Abstract
A lot of demerits are associated with sexual selection in certain animals. Top of the list is the fact that this process has led to the development of certain characteristics which are costly to the carrier even to the point of increased chances of death by predation. This review however weighs the advantages derived from this process of sexual selection against this major argument and tries to point out that the benefits (individual and population benefits) derived from the development of these structures compensates for the cost incurred by its presence.
Introduction
Sexual selection is a special kind of natural selection within population in which members of one sex compete with other members of the same sex (intrasexual selection) to have access to mates of the other sex to mate with (intersexual selection). This selection permits some members of the population to have more reproductive success than other members of the same population because they possess certain traits lacking in the “less successful” ones. Sexual selection is a powerful force, strong enough to produce traits in organisms which not only gives them competitive advantage but also makes them more prone to attack from potential predators as can be seen among peacocks whose brightly coloured tail feathers attract not only members of the opposite sex but potential predators as well.
Brief history of Sexual Selection theory
The discovery of this theory of sexual selection has been attributed to Charles Darwin. In 1871, Darwin published a book “The descent of man and selection in relation to sex” where he proposed and justified this theory as a result of his observation that natural selection was not responsible for certain non-survival adaptations which he observed in some animals. This theory was further developed in the early 20th century by Ronald Fisher who gave insight into a process known as “Fisherman Runaway Model of Sexual Selection”.
A short review of past works
A lot has been said about sexual selection with many authors pointing out its major disadvantage which is higher chances of death as a result of the development of elaborate ornaments which are detrimental to survival and also sometimes have negative effects on reproductive success (Darwin, 1871). This trend, if genetically based can over the years lead to the further elaboration of such selected traits such as elaborate courtship behaviour, prominent plumage etc. in a population (Fisher, 1930). This review is aimed at showing counterarguments of these concerns.
Higher risk of death but greater reproductive success
As seen in the introduction above, traits which are preferred by female mates are sometimes costly to carry. However, several scientists including Charles Darwin (Darwin, 1871) have suggested that this cost is more than compensated for by its consequent reproductive success. In other words, if the gene coding for longer tails in the peacock has an increased chance of selection by females, its representation in the population will be increased in spite of its attendant high cost of survival and eventually the gene becomes fixed. That is, males will have longer tails even if it decreases their chances survival.
Handicap, biological fitness and choice
Furthermore, Zahavi, 1975 proposed the “Handicap Principle”. According to this principle, possession of handicapping morphology/behaviour signals biological fitness. He suggested that these traits which are often costly to develop and maintain but come about through intersexual selection confers handicaps on the selected individuals since their presence tests the quality of the males, hence male which are able to live with such handicap are seen as more fit by female mates (Zahavi, 1975). This can be said to go a long way guiding the female mate as to which male is the most suitable to father her offspring. As an example, the peacock tail which makes it more vulnerable to predation signals to a potential mate that it is fit because it is able to survive regardless of the cumbersome tail.
Undesirable genes elimination
In addition to assisting in choice making, the presence of this handicap goes a long way in eliminating or to a large extent, reducing the amount of undesirable genes in the population. This is based on the assumption that high quality males are able to survive better and longer despite these handicaps than the low quality males, thereby allowing them to mate with more females, and as a result, increase the spread of their desirable genes in the population (Zahavi, 1975). This would invariably produce high quality offspring.
These characteristics also present certain benefits to the females and the population
The possession of elaborate characteristics in males, beyond its “apparent disadvantages” also present certain benefits not only to the females but to the population (subsequent generations) as well (Andersson 1994 and Møller, 1994a). To obtain these benefits, females are often selective in choosing mates and prefer males with elaborate or obvious secondary sexual characters because they indicate to the females that such males are able to provide such benefits. These benefits could be direct (Kirkpatrick 1996; Iwasa and Pomiankowski 1999) or indirect (Andersson 1994; Jennions and Petrie 2000).
Direct benefits are the material benefits obtained by the female in the present generation and may be in the form of paternal care by the male, improved courtship feeding, anti-predator behaviour of the male, higher quality of a breeding territory or absence of directly transmitted disease and parasitic infections (Hoelzer 1989; Hamilton 1990; Andersson 1994; Møller 1994a). On the other hand, indirect (genetic) benefits include enhanced viability of offspring as a result of genetically based viability or resistance to parasites, increased attractiveness of sons to females in subsequent generations (Fisher 1930; Hamilton and Zuk 1982; Heywood 1989). These benefits (both direct and indirect) go a long way in promoting the survival and sustenance of the population.
Conclusion
In conclusion, females screen among males of varying genetic quality selecting the fittest for reproduction (Eberhard, 1996). This is necessary because females are surrounded by plenty willing males, therefore it is very important to choose the fittest mate especially as offspring number is not limited by the number of mating. As a result of research, we are well aware now that females make use of information signalled by sexually selected traits to decide which males are most suitable to be the father of their offspring (Andersson, 1994). This invariably acts as a filter to eliminate harmful genetic mutations thereby helping populations to thrive and avoid extinction in the long run.
Recommendations for further work
Much has been done by way of researching into sexual selection with certain population benefits of it becoming better understood. Sexual selection has also become very useful in explaining certain aspects of animal and indeed human behaviour as well as the evolution and elaboration of some characteristics such as evolution of ornaments (Petrie, 1994; Basolo, 1990), mate guarding (Birkhead, 1979) in certain animals e.g. the great crested flycatchers, the elephant seal and even among humans (Buss, 2002). A lot however still needs to be done to better understand the role played by sexual selection in the evolution process as well as the effects of natural selection on sexual selection driven evolution.
References
Andersson, M. (1994). Sexual Selection. Princeton University Press, Princeton N.J.
Basolo, A. (1990). Female preference predates the evolution of the sword in swardtail fish. Science, 250: 808 – 810.
Birkhead, T.R. (1979). Mate guarding in the magpie Pica pica. Animal Behaviour, 27:866 – 874.
Buss, D.M. (2002). Human Mate Guarding. Neuroendocrinology Letters Special Issue. Suppl. 4: 23-29
Darwin, C. (1871). The descent of man and selection in relation to sex. London, Murray.
Eberhard, W.G. (1996). Female Control: Sexual Selection by Cryptic Female Choice, Princeton University Press.
Fisher, R.A. (1930). The Genetical Theory of Natural Selection. ISBN 0-19-850440-3
Hamilton, W.D. (1990). Mate choice near and far. Am Zool, 30:341 – 352.
Hamilton, W.D. and Zuk, M. (1982). Heritable true fitness and bright birds: a role for parasites? Science, 218:384–387
Heywood, J.S. (1989). Sexual selection by the handicap mechanism. Evolution, 43:1387–1397.
Hoelzer, G.A. (1989). The good parent process of sexual selection. Anim Behav, 38:1067–1078
Iwasa, Y. and Pomiankowski, A. (1999). Good Parent and Good Genes Models of Handicap Evolution. Journal of Theoretical Biology, 200, 97-109.
Jennions, M.D and Petrie, M. (2000). Why do females mate multiply? A review of the genetic benefits. Biol Rev Camb Philos Soc, 75:21–64.
Kirkpatrick, M. (1996). Good genes and direct selection in the evolution of mating preferences. Evolution, 50:2125–2140
Møller, A.P. (1994a). Sexual selection and the barn swallow. Oxford University Press, Oxford
Petrie, M. (1994). Improved growth and survival of offspring of peacocks with more elaborate trains. Nature, 371:598 – 599.
Zahavi, A. (1975). Mate selection—a selection for a handicap. Journal of Theoretical Biology, 53 (1): 205–214.
