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In order to leave an evolutionary legacy survival is not enough. Individuals must also reproduce. Over 90% of species reproduce sexually, meaning two individuals from each sex must mate in order to produce offspring. Reproduction is expensive and can exert an additional evolutionary pressure. Darwin defined this pressure as sexual selection. Sexual selection operates through some members of a species having an advantage over others in terms of mating. It is the selection for traits that are solely concerned with increasing the mating success of an individual.
Darwin’s findings in relation to sexual selection were published in his book The Descent of Man and Selection in Relation to Sex in 1871. Darwin observed that there are some characteristics that do not appear to help an organism adapt to its environment and are thus not explained by natural selection. He suggested that they feature in the process of sexual selection. He defined the process by saying that it ‘depends on the advantage which certain individuals have over other individuals of the same sex and species, in exclusive relation to reproduction’. His observations and analysis lead to the reasoning that sexual selection works in two main ways: either through competition among members of one sex for access to members of the other sex (combat) or through choice by members of one sex for certain members of the other sex (display).
Darwin proposed that this process of sexual selection would work in the following way. In the past when peacocks had ordinary colour and length tails, peahens (for some reason) showed a preference to mate with males with slightly longer and more flamboyant than average tails. Thus, the characteristic of slightly longer, more brightly coloured tails would be passed on to the next generation and over many generations the peacocks' tails would become longer and brighter. Thus, the ornate tail gives such an advantage in terms of mating success that it is selected for despite being a disadvantage in terms of general survival. Darwin thus argued that these flamboyant male characteristics were not, as believed at the time, due to a designer who had an aesthetic sense, but due to the need to attract a mate. Other examples where males are more striking than the females are found in fish, lizards and spiders.
Darwin’s main evidence for sexual selection was the fact that he found, from a comparison of a great number of species, that there is a greater difference between the sexes (greater sexual dimorphism) in polygynous species than monogamous. A polygynous species is a species where one male mates with several females, whereas in monogamous species a single male pairs with a single female. Darwin reasoned that in polygynous species secondary sexual characters will be more developed to enable the males to gain access to more females (via combat or display). Therefore, polygynous species should, and were indeed found to, have greater sexual dimorphism.
Darwin also carried out various experiments to test sexual selection. These included plucking game cocks’ tail feathers, cutting the “eyes” out of a peacock’s tail and shortening or artificially enhancing tail length and then, in each case, testing the effect on mating success.
Written by Emily Brown
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