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Psychology 3229A/B
Scott Mac Dougall- Shackleton

Chapter 3: Sexual Selection Darwin and Sexual Selection • Darwin argued that features which helped you to breed might paradoxically sometimes be selected for, even up to the point of shortening your life • Sexual selection applies to those characteristics that provide individuals with advantages in gaining access to mates Intrasexual and Intersexual Selection • Darwin outlined how this competition for mates might take two different forms • Intrasexual selection consists of individual competing with members of their own sex for access to the opposite sex • Intersexual selection consists of members of one sex attempting to impress members of the other • Intrasexual selection is generally regarded as being responsible for males developing weapons to compete with each other such as large teeth and horns and a lower threshold for aggression when compared to their female counterparts • Intersexual selection, however, is believed to lead to the evolution of sexual ornamentation such as brighter plumage and courtship display in males in order to impress females • For most species we can equate intrasexual selection with male-male competition and intersexual selection with female choice Do Females Have a Choice? • Darwin was unable to explain why female preference should arise • Anumber of theories were advanced which attempted to answer the question ‘why should they choose gaudy males who would surely attract greater attention from predators?’ Theories of Sexual Selection • Sexy Males and Parental Investment • Fisher argued that ancestral females would have been attracted to males which had, for example, well maintained tail feathers since these might signal that they are likely to be good flyers • Fisher, then, the most important trait that a male can offer a potential mate is his genes for being attractive • This suggests that once a feature has been chosen by females it may become progressively exaggerated with each generation since females will constantly be looking for individuals with the largest, brightest example of that feature • Fisher called this ‘runaway selection’ • Parental Investment • Trivers suggested that sexual selection is directly related to asymmetries between the sexes in the amount of effort that each parent puts into raising the offspring • He called this effort parental investment • For most species this disparity of numbers means that, while females are limited in having a comparatively small number of eggs that may be fertilized during their lives, males are limited only by the number of matings they are able to accomplish Chapter 3: Sexual Selection • Since females invest so much in their offspring, Trivers argued that they should be very choosy about which males allow to fertilize their eggs • For males, however, Trivers predicted their should be far less discrimination and choosiness • Trivers suggested that choosiness in females is a direct result of their greater investment in offspring • Female Choice and MaleAdornment • Zahavi suggested that males might develop ornaments not to look attractive but perversely, as an impediment in order to demonstrate their abilities to survive despite having such a handicap • Hence to Zahavi male adornments allow females to assess their ability to survive and hence they are real signals of genetic quality, known as the handicap hypothesis • The Parasite Theory and Honest Signaling • Hamilton and Zuk proposed that male adornments evolved to demonstrate to females that they are free from parasites • They call this the parasite theory of female choice • So the argument goes, males which develop an elaborate healthy feature should be chosen by females since they are likely to pass on healthy genes to their offspring • In order for this theory to stand up it is necessary for males to be honest signalers • This means hat, for females to benefit from the signals that male ornaments provide, it is necessary that these features really do correlate with parasite resistance rather than simply appearing to do so • Evaluating Theories on Female Choice and MaleAdornment • Currently the main sticking point surrounding sexual selection theory is whether males’ ornamentation has evolved simply to make them attractive to females or whether it serves as a signal of real quality • Matt Ridley refers to this debate as the Fisherians vs. good-geners • For the ‘good geners’it is necessary to demonstrate that male adornments are widespread real signals of health and disease resistance in the animal kingdom • For the ‘Fisherians’it is necessary to demonstrate tha
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