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.

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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
Draft translation from: Evoluční biologie, 2. vydání (Evolutionary biology, 2nd edition), J. Flegr, Academia Prague 2009. The translation was not done by biologist, therefore any suggestion concerning proper scientific terminology and language usage are highly welcomed. You can send your comments to flegratcesnet [dot] cz. Thank you.