a201-11f-17-MatingSexualSelection - Introduction to...

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Introduction to Biological Anthropology: Notes 17 Mating: Sexual selection Copyright Bruce Owen 2011 - Sexual selection : selection that favors traits that increase male success in mating - Sexual selection is just a kind of natural selection - but it is unusual in that it often favors traits that - are mostly or only expressed in males - and that seem to be useless or even harmful to the males’ survival - the huge tails of male peacocks, for instance - that make it hard for the peacocks to escape predators - but make them more attractive to female peacocks - or the nose of male proboscis monkeys - sexual selection favors traits like these if - if they improve the males’ reproductive success by increasing matings - more than they harm reproductive success by increasing predation or other costs - two kinds of sexual selection: - intrasexual selection: selection that occurs due to differences in success at mating that result from interactions between members of the same sex - the most common form is male-male competition - such as males fighting each other for access to females - the outcome of the interaction of the males determines which male mates; the female is not involved in the decision - classic example: intrasexual selection in baboons - led to large bodies and large canines among males - because bigger males with larger canines can drive other males away, so they get more access to females, so they leave more offspring - intersexual selection: selection that occurs due to differences in success at mating that result from interactions between members of opposite sexes - the most common form is female choice - such as females picking the most desirable male to mate with, based on some visible trait - classic example: intersexual selection among peacocks - led large tails among males, because the female peacocks prefer to mate with males with large tails, so the large-tailed males mate more often, so they leave more offspring - sexual selection can exert a much stronger evolutionary effect than selection based on different rates of survival - males who do not mate frequently may produce absolutely no offspring - this means absolutely none of their genes go into the next generation - from an evolutionary point of view, leaving no offspring is equivalent to dying young - this causes very severe selection against traits that lead to having no offspring - yet, it is common for males to have few or no offspring - meanwhile, males who mate often may produce many, many offspring
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Intro to Biological Anthro F 2011 / Owen: Sexual selection p. 2 - dramatically increasing the frequency of the alleles and traits in the next generation - example: imagine a species in which 80% of the males produce no offspring, and the remaining 20% father the entire next generation - these are realistic figures for many animals, from lions to baboons - the effect of this difference in just one generation would be equivalent to a drought that killed off 8 out of 10 males - a very drastic selection event
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a201-11f-17-MatingSexualSelection - Introduction to...

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