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One of the most significant ideas of Charles Darwin was the theory of sexual selection that he first published in The Decent of Man in 1871. It encapsulated his conviction that evolution resulted from differential reproduction rather than from differential survival (Miller, 2000). Unfortunately, Darwin's opinionated and conservative contemporaries mocked the evolutionary importance of female choice and disregarded it (Miller, 1998). Subsequently, the theory of sexual selection was neglected for nearly a century, whereas at present day it is one of the most researched areas of evolutionary biology (Andersson & Iwasa, 1996).Sexual selection theory and its development
Natural selection is the variation in the survival rate of organisms that arises from their differential ability to adapt to their environment (Miller, 1998). However, it is not sufficient to account for costly traits that do not enhance survivorship but rather jeopardize it. Darwin argued that in species with sexual reproduction traits that improved an organism's chances of mating success would be selected regardless of their negative effects on survival (Miller, 2000). These costly traits are mainly present in males as secondary sexual characteristics that are not actually needed for reproduction as such; however, they may give an advantage over other contestants in mate competition (Ridley, 1993). The most famous example of secondary sexual characteristics is peacock's tail but also enlarged male body size and genitals etc (Ridley, 1993). Darwin's sexual selection theory suggests that the disadvantages of having elaborate secondary sexual characteristics pay off because they convey a benefit in gaining access to females and increase reproductive success (Ridley, 1993). The disadvantages of pronounced secondary sexual characteristics include higher predation threat, increased energetic costs due to larger body size and competition that may lead to injuries or death; therefore, the extent of those traits is limited by their cost and by sexual selection itself if one favoured trait starts to compromise another (Andersson & Iwasa, 1996). Although Darwin never investigated the origins of female mate preferences, she discriminated between female choice and male competition (Miller, 1998). Alfred Russell Wallace, who discovered natural selection at the same time as Darwin was one of the many to criticize his sexual selection theory. He suggested that exaggerated male ornaments and traits did not have an adaptive purpose and did not arise from female choice but resulted from vitality and good genes that allowed males to spend extra energy and resources on display (Miller, 1998). Wallace further argued that females were under stronger natural selection to have less ornamentation to avoid attention from predators because females spend lot of their time in close proximity with their offspring (Miller, 2000).
Ronald Fisher proposed in 1930 that mate selection criteria were biological and thus under natural selection (Miller, 1998). He suggested that male sexual ornaments served as indicators of high fitness and good genetic quality and would be selected by females (Miller, 2000). Furthermore, he coined the term runaway sexual selection, which is essentially an evolutionary feedback mechanism where female preferences reinforce and perpetuate the traits selected for in males (Andersson & Iwasa, 1996). In Fisher's model the male trait was not deleterious at the start but with females preferring a particular characteristic it passed its optimal cost-benefit ratio and ultimately led to phenomenon such as peacock's tail (Ridley, 1993). In the case of runaway selection females choose males with the preferred trait, which will be passed on to the offspring who will then also possess the trait, making them more attractive mates; and subsequently increasing the number of the female genes in the new generation (Miller, 1998). On the other hand, according to Zahavi's handicap theory, which also tries to explain mate choice criteria, only males with good genes can survive a handicapping trait and females choose them as mates (Zahavi, 1975). Zahavi argues that selection will only favour handicap traits in males with good genes; thus, making the high cost of handicapping characteristic a reliable indicator of male's quality (Ridley, 1993). In any case, females benefit from having a better-quality offspring if they can use visual cues in detecting the quality of a potential male (Dawkins, 1989).
Trivers (1972) was the first to explain the different intensity of sexual selection in males and females through unequal amount of parental investment. The production of gametes is more costly than that of sperm and females invest more resources into offspring, therefore, females must be choosier because by mating with high-quality male they enhance the survivorship of their offspring (Miller, 1998). Trivers proposed that the number of available females limits male reproduction success, thus, males have to compete for the females (Miller, 1998). For example, there is a great difference in body size between male and female elephant seals where one male can guard circa forty females, resulting in strong male-male competition (Le Boeuf, 1974). Trivers theory can be applied to species like pharalorpes and wading birds where the parental investment is reversed with males taking care of the offspring and females being stronger, more aggressive and competing for the males (Jenni, 1974). In addition to Trivers' differential parental investment idea, Bateman's gradient also explains the differential intensity of sexual selection in males and females and it has been incorporated into the sexual selection theory (Andersson & Iwasa, 1996). In his studies with Drosophila, Bateman showed that sexual selection is typically stronger in males because the number of offspring fathered by a male increases proportionally with the number of females, whereas the amount of offspring remains the same for the female regardless the number of males she mates (Andersson & Iwasa, 1996).
Moreover, environmental factors have a strong effect on the development of mating systems and ultimately on the intensity of sexual selection (Emlen & Oring, 1977). The presence of polygamous and monogamous mating systems depends on environmental factors such as the availability of receptive mates and the distribution of resources in time and space, which affects their defensibility (Emlen & Oring, 1977). The ability of a male to protect territory or other resources attracts more females and causes differences in the mating success of males (Emlen & Oring, 1977). Emlen and Oring (1977) point out that the more one sex manages to monopolize resources the stronger becomes sexual selection and the more likely is the development of polygamous mating system. They also claim that polygamy is more prevalent in species with unequal parental investment, in which case one sex can devote time and energy on defending resources (Emlen & Oring, 1977). Darwin argued that secondary sexual characteristics are more developed in polygamous species because selection for the traits that enhance reproduction is greater; and, provided evidence from the comparison of polygamous and monogamous species by showing that in the former males tend to have brighter colouring and larger bodies, whereas in the latter the difference between males and females is less significant (Ridley, 1993). Furthermore, the mating system can differ between populations of the same species due to several factors that can alter the potential of monopolization, for example, variations in environmental setting, population structure and density, amount and distribution of resources (Emlen & Oring, 1977).
Andersson and Iwasa (1996) claim that sexual selection operates through several mechanisms: firstly, female and male choice of mate that has been demonstrated in numerous studies acts to favour traits that attract mates from the opposite sex; secondly, contests that can take the form of direct fighting and favour traits such as large body size, physical stamina, weaponry and other characteristics that enhance fighting ability in the competing sex; thirdly, endurance rivalry that promotes traits to retain reproductive activeness for longer to increase the possibility of mating. In addition to infanticide, coercion and sperm competition they also suggest scramble competition that promotes traits that help in finding a mate before others, such as earlier maturation or better locomotion skills (Andersson & Iwasa, 1996). As Andersson and Iwasa (1996) point out, the majority of research has concerned mate choice and mate competition, whereas other mechanisms of sexual selection remain poorly examined.
To sum up it can be said that sexual selection theory was neglected because of its concept that was too innovative at the time; also, there were difficulties in mathematically analysing sexual selection, which by now have been overcome (Miller, 2000). As it was in exile for the most part of 20th century when a lot of science and humanities subjects were advanced without taking sexual selection into account, many theories may need to be revised (Miller, 1998). At present day sexual selection is generally accepted as an important factor influencing evolution and subject of numerous research projects (Andersson & Iwasa, 1996).
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