After introducing sexual selection in The Descent of Man, Darwin explained how mate rivalry might take two forms. Individuals compete with members of their sex for access to the opposite sex in intrasexual selection. In most circumstances, this involves males competing for access to females.
Intersexual selection, on the other hand, consists of individuals of one sex striving to impress members of the other; in this situation, the emphasis is on females because they often need to be impressed before they will agree to mate. When compared to their female counterparts, the intrasexual selection is thought to be responsible for males acquiring weapons to compete with one other, such as more enormous fangs and horns, larger musculature, and a lower tolerance for violence.
Intersexual selection, on the other hand, consists of individuals of one sex striving to impress members of the other; in this situation, the emphasis is on females because they often need to be impressed before they will agree to mate. When compared to their female counterparts, the intrasexual selection is thought to be responsible for males acquiring weapons to compete with one other, such as more enormous fangs and horns, larger musculature, and a lower tolerance for violence.
In 1930, the English geneticist Ronald Fisher focused on the link between female choice and masculine decoration, reigniting interest in sexual selection. Fisher (1930) contended that ancient females would have been drawn to males with well-maintained tail feathers, which might indicate that they would be strong fliers.
Furthermore, the fact that such males had the time to maintain such feathers demonstrated more excellent foraging and other talents that would benefit survival. According to Fisher, females' feature choice must have some initial survival benefit. However, once the tail feathers (or whatever trait) were chosen, they might grow accentuated and evolve beyond their initial function. Females who preferentially mate with men with these inherited beautiful qualities would create more handsome sons since their kids are more likely to carry these genes. Females would be more inclined to choose such boys in the future.
According to Fisher, the essential attribute a guy can give a potential mate is his genes for attractiveness. This implies that once females have picked a character, it may get further accentuated with each generation since females will always be hunting for the most prominent, brightest example of that feature. Fisher referred to this as 'runaway selection' since the feature deviates from its intended role and is chosen only for its appealing attributes.
In this approach, Fisher was able to explain the enormous tail feathers seen in peacocks today. According to Fisher, such superplumes would eventually stop being exaggerated because of the survival costs; natural selection would hold runaway selection in control. In other words, there will be a trade-off between enticing females and the costs of attracting predators.
Bill Hamilton and his colleague Marlene Zuk argued that male adornments developed to show females that they are parasite-free. This is known as the parasite theory of female choice. Parasites can range from tapeworms and fleas to microbiological life forms like bacteria and viruses. Given that parasites kill more people than predators or competing conspecifics, the reasoning goes that females should choose males with extensive healthy features because they are more likely to pass on healthy genes to their kids. Put another way, if guys with the fewest parasites can make the most elaborate decorations, and if females choose males, their children should be disease resistant.
The parasite explanation varies from Zahavi's in that, while both rely on males providing accurate quality signals, the ornament is viewed as a development to convey health rather than as a hindrance directly. The consequence is clear: the more parasites you have in your ancestors, the more pressure you will feel to express your lack of them. The Hamilton-Zuk parasite idea poses a fundamental challenge concerning animal communication signals. Males must be honest signallers in order for this argument to hold water. This indicates that for females to benefit from the signals provided by male ornaments, these traits must correspond with parasite resistance rather than merely appearing to do so.
One area of research that supports Fisher's runaway selection concept is female sensory bias. Female sensory bias indicates that women pay special attention to specific male characteristics, and this attention bias is inherited as part of sexual selection. Unsurprisingly, female swordtail fish prefer males with the longest sword (an extension of the tail fin only males of the species have). Both Fisherians and suitable genres would expect this. What was striking was the discovery that females of a closely related species called platyfish, in which neither sexes have a sword, prefer males with a long sword.
In other words, these females prefer men of another species because they share the same sensory bias as their near relatives. The rationale for this strange favoritism is that male platyfish used to have a sword, which they lost pretty recently in evolutionary terms; nonetheless, females retain the sensory preference for this trait. According to Basolo, the sword of the swordtail fish is not a genuine health advantage but an attractive masculine trait.
However, if female platyfish have a sensory predilection for a sword-like tail, how can we explain its disappearance in their own species' males? Haines and Gould may have found the solution by researching guppies, a cousin of both swordtails and platyfish. They discovered that guppies with shorter tails could outrun a simulated predator better than those with longer tails, even though females preferred the longer-tailed males! Perhaps the progenitors of these three related species had to deal with distinct types of predator pressure.
Following an initial female preference for long tails in the common ancestor of all three fish species, platyfish and guppies may have relocated to places where the value of a shorter tail exceeded the benefits of having a sexy long tail due to selective predator pressure. This scenario may be an excellent example of how natural and sexual selection may tug in opposite ways and how men may be forced to make concessions.
If Basolo's research supports the Fisherians, the suitable genres may call on various studies to support their position. In tropical birds, for example, Zuk established that the amount of parasites a species possesses is precisely proportionate to the degree of ornamentation found in its males. Furthermore, only males with parasite resistance can produce the most eye-catching ornaments. Indeed, it is now recognized that flamboyant species are particularly disturbed by parasites among various animals, including fishes, frogs, insects, and birds. Females within these species significantly prefer males with the fewest parasites.
Darwin introduced the concept of sexual selection in 1871, which leads to features that help individuals gain access to mates and take two forms - intrasexual and intersexual selection. For females, sexual selection leads to choosiness over mates. Ronald Fisher suggested that ancestral females selected such features, while Amotz Zahavi proposed that males develop elaborate features to demonstrate their survival ability.
Hamilton and Zuk suggested that male adornments have evolved to demonstrate their lack of parasites to females. Many animal species reproduce asexually, but sexual reproduction has several costs that asexual reproduction lacks.
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