In Malthusian Relativity there is sexual selection on the level of sexual reproduction. This form of sexual selection arises from the density dependent competitive interactions that selects for a female choice for interactively superior males and a male choice for females that allocate the largest fraction of the male genome to the offspring. This causal relationship, where sexual selection arises from secondary sex traits of interactive quality and selects directly on the level of sexual reproduction, is opposite to the causal relation originally proposed by Darwin (1859, 1871) . According to the original proposal and apparently all subsequent studies (Andersson, 1994), sexual selection is an intermediate form of selection that arises from sexual reproduction and selects on secondary sex traits by means of mate choice or intrasexual contest for the opposite sex.

The models that support Darwin's sexual selection hypothesis are usually divided into the Fisherian run away process (Fisher, 1930) and indicator mechanisms (e.g., Fisher, 1915; Williams, 1966b; Maynard Smith, 1976; Zahavi, 1975; Hamilton, 1982; Hamilton and Zuk, 1982; Heywood, 1989; Hoelzer, 1989; Grafen, 1990, reviewed Andersson, 1994), also known as the good gene or handicap hypothesis. Several authors have shown that these two sexual selection mechanisms may lead to the evolution of energetically costly secondary sex traits. But most of the predictions are evolutionarily unstable in the way that the mechanisms fail to explain the evolution of energetically costly secondary sex traits at the expense of energetically non-costly secondary sex traits. If we allow for genetic variation in the energetic cost of the secondary sex trait, and costly and non-costly traits are potentially equally suited as indicators for female choice, then the models predict the evolution of non-costly secondary sex traits. This contrast to many species that have an energetic cost associated with secondary sexual characters (Andersson, 1994). Energetically costly secondary sex traits that evolve by density dependent competitive interactions for other resources than mates, however, tend to be evolutionarily stable. The requirement is that the individuals that invest less energy in the secondary sex traits tend to lose during competitive interactions with individuals investing more energy. This condition is fulfilled when the secondary sex traits are direct measures of interactive ability.

References

• Andersson, M. B. (1994). Sexual selection. Princeton: Princeton University Press.
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