Darwin conceded that natural selection could not account for all evolutionary change. Even if modern concepts of mutation, gene flow, and genetic drift are considered, they cannot readily explain traits like the peacock's cumbersome tail, which baffled Darwin into quipping that the mere sight of it nauseated him.
To account for puzzling sorts of excessive and potentially handicapping traits, Darwin conceived of another type of selection, sexual selection, where the males and/or females of a population select mating partners based on characteristics which do not always have a conspicuous link to fitness.
Sexual selection is shaped by the difference between males and females in reproductive rates. Male mammals have a higher rate than females; that is, they can produce more offspring per unit of time. Females must wait to conceive again for weeks or months while carrying a fetus to term, but moments after conception males can conceive again with a different female.
There are also differences between sexes within species in parental investment strategies. In some species the male invests little in the offspring beyond his initial contribution of sperm, while the female is the sole caretaker from conception until the offspring is mature. Because the female carries the fetus and produces the milk, the female strategy is to invest largely in time, health, nutrition, and energy. Because the male does not carry the fetus or produce milk, the male strategy is best if he mates with numerous partners. The male strategy is further adaptive since male paternity is unknown without genetic testing. Males have alternative strategies in species that form monogamous pair-bonds, like gibbons, where males contribute largely to the offspring through territory protection and mate guarding.
In systems with sexes that have differing strategies, females tend to be the choosy sex. They make the most parental investment, take the most risk, and are the limiting resource for male reproductive success. Therefore, males are the competitive sex. They compete with other males for access to females. From an evolutionary standpoint, this scenario favors phenotypes that give a mating advantage to males, even if they lower their chances of survival. The most successful males are, for instance, the most aggressive, the strongest, the most beautiful, or the longest in canine teeth (competitive weaponry). The most successful females in such a system are the best choosers of males. Part of Darwin's difficulty in conceiving of sexual selection was cultural; he understood male competition but had a weaker grasp of female choice.
Sexual dimorphism—the differences in the size, color, or anatomy between males and females of a species—is exaggerated in species where mating strategy differences are exaggerated and mate competition is high. In monogamous apes like gibbons, the males and females are very hard to differentiate without close inspection. Their canine sizes and body sizes are the same and males have small testes because the need to produce copious amounts of sperm is diminished in the absence of competition with males.
Chimpanzees, however, have much larger testes because they compete for females in a multi-male, multi-female mating system. Chimpanzee males and females show differences in body size and canine size, with males having larger traits over all. In the gorilla system, males and females show even more body size dimorphism than chimpanzees. The dominant gorilla males are known as "silverbacks" because they display silver shocks of hair down their backs. Silverbacks usually have sole access to a group of females. Conversely, testes size in gorillas is very small which may correlate to success in preventing other males from mating with the females in the group.
Females choose mates that offer direct benefits to her like protection, food, and parental care. They can also gauge a male's fitness indicators which signal whether or not he has good genes that will increase the fitness of her offspring. Such indicators in humans have been shown to be as simple as clear eyes, shiny hair, glowing skin, a symmetrical face and body, dancing ability, and so on, but fitness indicators can also seem frivolous, silly, or even burdensome like the peacock's tail. The evolution of such traits are explained by Zahavi's handicap principle which states that traits can evolve that have no apparent function other than to advertise the fitness of the individual. That is, some traits simply provide means for bragging that the individual is so strong or so healthy that it can overcome a handicap.
Fitness indicators, whether they are handicaps or not, evolve through a process called runaway selection, that was conceived by R.A. Fisher in 1930. Say, for instance that a female has a preference for a larger than average nose. She will mate with large-nosed males and produce large-nosed offspring. Her sons will raise the average nose size in the population and her daughters will carry her gene for preferring large noses. If the cycle is allowed to continue and if large noses are indeed correlated to fitness, females will select for larger and larger noses, thus increasing the average nose size of the population. Runaway selection accounts for the evolution of sexually dimorphic traits as well as the preference for them.
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