitness. As such, our definition is not contingent on the action of any other forms of selection. That said, we expect that sexual selection will comprise only some of the variation in fitness in a given population at a given time.

THE RELATIONSHIP BETWEEN NATURAL SELECTION AND SEXUAL SELECTION

By being agnostic to other components of fitness, where does our definition place sexual selection with regards to natural selection? The relationship between natural and sexual selection has been long debated, from Darwin onwards (for example Darwin 1871; Fisher 1930; Arnold 1983; Andersson 1994; Klug et al. 2010; Alonzo and Servedio 2019). As mentioned above, a particularly useful conceptualization was provided by Endler (1986). He suggested that broad-sense natural selection should be taken to comprise all possible components of fitness (a sort of complete compendium of Darwinian selection possibilities), whilst narrow-sense natural selection should be taken to comprise things like viability and fecundity selection, as these have been more traditionally considered to be “natural selection,” and the main focus of Darwin’s The Origin of Species.

When theorists have considered natural selection acting to oppose sexual selection during the development of a sexual ornament, it is usually in terms of something like viability selection (as an ornament gets bigger, so natural selection via predation acts to limit further exaggeration, for example). If we take Endler’s view, then sexual selection sits within broad-sense evolution by natural selection but is separate from narrow-sense natural selection. However, there is not necessarily a “correct” answer here: we give things names as and when it is useful to do so. We should remember that although partitioning up fitness into different components may be useful, there will be alternative ways to partition fitness. Our definition clarifies a partitioning of fitness that goes back to Darwin. We should also remember that partitioning fitness in theory is more straightforward than doing so in practice. We should not be surprised when the empirical utility of partitioning out fitness is compromised, as when fitness components align and become indistinguishable. This would happen if access to mates correlated perfectly with feeding ability or longevity, for example. Indeed, we should expect such alignments to occur in real mating systems in real ecologies. However, we should not take those failures too seriously either, as concepts such as sexual selection and viability selection still have useful conceptual work to do (for further discussion, see Shuker 2010).

ALTERNATIVE DEFINITIONS

Here we consider a number of alternative definitions of sexual selection. In Table 2 we provide a flavor of various definitions by a range of authors, from Darwin onwards (for a similar sample, see Alonzo and Servedio 2019). Some of those definitions overlap or restate our definition, and we do not consider those further. We appreciate that our survey is by no means comprehensive, but we nonetheless hope that we cover the major groups of alternative definitions, minor differences in wording notwithstanding.

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. 
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. 

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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. 
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. 

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First, a definition of sexual selection has been proposed that limits itself to mate choice by females or otherwise (Roughgarden et al. 2006). However, as pointed out above, such a definition of sexual selection is far too narrow, as it excludes intrasexual selection via both mating and sperm and egg competition. In particular, it neglects a very substantial body of work that has traditionally sat within the compass of sexual selection: that is intrasexual (such as male–male) contest competition for access to gametes (Andersson 1994; Hardy and Briffa 2013). This component of sexual selection influences traits that are unlikely to have been favored by narrow-sense natural selection, such as horns and antlers (Andersson 1994; Emlen 2008, 2014; see in particular the discussion in McCullough et al. 2016). Focusing on mate choice, although ignoring the intrasexual competition, also lacks conceptual coherence, as it focuses on one group of mechanisms that might influence or mediate within-sex mating competition, whilst not including others (a point made, albeit rather cryptically, by Fisher 1930, p. 131–132).

Second, sexual selection is sometimes defined in terms of variation in reproductive success (for example in textbooks such as Avassar et al. 2013; Clark et al. 2018). Whilst this is understandable in some ways, because mating and reproduction seem to go hand in hand, reproductive success is actually a much broader concept, representing an organism’s direct fitness (in an inclusive fitness framework) through all direct fitness components, including those related to longevity, fecundity, and parental care. Informally, one has to survive to reproduce, as well as find mates, so reproductive success per se is too broad a concept to separate out sexual selection from other aspects of narrow-sense natural selection; all direct fitness collapses into reproductive success. Unfortunately, perhaps the most-repeated of Darwin’s definitions of sexual selection carries with it the sense of “reproductive success,” stating as it does that sexual selection “depends on the advantage which certain individuals have over other individuals of the same sex and species, in exclusive relation to reproduction” (Darwin 1871, p. 256; Table 2).

Taken at face value, this definition seems to focus on reproduction, rather than competition for mates (and then gametes). However, Darwin, in the preceding pages, discussed the difference between primary and secondary characters, noting that primary characters – which are needed to reproduce at all – are not the target of sexual selection, but secondary sexual characters are. Darwin also noted that it is “scarcely possible to decide” how to identify primary versus secondary sexual characters or separate out the effects of natural or sexual selection. So, the difficulties in ascribing forms of selection to traits were appreciated from the very outset of the intellectual history of sexual selection (Darwin 1871, p. 254; see also Fisher 1930, p. 132).

The relationship between reproductive success and sexual selection has been brought into renewed focus in recent years through a number of papers that have sought to redraw, or at least reexamine, ways in which sexual selection may influence females (Clutton-Brock 2007, 2009, 2010, 2017; Rosvall 2011; Stockley and Bro-Jørgensen 2011; Tobias et al. 2012). From a series of observations of – mostly – vertebrates, the idea that individuals of a given sex compete between themselves for resources, or social status, crucial for reproduction has been suggested to be a form of sexual selection, with the idea that “reproductive competition” comes to replace or broaden the notion of “competition for mates.” The problem with bringing female-female competition for reproductive resources, or indeed male–male competition for such resources, into sexual selection is that much of what all organisms do – male or female – is to compete for resources that sooner or later contribute to variation in reproductive success. In the broadest terms, this means that natural and sexual selection perfectly coincide, and the latter term becomes meaningless (Clutton-Brock 2007, 2009; Shuker 2010). If we wished to be less broad, then we would have to decide, a priori, how “close” to reproduction the resource was, or the competition for it was, when saying whether or not that competition engenders natural or sexual selection. Given that resource acquisition, and the life-history decisions that underlie how resources are allocated and competed for, may play out over the long-term (for example early-life effects: Lindström 1999; Jonsson and Jonsson 2011), teasing resource competition apart to assign to sexual or natural selection would be challenging. Instead, we suggest that components of “natural selection” are about being able to enter and remain in the fertilization game, where competition for access to mates and their gametes occurs, and that “sexual selection” is about how well you succeed in that game when compared with other same-sex contestants. Whilst fully recognizing the impact sexual selection has on the fitness of both sexes, we consider that competition for access to gametes is the focus of sexual selection, and that competition for resources – whether directly required for reproduction or not – is the focus of other components of fitness, unless the resource itself influences access to gametes. We will return to this topic below.

Another definition of sexual selection is again drawn from a particular reading of Darwin. Authors such as Padian and Horner have argued that sexual selection is characterized by sexual dimorphism of secondary sexual traits (Padian and Horner 2011a, 2011b, 2014a, 2014b; but see Knell et al. 2013; Borkovic and Russell 2014; Clutton-Brock 2017 also discusses the links between sexual dimorphism and sexual selection). Basing this idea on the detailed discussions of dimorphism given by Darwin, sexual selection is then defined either as the driver of sexual dimorphism, or only as occurring when there is sexual dimorphism (for example Padian and Horner 2014b). Put another way, without sexual dimorphism, sexual selection cannot act (or be said to have acted). This view of sexual selection, which we reject, has been discussed in particular in the paleontological literature, where evidence for sexual selection is notoriously difficult to find (for example Knell et al. 2013; Mallon 2017; Hone and Mallon 2017; O’Brien et al. 2018). However, that search is made harder still by stipulating a priori that fossils need to exhibit sexual dimorphism before sexual selection as a mechanism can come into play. Whilst there is much that could be discussed here, we will simply make the following points. First, there is clear evidence that sexual dimorphism can arise through narrow-sense natural selection on the sexes favoring ecological displacement (Shine 1989; Fairbairn et al. 2007). Second, the requirement for sexual dimorphism would mean that sexual selection cannot occur in isogamous species with separate mating types, or indeed in simultaneous hermaphrodites. Third, there is clear evidence of sexual selection in species that are sexually monomorphic, such as mutual mate choice for head ornaments in crested auklets (Jones and Hunter 1993). All in all, the requirement for sexual dimorphism for sexual selection to be said to be occurring is overly restrictive and lacks logical coherence when considering the competition for gametes in monomorphic species.

Along similar lines, but not so closely tied to sexual dimorphism per se, another alternative definition focuses on the different patterns of selection that arise on the two sexes (Carranza 2009), taking its cue from the title of Darwin’s book (1871: “selection in relation to sex”). As noted above, we should be careful in reading too much into the title of that book, and indeed the phraseology Darwin used at times, given the social circumstances under which he was writing, and the pressures applied by his publisher (Dawson 2007). Nonetheless, it is still not clear that viewing sexual selection in terms of differential patterns of selection on males and females is useful. Carranza (2009) argued for a definition of sexual selection based on the differences between the sexes in the action of selection. We think that this definition is a long way from Darwin’s conception of sexual selection and sits more neatly alongside the current body of sexual conflict theory (Parker 1979; Arnqvist and Rowe 2005). As a definition of sexual selection – at least as usually envisaged – it is problematic as the sexes may experience different patterns of selection on traits unrelated to reproduction (that is ecological causes of sexual dimorphism, as discussed above: Shine 1989). The corollary of this, as Carranza himself suggests, is that once we have separate sexual functions, then nearly all selection may be sexual selection. As such, sexual selection swallows natural selection (much as we saw natural selection swallowing sexual selection above). Therefore, whilst sexual selection may contribute to the opposing patterns of selection that arise on males and females (that is, sexual conflict), sexual conflict is most decidedly not the same thing as sexual selection (Shuker 2010; Kokko et al. 2014).

The final alternative definition of sexual selection we wish to consider explicitly is of a somewhat different nature: defining sexual selection in terms of one way in which it may be measured (Shuster and Wade 2003; see suggestion in Roughgarden et al. 2015). Shuster and Wade (2003; henceforth S&W) summarized a research programme initiated by Wade and coworkers that sought to quantify the differences between males and females in the variance in the number of mates each sex obtained (Wade 1979; Wade and Arnold 1980; Wade 1995). More formally, after Crow (1958, 1962), they show that the difference between males and females in the total opportunity of selection (Imales - Ifemales) is equal to what is termed Imates, where I represents the opportunity for selection. S&W state that Imates gives a “standardized measure of the intensity of sexual selection on males and the sex difference in strength of selection” (Shuster and Wade 2003, p. 29). Importantly, as the authors also note, the opportunity for selection is just that, only the opportunity. As such, Imates offers only an upper limit on selection.

The Imates approach has been the subject of a number of critiques and rebuttals down the years (for example Sutherland 1985; Downhower et al. 1987; Shuster and Wade 2003; Klug et al. 2010; Krakauer et al. 2011; Jennions et al. 2012; Henshaw et al. 2016). We do not wish to rehearse that debate here. Rather we wish to argue against the use of this measure as a definition of sexual selection. First, it is true that one cannot measure what one cannot define. But to define something by its measurement is a different thing entirely, risking circularity and reification. Second, and more problematic, is that the opportunity for sexual selection may also include random processes that influence the variance in obtaining gametes within a sex, so it may be nonzero even in the absence of any form of selection.

THE ROLE OF FEMALES IN SEXUAL SELECTION

Before we challenge our definition in the face of some real-world complications, we need to address the recent literature that has attempted to redraw the scope of sexual selection that we have outlined above, in particular in terms of accommodating female “reproductive competition” within a framework of sexual selection. Clearly females can and do compete for male gametes, but as noted above females also compete for other resources required for reproduction, and it is “reproductive competition” in this context that authors such as Clutton-Brock (see above) have suggested should be included with sexual selection. The argument is that by neglecting this kind of female–female competition, we are missing something important about sexual selection. We disagree and here we extend our argument introduced above.

First, our definition is explicitly sex and sex-role neutral: there is nothing in our definition that precludes females, or indeed hermaphrodites, or isogamous species, from the action of sexual selection. A recent example of females competing for access to male gametes comes from the common glowworm, Lampyris noctiluca (Hopkins et al. 2015; Borshagovski et al. 2019; Figure 1a–c). In this species, the female attracts a male by glowing. There is no resource other than sperm that successful females gain, and far from all females manage to attract a male. Thus, clearly female ability to attract males by their glow is a sexually selected trait.

Figure 1

Female competition for (a–c) gametes in the common glow worm, and (d–e) resources required for reproduction in the parasitoid wasp Goniozus legneri. (a) A female Lampyris noctiluca glowing to attract mates (photo: Jouni Valkeeniemi). (b) Experimental evidence that male L. noctiluca are attracted to brighter green LED lights, used here to mimic female displays (High brightness and Low brightness are 12.6 × 1012 and 7.0 × 1011 photons cm−2 s−1 respectively), whilst females with larger lanterns are also more fecund (c). (d) Female G. legneri fight for possession of a host on which to lay eggs, which can lead to vigorous struggles (inset; photos: Sonia Dourlot). (e) These contests are determined by differences in aggression between the owner and intruder, with the more aggressive individual, that initiates more interactions, winning the resource. The fitted line is from a logistic regression. (b-c) Figures redrawn from Open Access Data provided by Hopkins et al. (2015) under a CC0 1.0 Universal Public Domain Dedication license. (d–e) Figures reproduced from Goubault et al. (2006) with permission of The Royal Society.

Figure 1

Female competition for (a–c) gametes in the common glow worm, and (d–e) resources required for reproduction in the parasitoid wasp Goniozus legneri. (a) A female Lampyris noctiluca glowing to attract mates (photo: Jouni Valkeeniemi). (b) Experimental evidence that male L. noctiluca are attracted to brighter green LED lights, used here to mimic female displays (High brightness and Low brightness are 12.6 × 1012 and 7.0 × 1011 photons cm−2 s−1 respectively), whilst females with larger lanterns are also more fecund (c). (d) Female G. legneri fight for possession of a host on which to lay eggs, which can lead to vigorous struggles (inset; photos: Sonia Dourlot). (e) These contests are determined by differences in aggression between the owner and intruder, with the more aggressive individual, that initiates more interactions, winning the resource. The fitted line is from a logistic regression. (b-c) Figures redrawn from Open Access Data provided by Hopkins et al. (2015) under a CC0 1.0 Universal Public Domain Dedication license. (d–e) Figures reproduced from Goubault et al. (2006) with permission of The Royal Society.

Second, we do not consider that there is anything particularly special, intrinsically better, or more interesting, about sexual selection compared with other forms of selection. As such, we see no detriment – theoretically or empirically – in whether certain forms of competition are or are not included within the scope of sexual selection. What is more important is a coherent and robust definition of sexual selection and the resulting body of work. For instance, that female Goniozus wasps fight over access to hosts, and that this competition determines their ability to reproduce (Goubault et al. 2006, 2007; Figure 1d–e) resulting in narrow-sense natural selection, is just as fascinating as male red deer competing over access to a harem of females (Clutton-Brock et al. 1982) resulting in sexual selection. All organisms compete for the opportunity to reproduce, throughout their development, competing for the necessary food and other resources prior to reproductive maturity, fighting off parasites and predators, and so on. For female Goniozus wasps (Figure 1d), the final arena of reproductive competition shares much in common with contest competition over access to gametes, but what is being competed over is different: it is not gametes, but rather the resources needed for offspring to develop, in their case lepidopteran larvae. In this regard, the resulting selection is akin to selection on mammals to supply young with food and protection, the selection that we usually ascribe to narrow-sense natural selection. Apparent similarities (for example, in terms of contests) should not get in the way of the underlying logic of what is being competed for and why. As a hypothetical test (similar to the logic of potential reproductive rates: Clutton-Brock and Parker 1992), we may consider what would happen if an individual were given access to an extra set of gametes, what would happen to their fitness? In the red deer case, it would clearly increase male fitness, whereas in the female wasp case it would not, as the latter are limited by hosts, not gametes.

Another example is reproductive dominance. Reproductive dominance can achieve two aims. First, it can prevent other same-sex individuals from producing gametes at all. Well-known examples are found in animals that live in close-knit communities. For example, in ants and meerkats, dominant females may prevent other females in the colony from developing eggs or evict subdominants (for example Bourke and Franks 1995; Young et al. 2006; Clutton-Brock et al. 2008). In these instances, reproductive dominance is primarily about policing the opportunity to reproduce, limiting competition for the resources that offspring need, and so maximize relative fecundity of the dominant females. It is not about access to opposite-sex gametes, as subordinates have been precluded from even entering into that competition. Our conclusion, therefore, is that it falls under narrow-sense natural selection, and reproductive competition is a useful label to describe it (for example Clutton-Brock 2017).

Second, reproductive dominance can prevent other reproductively competent individuals in the group from mating and competing for fertilisation. In chimpanzees, all mature males are sexually competent, and indeed may attempt to copulate with a female on “consortships” away from the group, but the dominant alpha male will otherwise police the sexual behavior of subordinate males. Here, the reproductive dominance is sexually selected, as the dominant male is preventing subordinates from accessing opposite-sex gametes, subordinates who are otherwise able to inseminate females. Therefore, to repeat the hypothetical experiment, if a subdominant meerkat female is given (secretly) free access to male gametes, then it presumably will not improve her fitness, as she does not have sufficient status in the group to reproduce, regardless of access to gametes, whereas a free mating for a chimpanzee male is free fitness.

Similarly, in harem-holding sequential hermaphrodites, such as many wrasses, the largest and most dominant individual in a shoal becomes a male, whereas all the subdominant individuals reproduce as females. Here, all individuals are able to reproduce, but by being dominant, the male function gets exclusive access to the gametes of the other sex (hence sexual selection). Here the competition for access to gametes becomes tied up with which is the optimal sex to be if dominant.

We do not pretend that the dividing line will necessarily be easy to draw in real organisms in all cases. In the pipefish Syngnathus typhle, males provide care by carrying the embryos in a brood pouch on the tail. In this species, the hypothetical experiment above has already been carried out, and it shows that females benefit more from free access to the opposite sex and its gametes than males do, and that, on average, female egg production is almost twice as high as the male capacity to care for the eggs (Berglund et al. 1989), creating female-female competition for mating opportunities with males (Rosenqvist and Berglund 2011). Fish typically grow throughout their life and this species is no exception. Experimental work shows that small (young) females with free access to males reduce egg production and instead invest more in growth in the presence of an enclosed but visible large female, compared with when kept in the visual presence of a small female, or in the absence of other females (Berglund 1991). Arguably, this is a result of intrasexual dominance by the large female, combined with a male preference for larger females that produce more and larger eggs, favoring a life-history decision by the small female to allocate her resources to increasing her odds at gaining mating success later in life. We view this as an example of female-female competition for mates, in which success is achieved, not through combat, but through reproductive dominance and competition in attractiveness. Nevertheless, because males provide both gametes and care, females compete for both gametes to fertilize her eggs (sexual selection) and for male care (natural selection; male care increases the growth and survival of the female’s offspring: Nygård et al. 2019).

We reiterate that we are in no way seeking to exclude females from sexual selection, nor to give males special status in terms of the action of sexual selection. Indeed, after Eberhard (1996), we agree that females may still remain under-appreciated in terms of how females influence sexual selection on males, and in turn how sexual selection acts on females. For instance, the awareness that male mate choice (leading to sexual selection on females) is much more common than we used to think attests to this (Bonduriansky 2001; Edward and Chapman 2011). However, that does not mean that female-female reproductive competition should necessarily be considered sexual selection by default. For us, what matters is what is being competed over. It is competition for fertilizations that yields sexual selection, and by making this distinction we can recognize shared or different routes to fitness for females and males (Figure 2).

Figure 2

Sours: https://academic.oup.com/beheco/advance-article/doi/10.1093/beheco/arab055/6344800
Darwin Day Special! What is Sexual Selection?

Sexual selection

Mode of natural selection involving the choosing of and competition for mates

This article is about the evolutionary concept. For the artificial selection of the sex of offspring, see sex selection.

Sexual selection is a mode of natural selection in which members of one biological sexchoose mates of the other sex to mate with (intersexual selection), and compete with members of the same sex for access to members of the opposite sex (intrasexual selection). These two forms of selection mean that some individuals have greater reproductive success than others within a population, for example because they are more attractive or prefer more attractive partners to produce offspring.[1][2] In general, males benefit from frequent mating and monopolizing access to a group of fertile females. Females can have a limited number of offspring and maximize the return on the energy they invest in reproduction.

The concept was first articulated by Charles Darwin who wrote of a "second agency" of selection, in which competition between mate candidates could lead to speciation.[3] The theory was given a mathematical basis by Ronald Fisher in the early 20th century. Sexual selection can lead males to extreme efforts to demonstrate their fitness to be chosen by females, producing sexual dimorphism in secondary sexual characteristics, such as the ornate plumage of birds such as birds of paradise and peafowl, or the antlers of deer, or the manes of lions, caused by a positive feedback mechanism known as a Fisherian runaway, where the passing-on of the desire for a trait in one sex is as important as having the trait in the other sex in producing the runaway effect. Although the sexy son hypothesis indicates that females would prefer male offspring, Fisher's principle explains why the sex ratio is most often 1:1. Sexual selection is also found in plants and fungi.[4][5][6]

History[edit]

Darwin[edit]

Sexual selection was first proposed by Charles Darwin in The Origin of Species (1859) and developed in The Descent of Man and Selection in Relation to Sex (1871), as he felt that natural selection alone was unable to account for certain types of non-survival adaptations. He once wrote to a colleague that "The sight of a feather in a peacock's tail, whenever I gaze at it, makes me sick!" His work divided sexual selection into male-male competition and female choice.

... 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.[7]

... when the males and females of any animal have the same general habits ... but differ in structure, colour, or ornament, such differences have been mainly caused by sexual selection.[8]

These views were to some extent opposed by Alfred Russel Wallace, mostly after Darwin's death. He accepted that sexual selection could occur, but argued that it was a relatively weak form of selection. He argued that male-male competitions were forms of natural selection, but that the "drab" peahen's coloration is itself adaptive as camouflage. In his opinion, ascribing mate choice to females was attributing the ability to judge standards of beauty to animals (such as beetles) far too cognitively undeveloped to be capable of aesthetic feeling.[9]

Ronald Fisher[edit]

Ronald Fisher, the Englishstatistician and evolutionary biologist developed a number of ideas about sexual selection in his 1930 book The Genetical Theory of Natural Selection including the sexy son hypothesis and Fisher's principle. The Fisherian runaway describes how sexual selection accelerates the preference for a specific ornament, causing the preferred trait and female preference for it to increase together in a positive feedback runaway cycle. In a remark that was not widely understood[10] for another 50 years he said:

... plumage development in the male, and sexual preference for such developments in the female, must thus advance together, and so long as the process is unchecked by severe counterselection, will advance with ever-increasing speed. In the total absence of such checks, it is easy to see that the speed of development will be proportional to the development already attained, which will therefore increase with time exponentially, or in geometric progression. —Ronald Fisher, 1930

This causes a dramatic increase in both the male's conspicuous feature and in female preference for it, resulting in marked sexual dimorphism, until practical physical constraints halt further exaggeration. A positive feedback loop is created, producing extravagant physical structures in the non-limiting sex. A classic example of female choice and potential runaway selection is the long-tailed widowbird. While males have long tails that are selected for by female choice, female tastes in tail length are still more extreme with females being attracted to tails longer than those that naturally occur.[11] Fisher understood that female preference for long tails may be passed on genetically, in conjunction with genes for the long tail itself. Long-tailed widowbird offspring of both sexes inherit both sets of genes, with females expressing their genetic preference for long tails, and males showing off the coveted long tail itself.[10]

Richard Dawkins presents a non-mathematical explanation of the runaway sexual selection process in his book The Blind Watchmaker.[10] Females that prefer long tailed males tend to have mothers that chose long-tailed fathers. As a result, they carry both sets of genes in their bodies. That is, genes for long tails and for preferring long tails become linked. The taste for long tails and tail length itself may therefore become correlated, tending to increase together. The more tails lengthen, the more long tails are desired. Any slight initial imbalance between taste and tails may set off an explosion in tail lengths. Fisher wrote that:

The exponential element, which is the kernel of the thing, arises from the rate of change in hen taste being proportional to the absolute average degree of taste. —Ronald Fisher, 1932[12]

The female widowbird chooses to mate with the most attractive long-tailed male so that her progeny, if male, will themselves be attractive to females of the next generation—thereby fathering many offspring that carry the female's genes. Since the rate of change in preference is proportional to the average taste amongst females, and as females desire to secure the services of the most sexually attractive males, an additive effect is created that, if unchecked, can yield exponential increases in a given taste and in the corresponding desired sexual attribute.[10]

It is important to notice that the conditions of relative stability brought about by these or other means, will be far longer duration than the process in which the ornaments are evolved. In most existing species the runaway process must have been already checked, and we should expect that the more extraordinary developments of sexual plumage are not due like most characters to a long and even course of evolutionary progress, but to sudden spurts of change. —Ronald Fisher, 1930

After Fisher[edit]

Since Fisher's initial conceptual model of the 'runaway' process, Russell Lande[13] and Peter O'Donald[14] have provided detailed mathematical proofs that define the circumstances under which runaway sexual selection can take place. Alongside this, biologists have extended Darwin's formulation; Malte Andersson's widely-accepted[15] 1994 definition is that "sexual selection is the differences in reproduction that arise from variation among individuals in traits that affect success in competition over mates and fertilizations".[11][15] Despite some practical challenges for biologists, the concept of sexual selection is "straightforward".[15]

Theory[edit]

Reproductive success[edit]

Extinct Irish elk(Megaloceros giganteus). These antlers span 2.7 metres (8.9 ft) and have a mass of 40 kg (88 lb).

The reproductive success of an organism is measured by the number of offspring left behind, and their quality or probable fitness.[16]

Sexual preference creates a tendency towards assortative mating or homogamy. The general conditions of sexual discrimination appear to be (1) the acceptance of one mate precludes the effective acceptance of alternative mates, and (2) the rejection of an offer is followed by other offers, either certainly or at such high chance that the risk of non-occurrence is smaller than the chance advantage to be gained by selecting a mate. Bateman's principle states that the sex which invests the most in producing offspring becomes a limiting resource for which the other sex competes, illustrated by the greater nutritional investment of an egg in a zygote, and the limited capacity of females to reproduce; for example, in humans, a woman can only give birth every ten months, whereas a male can become a father numerous times in the same period.[17] More recently, researchers have doubted whether Bateman was correct.[18]

Modern interpretation[edit]

Darwin's ideas on sexual selection were met with scepticism by his contemporaries and not considered of great importance until in the 1930s biologists decided to include sexual selection as a mode of natural selection.[19] Only in the 21st century have they become more important in biology;[20] the theory is now seen as generally applicable and analogous to natural selection.[21]

A ten-year study, experimentally varying sexual selection on flour beetles with other factors held constant, showed that sexual selection protected even an inbred population against extinction.[22]

The handicap principle of Amotz Zahavi, Russell Lande and W. D. Hamilton, holds that the male's survival until and through the age of reproduction with seemingly maladaptive traits is taken by the female as a signal of his overall fitness. Such handicaps might prove he is either free of or resistant to disease, or that he possesses more speed or a greater physical strength that is used to combat the troubles brought on by the exaggerated trait.[23][24][25] Zahavi's work spurred a re-examination of the field and several new theories. In 1984, Hamilton and Marlene Zuk introduced the "Bright Male" hypothesis, suggesting that male elaborations might serve as a marker of health, by exaggerating the effects of disease and deficiency.[26]

In 1990, Michael Ryan and A.S. Rand, working with the Túngara frog, proposed the hypothesis of "Sensory Exploitation", where exaggerated male traits may provide a sensory stimulation that females find hard to resist.[27] In the late 1970s, Janzen and Mary Willson, noting that male flowers are often larger than female flowers, expanded the field of sexual selection into plants.[28]

More recently, the field has grown to include other areas of study, not all of which fit Darwin's definition of sexual selection. A "bewildering"[29] range of models variously attempt to relate sexual selection not only to the fundamental[29] questions of anisogamy and parental roles, but also to mechanisms such as sex ratios, parental care, having sexy sons, sexual conflict, and the "most-debated effect",[29] namely mate choice.[29]

Elaborated characteristics that might seem costly for their bearers (e.g., the tail of the swordfish Xiphophorus montezumae) do not always have an energetics, performance or even survival cost; this may be because "compensatory traits" have evolved in concert with the sexually selected traits.[30]

Toolkit of natural selection[edit]

Sexual selection may explain how characteristics such as feathers had survival value at an early stage in their evolution. Geoffrey Miller proposes that the feathers of proto-birds like Archaeopteryx were originally sexual ornaments. The earliest proto-birds such as Protarchaeopteryx had well-developed feathers but no sign of the top/bottom asymmetry that gives wings lift. One proposal is that the feathers served as insulation, helping females incubate their eggs. But if proto-bird courtship combined displays of forelimb feathers with energetic jumps, then the transition from display to aerodynamic functions could have been relatively smooth.[31]

Sexual selection sometimes generates features that may help cause a species' extinction, as has been suggested[31] for the giant antlers of the Irish elk (Megaloceros giganteus) that became extinct in Pleistocene Europe.[32] Or it may do the opposite, driving species divergence—sometimes through elaborate changes in genitalia[33]—such that new species emerge.[34][35][36]

Sexual dimorphism[edit]

Main article: Sexual dimorphism

Sex differences directly related to reproduction and serving no direct purpose in courtship are called primary sexual characteristics. Traits amenable to sexual selection, which give an organism an advantage over its rivals (such as in courtship) without being directly involved in reproduction, are called secondary sex characteristics.

In most sexual species the males and females have different equilibrium strategies, due to a difference in relative investment in producing offspring. As formulated in Bateman's principle, females have a greater initial investment in producing offspring (pregnancy in mammals or the production of the egg in birds and reptiles), and this difference in initial investment creates differences in variance in expected reproductive success and bootstraps the sexual selection processes. (Pipefish and Wilson's phalarope are classic examples of sex role reversal.[37]) Also, unlike a female, a male (except in monogamous species) has some uncertainty about whether or not he is the true parent of a child, and so is less interested in spending his energy helping to raise offspring that may or may not be related to him. As a result of these factors, males can be expected to be more willing to mate than females, while females are expected to be the ones doing the choosing (except in cases of forced copulations, which has been observed in numerous species, including mammals, birds, insects and fish[38]). The effects of sexual selection are thus often more pronounced in males than in females.

Differences in secondary sexual characteristics between males and females of a species are referred to as sexual dimorphisms. These can be as subtle as a size difference (sexual size dimorphism, often abbreviated as SSD) or as extreme as horns and colour patterns. Sexual dimorphisms abound in nature. Examples include the possession of antlers by only male deer, the brighter coloration of many male birds in comparison with females of the same species, or even more distinct differences in basic morphology, such as the drastically increased eye-span of the male stalk-eyed fly. The peacock, with its elaborate and colourful tail feathers, which the peahen lacks, is often referred to as perhaps the most extraordinary example of a dimorphism. Male and female black-throated blue warblers and Guianan cock-of-the-rocks also differ radically in their plumage. Early naturalists even believed the females to be a separate species. The largest sexual size dimorphism in vertebrates is the shell dwellingcichlidfishNeolamprologus callipterus in which males are up to 30 times the size of females.[39] Many other fish such as guppies are sexually dimorphic. Extreme sexual size dimorphism, with females larger than males, is quite common in spiders and birds of prey.

The maintenance of sexual reproduction in a highly competitive world is one of the major puzzles in biology given that asexual reproduction can reproduce much more quickly as 50% of offspring are not males, unable to produce offspring themselves. Many non-exclusive hypotheses have been proposed,[40] including the positive impact of an additional form of selection, sexual selection, on the probability of persistence of a species.[22]

Male intrasexual competition[edit]

See also: Female intrasexual competition

Male-male competition occurs when two males of the same species compete for the opportunity to mate with a female. Sexually dimorphic traits, size, sex ratio,[41] and the social situation[42] may all play a role in the effects male-male competition has on the reproductive success of a male and the mate choice of a female. Larger males tend to win male-male conflicts due to their sheer strength and ability to ward off other males from taking over their females. For instance, in the fly Dryomyza anilis, size shows the strongest correlation to the outcome of male-male conflicts over resources like territory and females.[43]

Influencing factors[edit]

Sex ratio[edit]

Japanese medaka, Oryzias latipes

There are multiple types of male-male competition that may occur in a population at different times depending on the conditions. Competition variation occurs based on the frequency of various mating behaviours present in the population.[41] One factor that can influence the type of competition observed is the population density of males.[41] When there is a high density of males present in the population, competition tends to be less aggressive and therefore sneak tactics and disruptions techniques are more often employed.[41] These techniques often indicate a type of competition referred to as scramble competition.[41] In Japanese medaka, Oryzias latipes, sneaking behaviours refer to when a male interrupts a mating pair during copulation by grasping on to either the male or the female and releasing their own sperm in the hopes of being the one to fertilize the female.[41] Disruption is a technique which involves one male bumping the male that is copulating with the female away just before his sperm is released and the eggs are fertilized.[41]

However, all techniques are not equally successful when in competition for reproductive success. Disruption results in a shorter copulation period and can therefore disrupt the fertilization of the eggs by the sperm, which frequently results in lower rates of fertilization and smaller clutch size.[41]

Resource value and social ranking[edit]

Another factor that can influence male-male competition is the value of the resource to competitors. Male-male competition can pose many risks to a male's fitness, such as high energy expenditure, physical injury, lower sperm quality and lost paternity.[44] The risk of competition must therefore be worth the value of the resource. A male is more likely to engage in competition for a resource that improves their reproductive success if the resource value is higher. While male-male competition can occur in the presence or absence of a female, competition occurs more frequently in the presence of a female.[42] The presence of a female directly increases the resource value of a territory or shelter and so the males are more likely to accept the risk of competition when a female is present.[42] The smaller males of a species are also more likely to engage in competition with larger males in the presence of a female.[42] Due to the higher level of risk for subordinate males, they tend to engage in competition less frequently than larger, more dominant males and therefore breed less frequently than dominant males.[44] This is seen in many species, such as the Omei treefrog, Rhacophorus omeimontis, where larger males obtain more mating opportunities and mate with larger females.[45]

Winner–loser effects[edit]

A third factor that can impact the success of a male in competition is winner-loser effects.[46] Burrowing crickets, Velarifictorus aspersus, compete for burrows to attract females using their large mandibles for fighting.[46] Female burrowing crickets are more likely to choose winner of a competition in the 2 hours after the fight.[46] The presence of a winning male suppresses mating behaviours of the losing males because the winning male tends to produce more frequent and enhanced mating calls in this period of time.[46]

Effect on female fitness[edit]

Male-male competition can both positively and negatively affect female fitness. When there is a high density of males in a population and a large number of males attempting to mate with the female, she is more likely to resist mating attempts, resulting in lower fertilization rates.[41] High levels of male-male competition can also result in a reduction in female investment in mating.[44] Many forms of competition can also cause significant distress for the female negatively impacting her ability to reproduce.[41] An increase in male-male competition can affect a female's ability to select the best mates, and therefore decrease the likelihood of successful reproduction.[47]

However, group mating in Japanese medaka has been shown to positively affect the fitness of females due to an increase in genetic variation, a higher likelihood of paternal care and a higher likelihood of successful fertilization.[41] Exposure to environmental estrogens, such as some herbicides, can confuse female choice of males.[48]

In different taxa[edit]

In detail, see Sexual selection in birds, mammals, humans, scaled reptiles, amphibians, insects, spiders

See also: Major histocompatibility complex and sexual selection

SEMimage of lateral view of a love dart of the land snail Monachoides vicinus. The scale bar is 500 μm (0.5 mm).
Human spermatozoa can reach 250 million in a single ejaculation

Sexual selection has been observed to occur in plants, animals and fungi.[5] In certain hermaphroditicsnail and slug species of molluscs the throwing of love darts is a form of sexual selection.[49] Certain male insects of the order Lepidoptera cement the vaginal pores of their females.[50]

A male bed bug(Cimex lectularius) traumatically inseminates a female bed bug (top). The female's ventral carapaceis visibly cracked around the point of insemination.

Today, biologists say that certain evolutionary traits can be explained by intraspecific competition—competition between members of the same species—distinguishing between competition before or after sexual intercourse.

Illustration from The Descent of Manshowing the tufted coquetteLophornis ornatus: female on left, ornamented male on right

Before copulation, intrasexual selection—usually between males—may take the form of male-to-male combat. Also, intersexual selection, or mate choice, occurs when females choose between male mates.[51] Traits selected by male combat are called secondary sexual characteristics (including horns, antlers, etc.), which Darwin described as "weapons", while traits selected by mate (usually female) choice are called "ornaments". Due to their sometimes greatly exaggerated nature, secondary sexual characteristics can prove to be a hindrance to an animal, thereby lowering its chances of survival. For example, the large antlers of a moose are bulky and heavy and slow the creature's flight from predators; they also can become entangled in low-hanging tree branches and shrubs, and undoubtedly have led to the demise of many individuals[citation needed]. Bright colourations and showy ornamenations, such as those seen in many male birds, in addition to capturing the eyes of females, also attract the attention of predators[citation needed]. Some of these traits also represent energetically costly investments for the animals that bear them[citation needed]. However, one must also consider that intersexual selection can occur with an emphasis on resources that one sex possesses rather than morphological and physiological differences. For example, males of Euglossa imperialis, a non-social bee species, form aggregations of territories considered to be leks, to defend fragrant-rich primary territories. The purpose of these aggregations is only facultative, since the more suitable fragrant-rich sites there are, the more habitable territories there are to inhabit, giving females of this species a large selection of males with whom to potentially mate.[52]

After copulation, male–male competition distinct from conventional aggression may take the form of sperm competition, as described by Parker[53] in 1970. More recently, interest has arisen in cryptic female choice,[54] a phenomenon of internally fertilised animals such as mammals and birds, where a female can get rid of a male's sperm without his knowledge.

Victorian cartoonists quickly picked up on Darwin's ideas about display in sexual selection. Here he is fascinated by the apparent steatopygiain the latest fashion.

Finally, sexual conflict is said to occur between breeding partners,[55] sometimes leading to an evolutionary arms race between males and females. Sexual selection can also occur as a product of pheromone release, such as with the stingless bee, Trigona corvina.[56]

Female mating preferences are widely recognized as being responsible for the rapid and divergent evolution of male secondary sexual traits.[57] Females of many animal species prefer to mate with males with external ornaments—exaggerated features of morphology such as elaborate sex organs. These preferences may arise when an arbitrary female preference for some aspect of male morphology—initially, perhaps, a result of genetic drift—creates, in due course, selection for males with the appropriate ornament. One interpretation of this is known as the sexy son hypothesis. Alternatively, genes that enable males to develop impressive ornaments or fighting ability may simply show off greater disease resistance or a more efficient metabolism, features that also benefit females. This idea is known as the good genes hypothesis.

Bright colors that develop in animals during mating season function to attract partners. It has been suggested that there is a causal link between strength of display of ornaments involved in sexual selection and free radical biology.[58] To test this idea, experiments were performed on male painted dragon lizards.[59] Male lizards are brightly conspicuous in their breeding coloration, but their color declines with aging. Experiments involving administration of antioxidants to these males led to the conclusion that breeding coloration is a reflection of innate anti-oxidation capacity that protects against oxidative damage, including oxidative DNA damage.[59] Thus color could act as a "health certificate" that allows females to visualize the underlying oxidative stress induced damage in potential mates.

Darwin conjectured that heritable traits such as beards and hairlessness in different human populations are results of sexual selection in humans. Geoffrey Miller has hypothesized that many human behaviours not clearly tied to survival benefits, such as humour, music, visual art, verbal creativity, and some forms of altruism, are courtship adaptations that have been favoured through sexual selection. In that view, many human artefacts could be considered subject to sexual selection as part of the extended phenotype, for instance clothing that enhances sexually selected traits[citation needed]. Some argue that the evolution of human intelligence is a sexually selected trait, as it would not confer enough fitness in itself relative to its high maintenance costs.[60]

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Sources[edit]

  • Andersson, M. (1994) Sexual selection. Princeton University Press. ISBN 0-691-00057-3
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  • Miller, G. F. (1998) How mate choice shaped human nature: A review of sexual selection and human evolution. In: C. Crawford & D. Krebs (Eds.) Handbook of evolutionary psychology: Ideas, issues, and applications. Lawrence Erlbaum, pp. 87–129
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  • Rosenberg, J. & Tunney, R. J. (2008). Human vocabulary use as display. Evolutionary Psychology, 6, 538–549
Sours: https://en.wikipedia.org/wiki/Sexual_selection

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