XV.4 Sexual selection can produce morphological structures or behavioral patterns that reduce the viability of their bearer

The action of environmental selection can lead to the formation of structures or patterns of behavior with a positive adaptive value, i.e. those that either directly or indirectly improve the chances of survival of organisms in their natural environment. In contrast, the action of sexual selection can lead to structures or patterns of behavior that are detrimental for their bearers, i.e. reduce their viability {10856}.

            The extremely long feathers in the cocks of argus pheasants (Argusianus argus), which apparently greatly hinder their bearers in flying and moving over the ground, are frequently mentioned as an example. The about 40 kg antlers of the Pleistocene elk Megaloceros giganteus constitute another frequently mentioned example. With a span of 3.5 meters, this weapon for combat between males represented a considerable burden for their bearers, either as a mechanical obstacle to motion in the natural terrain or as a weight that the males had to constantly drag around, and also as the amount of organic matter that they had to grow each year. This example is now frequently thrown into doubt - Megaloceros lived in a landscape without forests and the size of its antlers related to its overall body weight was not greater than that of the other members of the deer species, etc. (Gould 1974). However, a definitive answer to such questions could only be provided by an experiment comparing the rate of growth of a population of elks with antlers and without antlers occurring under otherwise identical conditions. 

            However, the viability of individuals need not be reduced only by hypertrophic body structure. Males could also live to a lower age because of brighter coloring, which increases the risk of attack by a predator. Probably for this reason, secondary sex traits are expressed in some species only at the time of reproduction.

            It has been observed amongst extinct mammals that the length of existence of a species is negatively correlated with the body size of the individual species. Simultaneously, some authors are of the opinion that the main evolutionary motor for an increase in body dimensions in mammals lies just in sexual selection (Mclain 1993). It has been observed for birds artificially introduced on individual islands that the probability of successful introduction is substantially lower for species with sexual dimorphism than for species without marked sexual dimorphism (Mclain, Moulton, & Redfearn 1995). Newer studies performed on North American birds have shown that species with greater sexual dimorphism, to be more exact dichromatism (differences in coloration of males and females) more readily become locally extinct (Doherty et al. 2003).

            The disadvantageousness of some traits acquired through sexual selection has also been observed in intraspecific comparative studies. For example, it has been found that the sexually most attractive guppy males have the lowest viability (Brooks 2000; Godin & McDonough 2003) (Fig. XV.2). However, the results of meta-analyses have shown that the degree of expression of secondary sexual traits is generally positively correlated with the viability of males (Jennions, Moller, & Petrie 2001). This result can have a number of causes. To begin with, the degree of expression of sexual traits can be determined directly by the fitness of the males, so that only extremely fit males can allow themselves to form more obvious secondary sexual traits. In addition, in other species, the formation of secondary sexual traits need not be influenced by the fitness of the individual, but this fitness can fundamentally affect the probability of whether males with extremely accentuated secondary sexual traits survive in nature and whether they can be included in comparative studies.s

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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.