Sexual Selection

The sexual selection constitutes one of the two mechanisms of the Théorie of the evolution, that which is related to the “fight for the reproduction”, distinct and complementary to the “fight for survival” (or Natural selection).

Definition

The concept of sexual selection indicates the process according to which the competition between the individuals for the coupling is a factor of the evolution of some Trait S hereditary. It is about an intraspecific competition (between individuals of same a Espèce) and sex-dependant (she is exerted differently on the male and female individuals of same a Population). The sexual selection allows, for example, to explain:

of the phenomena of contrary appearance to the needs for survival, like the majestic tail of the Peacock
variety of the fauna of the Galapagos, where existed little the predatory ones; in such a case, the selection of reproduction takes the step on the selection of survival and can lead to the emergence of news Espèce S by sympatric speciation
the sexual Dimorphisme, i.e. the differences which exist enter the male S and the Femelle S of the same species

History of the concept

By publishing the Origin of the species (1859), Charles Darwin revolutionizes the Théorie of the evolution by proposing the mechanism of Natural selection based on the differential reproduction of the individuals according to their capacity to survive. Later, he adds another slope to his theory of the evolution, slope which he will baptize itself sexual selection , asserting that the competition between individuals for the sexuée reproduction can also be a major factor of evolution of certain unexplainable features within the only framework of the ecological selection, the most known example city by Darwin being that of the tail of the peacock.

In the England victorienne, Darwin had many difficulties of defending this aspect of his theory against his contemporaries, as vis-a-vis his colleague Alfred Wallace who had however simultaneously arrived at a similar formulation of the theory of the ecological selection. The majority of the scientists of the time indeed refused to see in the sexual selection another thing that a form of selection of survival related to the competition between the males of a species, refusing the idea that the choice of the females can exert any evolutionary pressure. The theory of the selection is seen of as much less accepted than Darwin develops it in a work entitled the Filiation of the man and the selection related to the sex where it defends as the idea as certain attributes human are pure products of the natural selection. It quotes dimophic features thus as the Barbe present at the only male individuals or the relatively weak human Pilosité compared with the others Mammifère S like examples of products of the sexual selection at the human being.

It is only during the 20th century, in particular thanks to work of Ronald Fisher in the Années 1930 but especially as from the years 1970, with work of Robert Trivers or Robert Williams that this aspect of the theory darwinienne will be fully accepted like a mechanism with whole share. Ronald Fisher contributes for example to renew the concept by applying statistical methods to the Génétique populations. It shows for example that a light skew in the preference of the females which would make them prefer males equipped with long feathers to males equipped with feathers shorter can lead to an evolutionary situation where from generation to generation, the plumage of the males increases because with each generation the most fertile males are which have the longest feathers in the population. In the same one, the skew preferably of the females is accentuated: that which chooses the most reproductive males are also that whose descendants will be most numerous. Thus in the demonstration of Fisher, a light skew in the preference of a sex for the attributes of the other can lead to an extreme situation up to the point where the reproductive advantage is compensated by the disadvantage in term of survival (risk to be located by a predator, for example).

During Years 1960, William Hamilton will continue work of Fisher. It will contribute for example to the Théorie of the red queen explaining the advantage of the sexuée Reproduction in what she makes it possible to resist in a more effective way the Parasite S.

The idea of unfavorable evolution will be thorough front by Amotz Zahavi which explains, in 1975, by its Théorie of the handicap that the fact why an attribute is a handicap involves precisely that he is preferred by the members of the other sex. Indeed, this handicap is also a Signal being the direct proof of the quality of the individual since this last is able to survive in spite of this handicap. Thus, for the species of the kind Peacock , plus the feathers of the tail are long, plus the male has success when well even they strongly handicap displacements of the animal.

Selection inter-sex and intra-sex

The sexual selection is particularly important among many groups of Vertébré S (Oiseau X, Mammifère S) at which the production of Gamète S by the female is relatively restricted. The access to the sexual partner is thus a big factor of the difference of fruitfulness between the individuals. The sexual competition can thus take two nonexclusive methods:

Competition for being more attracting for the members of the opposed sex, one speaks about intersexuelle selection
Compétition between individuals of the same sex to assume the access to the sexual partners, one speaks about intrasexuelle selection

The sexual selection in general results in increasing the sexual Dimorphisme. Many features, for example the size, the color, a powerful Musculature, can be not only one advantage for the collection of food while also constituting to be attractile the sexual important ones. The sexual selection is also responsible for the development of specific adaptations. These features, which give an advantage to an individual compared to his rivals at the time of the search for a sexual partner, are called secondary sexual characters . They can be several types:

morphological and Physiological S (bodies of seduction and/or duel between individuals of the same sex. Example: the wood of the stag)
ethologic (behavior of search of partner, defense of the Territory, bridal parade…)

The sexual differences directly related to the reproduction and not being used directly for research of a partner are called primary education sexual characters .

Sexual selection vs. ecological selection

The adaptations resulting from the sexual selection are independent even antagonistic with the adaptations determined by the ecological Sélection which is the intraspecific result of the competitions and interspecific for the food within the ecological niche and of the adaptation to survival (resistance to the Prédation, the Parasitisme, etc). For example the impressive one but cumbersome tail of the Paon is primarily useful for him for the parade in love. According to the Théorie of the handicap explains it is even because a feature is a risk factor for the survival which it will make the object of a sexual selection: individuals having interest to reproduce with those which are able to survive in spite of this handicap (which thus attests their capacities).

The combination of these two types of selections (sexual and ecological) on same the features is generally called Natural selection . In the general public, the natural selection is limited to its strictly ecological dimension and the sexual selection is sometimes simply ignored, but this seems to be a remainder of the prudery victorienne.

Pre selection and post-copulatoire

Even if it is less visible, the sexual selection can be prolonged after the act copulatoire. Indeed, before the coupling itself, at the time of the pre-copulatoire phase, the individuals are in competition to reach to the partner. But this competition does not cease after the Sex act took place. Thus, the female which is in general, at the Vertébré S, the sex which produces less Gamète S and for which the parental Investissement is highest can seek to reproduce with other males of the species. It is what is called the Polyandrie. In which case, the Spermatozoïde S of its various sexual partners will mix in his reproductive Système and will enter in competition (intra-sex) to fertilize the Ovule (S). From an evolutionary point of view, the female thus may find it beneficial to put in competition the gamètes several partners in order to reproduce with the best fertilisator. Consequently, it can be established a competition inter-sex, by which, one of the two partners (in general, the male) constrained the female not to have sexual relationships with other individuals.

The choice which the female via a selection exerts on the Gamète S males was baptized " choice female cryptique" .

Sexual selection at the human being

As soon as he proposes his theory of the sexual selection in 1871, Charles Darwin considers that this one applies as much to the mankind as with the other animal species. For example, the particular Pilosity of our species, quasi absent on the whole of the body but very present on the head (Barb and Hair X), constituted for Darwin a product of the sexual selection in the absence of selective advantage for survival. In the same way he explained the origin of the human Race S which did not seem to be the result of the natural selection. More recently, it was suggested that the Pénis of the man and in particular the Prépuce are the product of a competition post-copulatoire between the male individuals: the form of this Sexual organ is such as it acts as a vacuum cleaner which eliminates the Sperme possible other males which would have preceded it.

However, in the mankind, the mechanisms of the sexual selection remain difficult to highlight because of the cultural traditions which govern the sexual behaviors (Séduction, Sexual relation…) and of the intellectual capacities individual which superimpose on the conduits more Instinct ives. The research undertaken on the subject thus studied the phenomena associated with the possible forms with sexual selection in the mankind under two principal angles:

the intercultural Psychology which seeks in the sexual behaviors of the Universaux independent of the practices cultures;
the experimental Psychology, which studies the influence of various experimental factors on the choice carried out by the subjects questioned on their preference concerning a sexual partner.

The work undertaken by the evolutionary Psychologue David Buss near several tens of different cultures shows that, in the mankind, certain features are universally developed in a sexual partner: the kindness, intelligence, good health… In addition, certain physical characteristics are considered attracting more that others, like the Symétrie of the Visage or the Corps. But in addition to these factors common to both sexes, the theory of the selection predicts differences between the men and the women as for their preferences, because of the difference in the parental Investissement between the two sexes.

Indeed, in the majority of the cultures, the men are influenced by certain morphological criteria related to the Fertilité of the women, like a young person age or weak a Rapport size-hip. Contrary, with the eyes of the women, a partner is considered to be on average gravitational if it is older than themselves; compared to the men, the physical characteristics are less important than those related to the Social status.

The work undertaken in laboratory as seems to show as the human ones are also sensitive to different multiples factors in a more or less explicit way. A series of experiments thus highlighted that the human beings were sensitive to the odors. The women bring back being of better mood after having felt Stéroïde S produced by the glands Sudoripare S of the Aisselle S, such as one can collect them at adult men. Without seeing them, the women prefer the odors of the men whose morphology is symmetrical. Lastly, the odors seem also used (in way Implicite) to support diversity Génétique: an individual considers more pleasant one odor coming from a person of opposite sex whose system HLA has a greater dissimilarity with his.

If the dimorphism Anatomique between the two sexes suggests that the sexual selection has a significant role in the evolution of the anatomical features and Physiologique S of the human being, the consequences of this one on the sexual behaviors remain discussed. The most convincing elements come from the trans-cultural comparisons which show that the consequences Comportement ales of a form of sexual selection in the mankind seem quite universal -- what does not exclude from many variations as well between as inside even of each cultural tradition. On the other hand, the subjacent mechanisms neurocognitifs remain them still little explored.

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