Sexual selection is a mode of natural selection in which some individuals out-reproduce others of a population because they are better at securing mates. In 1858, Darwin described sexual selection as an important process driving species evolution and as a significant element of his theory of natural selection, but this concept was only named in his 1859 book On the Origin of Species. The sexual form of 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.
... 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.
Darwin greatly expanded his initial three-page treatment of sexual selection in the 1871 book The Descent of Man and Selection in Relation to Sex. This 900-page, two-volume work includes 70 pages on sexual selection in human evolution, and 500 pages on sexual selection in other animals. In summary, while natural selection results from the struggle to survive, sexual selection emerges from the struggle to reproduce.
The sexual struggle is of two kinds; in the one it is between individuals of the same sex, generally the males, in order to drive away or kill their rivals, the females remaining passive; whilst in the other, the struggle is likewise between the individuals of the same sex, in order to excite or charm those of the opposite sex, generally the females, which no longer remain passive, but select the more agreeable partners.
- 1 Concept
- 2 Modern Interpretation
- 3 Criteria for reproductive success
- 4 Exponential growth in female preference
- 5 Sexual dimorphism
- 6 Sexual selection as a toolkit of natural selection
- 7 Viability and variations of the theory
- 8 In humans
- 9 History and application of the theory
- 10 See also
- 11 Notes
- 12 References
- 13 Further reading
- 14 External links
The sexual selection concept arises from the observation that many animals had evolved features whose function is not to help individuals survive, but help them to maximize their reproductive success. This can be realized in two different ways:
- by making themselves attractive to the opposite sex (intersexual selection, between the sexes), or
- by intimidating, deterring or defeating same-sex rivals (intrasexual selection, within a given sex).
Thus, sexual selection takes two major forms: intersexual selection (also known as 'mate choice' or 'female choice') in which males compete with each other to be chosen by females; and 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. The limiting sex is the sex which has the higher parental investment, which therefore faces the most pressure to make a good mate decision.
For intersexual selection to work, one sex must evolve a feature alluring to the opposite sex, sometimes resulting in a "fashion fad" of intense selection in an arbitrary direction. Or, in the second case, while natural selection can help animals develop ways of killing or escaping from other species, intrasexual selection drives the selection of attributes that allow alpha males to dominate their own breeding partners and rivals.
Sexual selection sometimes generates monstrously absurd features that, in harder times, may help cause a species' extinction, as has been suggested for the giant antlers of the Irish elk (Megaloceros giganteus) that became extinct in Pleistocene Europe. However, sexual selection can also do the opposite, driving species divergence - sometimes through elaborate changes in genitalia - such that new species emerge.
Although the driving force for both sexes is reproductive success, the two genders have different ways to maximize it: males benefit from frequent mating, including by monopolizing access to a group of fertile females, because new matings make them increase the number of eggs they fertilize. Females have a limited number of offspring they can have which is not increased by mating more frequently. They maximize the return on the energy they invest in reproduction by seeing their offspring grow into healthy adults, such as sons with well-developed, sexually attractive features, which sire many descendants, or fecund daughters. In addition, males often invest less of their energy in each individual offspring whereas female energy expenditures on gestation and parental care being much higher. Females have much more reason to be "picky". They need a way to choose males that are most likely to produce high-quality offspring.
Today, biologists would 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.
- 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. 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".
- After copulation, male–male competition distinct from conventional aggression may take the form of sperm competition, as described by Parker in 1970. More recently, interest has arisen in cryptic female choice, a phenomenon of internally fertilised animals such as mammals and birds, where a female will get rid of a male's sperm without his knowledge.
Female mating preferences are widely recognized as being responsible for the rapid and divergent evolution of male secondary sexual traits. 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.
Criteria for reproductive success
The success of an organism is not only measured by the number of offspring left behind, but by the quality or probable fitness of the offspring: their reproductive fitness. Sexual selection increases the ability of organisms to differentiate one another 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 tendency towards assortative mating or homogamy, providing a system by which a group otherwise invaded by diverse genes is able to suppress their effects and diverge genetically.
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.
Example: intersexual selection
The conditions determining which sex becomes the more limited resource 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. Thus, '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).
The peacock provides a particularly well known example of intersexual selection, where ornate males compete to be chosen by females. The result is a stunning feathered display, which is large and unwieldy enough to pose a significant survival disadvantage. Biologists have suggested that the layers of the ornate plumage of males provide a means of demonstrating body symmetry, such that peahens are "trying" to discover the health of the male or the quality of his genes. Diseases, injuries, and genetic disorders may impair the body's symmetry.
Bird species often demonstrate intersexual selection, perhaps because - due to their lightweight body structures - fights between males may be ineffective or impractical. Therefore, male birds commonly use the following methods to try to seduce the females:
- Colour: Some species have ornate, diverse, and often colourful feathers.
- Song: Male birdsong provides an important way of protecting territory (intrasexual selection).
- Nest construction: In some species, males build nests that females subject to rigorous inspection, choosing the male that makes the most attractive nest.
- Dance: Males dance in front of females. Cranes provide a well-known example.
As a propagandist, the cock behaves as though he knew that it was as advantageous to impress the males as the females of his species, and a sprightly bearing with fine feathers and triumphant song are quite as well adapted for war-propaganda as for courtship. —Ronald Fisher, 1930
In some bird species, both the male and the female contribute a great deal to offspring-care. In these cases, the male and female will be continuously assessing each other based on sexual characteristics. In the blue-footed booby, the females tend to choose males with brighter blue feet, because birds with brighter feet are younger, and thus have greater fertility and ability to provide paternal care. When researchers put make-up on the males' feet to make them look duller after the laying of the first eggs, their mates consequently laid smaller second eggs, which shows that female boobies continuously evaluate their mates' reproductive value. Males also vary their behavior based on the females' foot color. Males mated to females with brighter feet are more willing to incubate their eggs.
Exponential growth in female preference
In species where intersexual selection is active, as in many polygamous birds, sexual selection acts by accelerating the preference that specific "fashion" ornaments attract, causing the preferred trait and female preference for it to increase together, explosively. While Darwin had been criticised for simply accepting female whims as given, Ronald Fisher grasped the underlying mechanism in The Genetical Theory of Natural Selection, in a remark that was not widely understood for another 50 years:
... 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
Fisher's runaway process causes a dramatic increase in both the male's conspicuous feature and in female preference for it, 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 (left). 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. 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 will inherit both sets of genes, with females expressing their genetic preference for long tails, and males showing off the coveted long tail itself.
Richard Dawkins presents a non-mathematical explanation of the runaway sexual selection process in his book The Blind Watchmaker. Females who 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 corresponded 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
The female widow bird will desire 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 who will 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.
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
Since Fisher's initial conceptual model of the 'runaway' process, Russell Lande and Peter O'Donald have provided detailed mathematical proofs that define the circumstances under which runaway sexual selection can take place.
Example: intrasexual selection
A good example of intrasexual selection, in which males fight for dominance over a harem of females, is the elephant seal - large, oceangoing mammals of the genus Mirounga. There are two species: the northern (M. angustirostris) and southern elephant seal (M. leonina) - the largest carnivore living today. Both species show extreme sexual dimorphism, possibly the largest of any mammal, with southern males typically five to six times heavier than the females. While the females average 400 to 900 kilograms (880 to 1,980 lb) and 2.6 to 3 meters (8.5 to 9.8 ft) long, the bulls average 2,200 to 4,000 kilograms (4,900 to 8,800 lb) and 4.2 to 5 meters (14 to 16 ft) long. The record-sized bull, shot in Possession Bay, South Georgia, on February 28, 1913, measured 6.85 meters (22.5 ft) long and was estimated to weigh 5,000 kilograms (11,000 lb). The maximum weight of a female is 1,000 kilograms (2,200 lb) with a length of 3.7 meters (12 ft).
Males arrive in the colonies before the females and fight for control of harems. Large body size confers advantages in fighting. The agonistic behaviour of the bulls gives rise to a dominance hierarchy, with access to harems and breeding activity being determined by rank. The dominant bulls or "harem masters" establish harems of several dozen females. The least successful males have no harems, but may try to copulate with a harem male's females when the dominant male is not looking. A dominant male must stay in his territory to defend it, which can mean months without eating, living on his store of blubber. Some males have stayed ashore for more than three months without food. Two fighting males use their weight and canine teeth against each other. The outcome is rarely fatal, and the defeated bull will flee; however, bulls suffer severe tears and cuts. Males commonly vocalize with a coughing roar that serves in both individual recognition and size assessment. Conflicts between high-ranking males are more often resolved with posturing and vocalizing than with physical contact.
In the case of intrasexual selection, adorned males may gain a reproductive advantage without the intervention of female preference. This advantage will be conferred by weapons used in the process of resolving disputes, such as those over territorial rights. The use of sexual ornamentation as a signaling device to create a dominance hierarchy among males, also known as a pecking order, allows struggle to proceed without excessive injury or 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.
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, 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 colorful 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 dwelling cichlid fish Neolamprologus callipterus in which males are up to 30 times the size of females. Many other fish such as guppies also exhibit sexual dimorphism. Extreme sexual size dimorphism, with females larger than males, is quite common in spiders.
Sexual selection as a toolkit of natural selection
Sexual selection may explain how certain characteristics (such as feathers) had distinct survival value at an early stage in their evolution. One recent theory sees evolution as an "adventure quest" in which species develop complexity and novelty by acquiring modular capabilities through chance encounters in an evolutionary game. But this still leaves open the question of how natural selection initiated each module.
Geoffrey Miller proposes that sexual selection might have contributed by creating evolutionary modules such as Archaeopteryx feathers as sexual ornaments, at first. The earliest proto-birds such as China's Protarchaeopteryx, discovered in the early 1990s, had well-developed feathers but no sign of the top/bottom asymmetry that gives wings lift. Some have suggested that the feathers served as insulation, helping females incubate their eggs. But perhaps the feathers served as the kinds of sexual ornaments still common in most bird species, and especially in birds such as peacocks and birds-of-paradise today. If proto-bird courtship displays combined displays of forelimb feathers with energetic jumps, then the transition from display to aerodynamic functions could have been relatively smooth.
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. 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 competition among males in a tournament species. The winner, which 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.
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. 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 observations and 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, Thierry Lodé 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.
Several workers have brought attention to the fact that elaborated characters that ought to be costly in one way or another for their bearers (e.g., the tails of some species of Xiphophorus fish) do not always appear to have a cost in terms of energetics, performance or even survival. One possible explanation for the apparent lack of costs is that "compensatory traits" have evolved in concert with the sexually selected traits.
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 contrasts in sexual selection acting along with natural selection were significant in the geographical differentiation in human appearance of some isolated groups as he did not believe that natural selection alone provided a satisfactory answer. As an example for this, he implicitly mentions Steatopygia in Khoisan women.
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 enhances sexually selected traits. The German anthropologist Ferdinand Fellman argues that the emergence of human self-consciousness is due to an extended sexual selection, termed "emotional selection", bridging the gap between animal sexual behaviour and human erotic love.
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). 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. 
An uncertain example: the giraffe
The evolutionary origins of the giraffe's (Giraffa camelopardalis) long neck are controversial. The long-accepted "competing browser's hypothesis" originally put forth by Charles Darwin has been put into question. Originally, scientists believed that the elongation of the giraffe's neck had been a result of natural selection acting in relation to foraging behavior, where it was supposed that longer necks enabled favored individuals to gather food inaccessible to other animals. But even though the giraffe’s overall height is about 6 meters, it still typically feeds at about 2 meters above the ground. Moreover, the giraffe's kudu, impala, and steenbok competitors do not feed above 2 meters and prefer feeding at shoulder level as well, rather than at the maximum height they could reach.
An alternative explanation for the origin of long necks in giraffe is sexual selection. Male giraffe often neck with other males to exhibit dominance. There are six criteria that need to be satisfied for the exaggerated neck to be classified as a result of sexual selection. The characteristic should be more exaggerated in one of the sexes; it must be used to indicate dominance; have no direct survival benefits; cost the organism in terms of survival or other factors (e.g., energetics); positive allometry should be observed. But evolutionary history shows that increased neck length is not correlated to increases in other parts of the body, which would be expected from foraging selection, so sexual selection may be a more satisfactory explanation. Studies have failed to resolve the causes involved: perhaps the neck was a result of both or other forces.
History and application of the theory
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.
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