17. Sexual Selection - Sexual Selection A form of natural...

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Sexual Selection
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A form of natural selection that depends “not on the struggle for existence in relation to other organic beings or to external conditions, but on a struggle between the individuals of one sex, generally the males, for the possession of the other sex.” Darwin, 1871
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“There are many other structures and instincts which must have been developed through sexual selection- such as the weapons of offence and the means of defence of the males for fighting with and driving away their rivals- their courage and pugnacity- their various ornaments- their contrivances for production of vocal or instrumental music- and their glands for emitting odours, most of these latter structures serving only to allure or excite the females.” - Darwin
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Sexual Selection is usually considered to have 2 aspects: Intrasexual Selection whereby members of one sex compete with each other for access to the other sex. (Male Competition) Intersexual Selection whereby individuals of one sex choose which individuals of the other to take as mates. (Female Choice)
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Female Choice
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Natural Selection has favored the evolution of 2 types of gametes: small and quick (sperm) large and relatively non-mobile (eggs)
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Bateman (1948) argued that males are more sexually competitive than females primarily because of the sexual asymmetry in gamete size. Males are limited by their success at inseminating females. Females are limited by their ability to produce gametes. Working in fruit flies ( Drosophila ) he showed that the intensity of sexual selection is greater on males due to their greater variability in mating success.
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However, Williams (1966) and Trivers (1972) showed that what controls the intensity of sexual selection and explains the evolution of sex differences in reproductive strategy is not just prezygotic investment by the sexes in gametes, but includes ALL the goods and services that contribute to the next generation. Nuptial Gifts Male Parental Care
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Aside: Seahorses (which are not actually horses!) are one of the best examples of male parental care since the males are the ones that incubate the embryos in a specialized brood pouch!
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So, it is not quite as simple as Male Competition/Female Choice Whichever sex contributes the MOST to the production and survival of offspring, sexually active members of this sex are in short supply and become the limiting factor. Furthermore: The more 1 sex contributes, the greater the competition should be among the sex that contributes least. The sex that contributes more has the greater risk in terms of fitness loss should the reproductive effort fail.
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For many, if not most species, this sets up a dramatic disparity or asymmetry and favors choice by the sex contributing the most. This can also be seen in physical as well as behavioral sexual dimorphism between the sexes
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Pervasive among ethologists is the idea that choices of mates is under some form of genetic control.
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