Sexual selection

The emergence of sexual reproduction and differentiation of individuals of one species into males and females led to the appearance of a new factor, sexual selection. For the individual, it is not important to simply survive to reproductive age, but it is also necessary to find a sexual partner (or the optimal sexual partner) for reproduction. Competition occurs amongst members of the same sex for a suitable sexual partner. This competition is generally accompanied by very intense selection, which is then termed sexual selection. The direction and intensity of sexual selection acting on both sexes can differ substantially. This leads, amongst other things, to different evolution of morphological traits in the two sexes, to the formation of secondary sexual traits (epigamic traits) and thus frequently to very marked sexual dimorphism. The existence of striking secondary sexual traits and the impossibility of explaining their emergence through the action of environmental selection led Darwin to differentiation of a second type of natural selection – sexual selection.
            The action of sexual selection can be extremely intense. It is not rare that, in a population consisting of half males and half females, a single male is the father of all the offspring. Thus, the other males have zero exclusive fitness. Selection pressures following from this type of selection can thus be stronger than the selection pressures of environmental selection and, as will be shown in this chapter, can lead to quite interesting phenomena.
; In most cases, sexual selection does not act with the same intensity on the members of the two sexes. Females are usually more selective and males less selective in choice of a sexual partner. Consequently, males are usually subjected to more intense sexual selection and thus more obvious secondary sexual traits tend to emerge amongst them. The cause of this asymmetry lies primarily in the different cost of production of microgametes and macrogametes and secondarily in the different cost of the two parent roles. Because a female invests more energy in the production of macrogametes from the very beginning of reproduction than a male into the production of microgametes, she must generally accept the future thankless role of the parent that invests more into feeding and bringing up the offspring than her partner. In organisms with internal fertilization, this tendency is further reinforced by the fact that the greatest part of the cost of reproduction, nutrition of the embryo, is, in its biological nature, not transferrable to the male. Asymmetry in expenditure of energy thus attains extreme values here. The fitness of the individual depends both on the number of offspring produced in a lifetime and also on their quality. Males can increase their fitness through greater sexual activity, while females basically do not have any reserves in this area and the numbers of their offspring do not increase with an increasing number of sexual partners (Fig. XV.1). Thus, males need not be as discriminating in the quality of their sexual partners and can even reproduce with females that are of inferior quality phenotypically or genotypically. In contrast, females must carefully differentiate the quality of the males with whom they will reproduce. The quality of their future offspring depends to the same degree on the quality of their own genes as on the quality of the genes of the male.
            Thus, the population contains a constant excess supply of males that are willing to reproduce over the supply of females that are willing to reproduce. This leads to strong intrasexual competition and thus more intense sexual selection amongst males.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more