Sexual selection

Illustration from The Descent of Man and Selection in Relation to Sex by Charles Darwin showing the Tufted Coquette Lophornis ornatus, female on left, ornamented male on right.

Sexual selection is the theory proposed by Charles Darwin that states that certain evolutionary traits can be explained by intraspecific competition. Darwin defined sexual selection as the effects of the "struggle between the individuals of one sex, generally the males, for the possession of the other sex".^[1] Biologists today distinguish between "male to male combat" or "Intrasexual Selection" (it is usually males who fight each other), "mate choice" or "Intersexual Selection" (usually female choice of male mates)^[2] and sexual conflict. Traits selected by male combat are called secondary sexual characteristics (including horns, antlers, etc.) and sometimes referred to as "weapons"; and traits selected by mate choice are called "ornaments". Much attention has been given to cryptic female choice,^[3] a phenomenon in internally fertilising animals such as mammals and birds, where a female will get rid of a male's sperm without his knowledge. The equivalent in male-to-male combat is sperm competition.

Females often prefer to mate with males with external ornaments—exaggerated features of morphology. These can plausibly arise because an arbitrary female preference for some aspect of male morphology initially increased by genetic drift, creating, in due course, selection for males with the appropriate ornament. 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.

Phylogeny of sexual selection and base conditions

Two male Giraffes,Giraffa camelopardalis reticulata necking in San Francisco Zoo.

The success of an organism is not only measured by the number of offspring left behind, but by the quality or probable success of the offspring: reproductive fitness. Sexual selection is the expansion on the ability of organisms to differentiate each other at the species level, interspecies selection.

The expansion of interspecies selection and intraspecies selection is a driving force behind species fission: the separation of a single contiguous species into multiple non-contiguous variants. Sexual preference creates a specialized tendency towards homogamy that provides a system by which a group constantly invaded by the diffusion of unfavourable genes may suppress ill effects.

When it is possible for it to be exercised usefully, 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 will be followed by other offers, either certainly, or at such high chance that the risk of non-occurrence will be smaller than the chance advantage to be gained by selecting a mate.

Intrasexual and intersexual selection

Male Northern Elephant Seals fight fiercely each year. Unsuccessful males will not mate at all, while successful males have harems of 30 to 100 females.

Sexual selection takes two major forms: intrasexual selection (also known as 'male–male competition') in which members of the less limited sex (typically males) compete aggressively among themselves for access to the limiting sex, and intersexual selection (also known as 'mate choice' or 'female choice') in which males compete with each other to be chosen by females.

With intrasexual selection it should be brought to mind that adorned males will gain reproductive advantage without the intervention of female preference and intersexual selection. This advantage will be conferred by weapons used in the process of resolving disputes, such as those over territorial rights. The use of male sexual ornamentation is primarily used in the search of asymmetries between rival males, contrary to what would seem most obvious (mortally wounding the opponent), since a high number of fatal combats over territory would result in a clear disadvantage. The use of sexual ornamentation is used as a signaling device (signalling theory) amongst males to create a dominance hierarchy, also known as a pecking order, without unneeded detriment and fatality. It is predominantly when two opposing males are so closely matched, as would be found in males not having established themselves in a dominance hierarchy, that asymmetries cannot be found and the confrontation escalates to a point where the asymmetries must be proved by aggressive use of ornamentation.

How often males will physically engage each other, and in what manner, can best be understood by applying game theory developed for biology, most notably by John Maynard Smith^[4].

In addition to conventional aggression, male–male competition may take the form of sperm competition.

However, 'sexual selection' typically refers to the process of choice (the limiting factor, which is typically females) over members of the opposite sex (the non-limited factor, typically males). This process is known as intersexual selection. Fisher pointed out that preference could be under genetic control and therefore subject to a combination of natural and sexual selection just as much as the qualities of the ornamentation 'preferred'.

An example of intersexual selection: the female peahen chose to mate with the male peacock who, in her mind, had the most beautiful plumage.

The conditions determining which sex is the limiting factor in intersexual selection can be best understood by way of Bateman's principle which states that the sex which invests the most in producing offspring becomes a limiting resource over which the other sex will compete. This can be most easily illustrated by the contrast in nutritional investment into a zygote between egg and sperm, and the limited reproductive capacity of females compared to males.

Geometric progression

In species where the reproductive success of one sex depends heavily on winning the concession of the other, as with many polygamous birds, sexual selection will act by increasing the degree of preference to which it is due, with the consequence that both the trait preferred and the intensity of preference will be increased together. This process causes a fervent and rapid evolution of both the conspicuous ornamentation and the preference for such, until arrested directly or indirectly for ecological reasons. Thus, in many cases a positive feedback loop of sexual selection is created, resulting in exorbitant physical structures in the non-limited sex, the most notorious example being the peacock (shown above). It is important to note that while a peacock may have exorbitant plumage, the peahen has even more exorbitant taste for such.^[5]

Initially to start the process there would need to be a correlation between the trait and higher fitness. For this example we use a hypothetical species of songbird, of the order Passeriformes. Tail length in these birds is correlated with fitness such that longer tails equate to higher fitness. As females of subsequent generations favor males with longer tails, the preference for them would cause them to be conspicuously favored after many generations.

The peahen will desire to mate with the most attractive Peacock so that her progeny, if male, will be attractive to females in the next generation. Additionally the Peacock will desire to mate with a Peahen that finds him attractive so that if the progeny is female, preference for his degree of ornamentation remains present in the next generation. Since the rate of change in preference is proportioned according to the highest average degree of taste amongst females, and that females desire to best other members of the sex, it creates an additive effect in the cyclical process that will yield exponential increases, in both sexes, if unchecked.

R.A.Fisher in The Genetical Theory of Natural Selection was the first to articulate this process in a game theoretic style treatment.

Since Fisher's initial conceptual model of the 'run-away' process, various others have continued the work on modeling an accurate mathematical proof. Notably R.Lande^[7] & P.O'Donald.

Sexual dimorphism

The Stag beetle demonstrates a classic case of sexual dimorphism. This plate is from Darwin's Descent of Man, with the male at top, female at bottom.

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. Classic examples of reversed sex-role species include the pipefish, and Wilson's phalarope. 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 will be 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 are typically more willing to mate than females, and so females are typically the ones doing the choosing (except in cases of forced copulations, which can occur in certain species of primates, ducks, and others). The effects of sexual selection are thus held to typically be 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 color patterns. Sexual dimorphisms abound in nature. Examples include the possession of antlers by only male deer, and the brighter coloration of many male birds, in comparison with females of the same species. The peacock, with its elaborate and colorful tail feathers, which the peahen lacks, is often referred to as perhaps the most extraordinary example of a dimorphism. The largest sexual size dimorphism in vertebrates is the shell dwelling cichlid fish Neolamprologus callipterus in which males are up to 30 times the size of females. Extreme sexual size dimorphism, with females larger than males, is quite common in spiders.

Viability and variations of the theory

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. Bright colorations 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; when a male peacock spreads its tail, it is beautiful, but very obvious (though this may actually be advantageous to the survival of the male's offspring and the breeding population as a whole; see below). Some of these traits also represent energetically costly investments for the animals that bear them. Because traits held to be due to sexual selection often conflict with the survival fitness of the individual, the question then arises as to why, in nature, in which survival of the fittest is considered the rule of thumb, such apparent liabilities are allowed to persist.

An often-cited theory published by R.A. Fisher in 1930 that attempts to resolve the paradox posits that such traits are the results of explosive positive feedback loops that have as their starting points particular sexual preferences for features that confer a survival advantage and thus "become established in the species." Fisher argued that such features advance in the direction of the preference even beyond the optimal level for survival, until the selection pressure of female choice is precisely counterbalanced by the resultant disadvantage for survival. Fisher further argued that the strength of the female preference tends to grow exponentially (leading to 'explosive' evolution of the characteristic) until finally checked by ecological selection, since the offspring of those females with the strongest preference typically fare better in reproducing than the offspring of females with weaker preferences. Any mutations for the preference opposite to the given characteristic, though tending to promote survival against ecological selection, nevertheless tend not to survive in the gene pool because male offspring that result from matings based on the preference are less sexually attractive to the majority of the females in the population, and thus infrequently chosen as mates. An equivalent way of expressing this is that if most females are looking, for example, for long-tailed males, then each female individually does better to select a long-tailed male, since then her male children are more likely to succeed. (The females do not actually have this thought process; this kind of "decision" is an evolutionarily stable strategy.)

Other theories highlight intrinsically useful qualities of such traits. Antlers, horns and the like can be used in physical defense from a predator, and also in show jousting or competition among males in a species. The winner, who typically becomes the dominant animal in the population, is granted access to females, and therefore increases his reproductive output. Antlers are not the only mechanism that can be used to counteract predation. Predators typically look for the eyes of their prey so they can attack that end of the creature. The conspicuousness of eyespots on many species of butterflies and fishes confuses predators and helps to prevent the prey from suffering serious damage.[1]

Another, more recently developed, theory, the Handicap principle of Amotz Zahavi, Russell Lande and W. D. Hamilton, holds that the fact that the male of the species is able to survive until and through the age of reproduction with such a seemingly maladaptive trait is effectively considered by the female to be a testament to his overall fitness. Such handicaps might prove he is either free of or resistant to disease, or it might demonstrate that this animal possesses more speed or a greater physical strength that is used to combat the troubles brought on by the exaggerated trait.

Zahavi's work spurred a re-examination of the field, which has produced an ever-accelerating number of 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. 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. In 1991, Anders Pape Møller, working with the tails of male barn swallows, introduced fluctuating asymmetry to the field. Fluctuating asymmetry, a concept previously invoked under natural selection, is based on the observations that healthier specimens have more left-to-right sided symmetry than less healthy specimens. Subsequently the theories of the "Gravity Hypothesis" by Jordi Moya-Larano et al. and "Chase Away" by Brett Holland and William R. Rice have also been added. In addition, 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.

In the past few years, the field has exploded to include many additional areas of study, not all of which are clearly included under Darwin's definition of sexual selection. These include cuckoldry, nuptial gifts, sperm competition, infanticide, physical beauty, mating by subterfuge, species isolation mechanisms, male parental care, ambiparental care, mate location, polygamy, and mechanisms that can only be called bizarre, including homosexual rape in certain male animals, cementing of females' vaginal pores by males in some lepidopteran insects, and insect penises specialized to remove any sperm packets from females which may have been deposited by previous suitors.

These theories are not mutually exclusive; combinations of them may also be considered.

Focusing on the effect of sexual conflict, as hypothezised by William Rice, Locke Rowe et Göran Arnvist^[8], Thierry Lodé^[9] underlines that the divergence of interest constitutes a key for evolutionary process. Sexual conflict leads to an antagonistic co-evolution in which one sex tends to control the other, resulting in a tug of war. Besides, the sexual propaganda theory only argued that mate were opportunistically lead, on the basis of various factors determining the choice such as phenotypic characteristics, apparent vigor of individual, strength of mate signals, trophic resources, territoriality etc and could explain the maintenance of genetic diversity within populations.

In humans

Charles Darwin conjectured that the male beard, as well as the relative hairlessness of humans compared to nearly all other mammals, are results of sexual selection. He reasoned that since, compared to males, the bodies of females are more nearly hairless, hairlessness is one of the atypical cases due to its selection by males at a remote prehistoric time, when males had overwhelming selective power, and that it nonetheless affected males due to genetic correlation between the sexes. He also hypothesized that sexual selection could also be what had differentiated between different human races, as he did not believe that natural selection provided a satisfactory answer.

Geoffrey Miller, drawing on some of Darwin's largely neglected ideas about human behavior, has hypothesized that many human behaviors not clearly tied to survival benefits, such as humor, music, visual art, verbal creativity, and some forms of altruism, are courtship adaptations that have been favored 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 enhance sexually selected traits.

Some hypotheses about the evolution of the human brain argue that it is a sexually selected trait, as it would not confer enough fitness in itself relative to its high maintenance costs (a quarter to a fifth of the energy and oxygen consumed by a human).^[10] Related to this is vocabulary, where humans, on average, know far more words than are necessary for communication. Miller (2000) has proposed that this apparent redundancy is due to individuals using vocabulary to demonstrate their intelligence, and consequently their “fitness”, to potential mates. This has been tested experimentally and it appears that males do make greater use of lower frequency (more unusual) words when in a romantic mindset compared to a non-romantic mindset, meaning that vocabulary is likely to be used as a sexual display (Rosenberg & Tunney, 2008).

The evolutionary biologist Richard Dawkins has speculated that the loss of the penis bone in humans, when it is present in other primates, may be due to sexual selection by females looking for an honest advertisement of good health in prospective mates. Since a human erection relies on a hydraulic pumping system, erection failure is a sensitive early warning of certain kinds of physical and mental ill health.^[11]

History and application of the theory

Victorian cartoonists quickly picked up on Darwin's ideas about the role of display in sexual selection. Here Darwin is fascinated by the apparent steatopygia in the new fashion for bustles. The woman asks him to "leave my emotions alone", a reference to Darwin's new book The Expression of the Emotions in Man and Animals.

The theory of sexual selection was first proposed by Charles Darwin in his book The Origin of Species, though it was primarily devoted to natural selection. A later work, The Descent of Man and Selection in Relation to Sex dealt with the subject of sexual selection exhaustively, in part because Darwin felt that natural selection alone was unable to account for certain types of apparently non-competitive adaptations, such as the tail of a male peacock. 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 two primary categories: male-male competition (which would produce adaptations such as a Bighorn Sheep's horns, which are used primarily in sparring with other males over females), and cases of female choice (which would produce adaptations like beautiful plumage, elaborate songs, and other things related to impressing and attracting).

Darwin's views on sexual selection were opposed strongly by his "co-discoverer" of natural selection, Alfred Russel Wallace, though much of his "debate" with Darwin took place after Darwin's death. Wallace argued that the aspects of it which were male-male competition, while real, were simply forms of natural selection, and that the notion of "female choice" was attributing the ability to judge standards of beauty to animals far too cognitively undeveloped to be capable of aesthetic feeling (such as beetles).

Wallace also argued that Darwin too much favored the bright colors of the male peacock as adaptive without realizing that the "drab" peahen's coloration is itself adaptive, as camouflage. Wallace more speculatively argued that the bright colors and long tails of the peacock were not adaptive in any way, and that bright coloration could result from non-adaptive physiological development (for example, the internal organs of animals, not being subject to a visual form of natural selection, come in a wide variety of bright colors). This has been questioned by later scholars as quite a stretch for Wallace, who in this particular instance abandoned his normally strict "adaptationist" agenda in asserting that the highly intricate and developed forms such as a peacock's tail resulted by sheer "physiological processes" that were somehow not at all subjected to adaptation.

Though Darwin considered sexual and natural selection to be two separate processes of equal importance, most of his contemporaries were not convinced, and sexual selection is usually de-emphasized as being a lesser force than, or simply a part of, natural selection.

The sciences of evolutionary psychology, human behavioral ecology, and sociobiology study the influence of sexual selection in humans, though these are often controversial fields. The field of epigenetics is broadly concerned with the competence of adult organisms within a given sexual, social, and ecological niche, which includes the development of mating competences, e.g., by mimicking adult behavior.

Criticism

Sexual selection has been criticised in a 2006 paper^[12] claiming that individuals did not compete strongly for mating opportunities, but rather the function of sex is mostly social. The paper, however, was heavily criticised for its factual inaccuracy, unwarranted assumptions, and neglect of recent research on sexual selection.^[13][14] Also, cultural critics have proposed that Darwin's ideas about sexual selection were strongly shaped by Victorian mores and may at times reflect a distinct chauvinistic bias.^[15]

See also

Notes

References

• Abraham, J.N. (1998) La Saboteuse: An Ecological Theory of Sexual Dimorphism in Animals. Acta Biotheoretica 46 23-35 [2]
• Abraham, J.N. (2005) Insect Choice and Floral Size Dimorphism: Sexual Selection or Natural Selection? J. Insect Behavior, 18 743-756 [3]
• Andersson, M (1994) Sexual selection. Princeton University Press. ISBN 0-691-00057-3
• Cronin, Helena (1991) The ant and the peacock: altruism and sexual selection from Darwin to today. Cambridge University Press.
• G Arnqvist & L Rowe (2005) Sexual conflict. Princeton University Press
• Darwin, C (1871) The Descent of Man and Selection in Relation to Sex. John Murray, London.

Eberhard, WG. (1996) Female control: Sexual selection by cryptic female choice. Princeton, Princeton University Press.

• Fisher, RA (1930) The Genetical Theory of Natural Selection. Oxford University Press, ISBN 0-19-850440-3, Chapter 6 [4]
• Lande, CF R (1981). Models of speciation by sexual selection on polygenic traits. Proc. Nat. Acad. Sci., USA, 78 3721-5 [5]
• Miller, GF (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.[6]
• Miller, GF (2000) The Mating Mind: How sexual choice shaped the evolution of human nature. Heinemann, London. ISBN 0-434-00741-2
• Rosenberg, J. & Tunney, R.J. (2008). Human vocabulary use as display. Evolutionary Psychology, 6, 538-549 [7]
• Lodé, Thierry (2006) La guerre des sexes chez les animaux. Eds Odile Jacob . ISBN 2-7381-1901-8

Further reading

Judson, Olivia (2003) Dr.Tatiana's Sex Advice to All Creation: Definitive Guide to the Evolutionary Biology of Sex. ISBN 978-0099283751

Jolly, Allison (2001) Lucy's Legacy - Sex and Intelligence in Human Evolution. ISBN 978-0674005402

Diamond, Jared (1997) Why is Sex Fun? The Evolution of Human Sexuality. ISBN 978-0465031269

External links

• Sexual Selection: Stanford University
• The Great Debate: Sexual Selection
• An Introduction to Sexual Selection: University of Siegen
• Intralocus Sexual Conflict Diminishes the Benefits of Sexual Selection
• Review of GF Miller's The Mating Mind