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AnisogamyDifferences between male and female sex cells (sperm and ova) which give rise to different mating strategies.Inter-sexual selectionPreferred mating strategy for the female – ‘quality over quantity’ means females are choosier and look for resources because consequences of a bad choice are more critical for her.Sexy sons hypothesis – female preference for a genetically ‘fit’ male ensures that desirable characteristics (e.g. height) are inherited by male offspring and propagated.Intra-sexual selectionPreferred mating strategy of the male – ‘quantity over quality’ means males compete to mate with as many females as possible, driving physical and psychological differences between males and females (dimorphism).Males are sensitive to indicators of youth and fertility in females because this suits their optimum mating strategy.In a lengthy article on the website of the journal Nature, Patricia Brennan of Yale University explains sexual selection and some associated concepts in detail.A summary of the article:Darwin initially failed to recognise the significance of sexual selection. He noted many examples in animal species of characteristics that seemed not to be adaptive (i.e. not beneficial to survival). In fact, some of these characteristics (such as the peacock’s tail) were positively threatening to the animal’s survival.Darwin’s theory of natural selection appeared unable to explain how these characteristics evolved. Eventually he realised that these features may threaten the animal’s survival, but they improve its reproductive success.Such characteristics evolved for two reasons:To increase the male’s chances of success in aggressive competition with other males (intra-sexual selection).To increase the male’s chances of being selected by the female (inter-sexual selection).The straight-laced Victorians of Darwin’s era readily accepted the notion of male competition. But the very idea that it is the female who determines mate choice was considered outrageous. It took another 80 years for it to be seriously considered.Reproductive behaviour is driven by anisogamy, the existence of two gametes that differ in size and form. Male gametes (sperm) are plentiful and female gametes (ova) are relatively scarce. Only one of each is needed to produce an offspring, so countless male sperm never fertilise an egg. Because ova require a lot of energy to produce, female reproduction is limited by the woman’s access to the resources needed to provide this energy (nutrition mainly). But because there are many more sperm than ova, male reproduction is limited by the man’s access to fertile females.The inevitable outcome of anisogamy is that males have to compete with each other for access to females, but a female can afford to be choosy and mate with only the ‘best’ (i.e. most genetically fit) male available. This choosiness brings direct benefits for the female. The fittest male can provide resources, protection from other males, help in raising children and even greater resistance to parasites and diseases (because the male is healthier than other males).The female also gains an important indirect benefit – her offspring inherit the very genes that made their father genetically fit. The male offspring who inherit these genes (from the male with the characteristics their mother preferred) are more attractive in turn to females of their generation, so their chances of reproducing are improved. This is – although Brennan doesn’t call it this – Fisher’s ‘sexy sons’ hypothesis.QuestionsWhy does the peacock’s tail appear to contradict the principles of natural selection?Can you think of or find any other male characteristics equivalent to the peacock’s tail in other species?Is there a characteristic or behaviour in human males that might improve his reproductive success? How?It is often said that male and female reproductive strategies are in conflict. Can you explain why?Explain the ‘sexy sons’ hypothesis.ReferencesBrennan, P. (2010) Sexual Selection. Nature Education Knowledge, 3, 79. ................
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