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Lecture

BGY Chapter_11.doc

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Biology
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BIOL 1020
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Chapter 11- SEXUAL Selection Sexual dimorphism- difference between the males and females of a species 11.1- Sexual Dimorphism and SEX • Natural selection cannot explain the differences between sexes of a species Sexual Selection- differential reproductive success due to variation among individuals in success at getting mates o A theory of evolution by sexual selection: If there is a heritable variation in a trait that affects the ability to obtain mates, then variants conducive to success will become more common over time Asymmetries in Sexual Reproduction • Mothers typically make a larger parental investment in each offspring than fathers Parental Investment- energy and time expended both in constructing an offspring and in caring for it o It is measured in fitness o Increases the reproductive success of the offspring receiving it o But also decreases the remaining reproductive success that the investing parent may achieve in the future by way of additional offspring • In more than 90% of mammal species, females provide substantial parental care and males provide little or none • In most animal species, neither parent cares for the young • Mated pairs of parents just make eggs, fertilize them, and leave them • When eggs are more expensive than ejaculates-when mothers make a larger parental investment than fathers-the factors limiting lifetime reproductive success will often be different for males versus females • A female’s potential reproductive success will be relatively small, and her realized reproductive success is likely to be limited more by the number of eggs she can make (or pregnancies she can carry) than by the number of males she can convince to mate with her • A male’s potential reproductive success will be relatively large, and his realized reproductive success is likely to be limited more by the number of females he can convince to mate with him than by the number of ejaculates he can make • Access to mates will be a limiting resources for males, but not for females o Under these circumstances, sexual selection (variation in mating success)- will be a more potent force in the evolution of males than in the evolution of females • To understand sexual dimorphism; we must quantify the relationship between number of mates and reproductive success for both males and females, is central to the theory of sexual selection Asymmetric Limits on Reproductive Success in Newts and Pipefish • Read Page 405-407 (top paragraph)- This example might be used on exam. Better safe than sorry. • In newts: heritable traits that are associated, in males, with failure to mate will tend to disappear, while heritable traits associated with mating success will tend to become common o Sexual selection is a more potent force in the evolution of males • In broad-nosed pipefish: heritable traits that are associated, in females, with failure to mate will tend to disappear, while heritable traits associated with mating success will become more common o Sexual selection is a more potent force in the evolution of females Behavioral Consequences of Asymmetric Limits on Fitness • In newts: for males reproductive success is limited by access to mates, and at any given time there are more males than females in the pond looking for love o In such cases, males will compete with each other for opportunities to fertilize eggs • For female newts: reproductive success is limited by capacity to make eggs, mating involves the commitment of a large investment, and there is an excess of willing partners o Therefore females will be selective about which partners they accept and which they reject • When sexual selection is strong for one sex and weak for the other we can predict that: o Members of the sex subject to strong sexual selection will be competitive o Members of the sex subject to weak sexual selection will be choosy Intrasexual selection- differential mating success among individuals of one sex due to interactions with members of the same sex (ex. Fighting with each other over the opposite sex); for example, differences in mating success among males due to male-male competition over access to females Intersexual selection- differential mating success among individuals of one sex due to interactions with members of the other sex; for example, variation in mating success among males due to female choosiness ; competition by advertising for their mates by singing, dancing, or showing off bright colours 11.2- Sexual Selection on Males: Competition • Sexual selection by male-male competition often occurs when individual males can monopolize access to females • Males may monopolize females through direct control of the females themselves or through control of some resource important to females, such as feeding territory or nest sites • Male-male competition can also occur for no apparent reason beyond simply impressing females • There are three forms of male-male competition: outright combat, sperm competition, and infanticide Combat • Outright combat is the most obvious form of male-male competition for mates • Intrasexual selection involving male-male combat over access to mates can favour morphological traits including large body size, weaponry, and armour • Male-male combat also selects for tactical cleverness • In iguanas: after sexual interaction, females lay their eggs and go and hide it far away from predators, guards the eggs for a few days, and then abandons it o Therefore parental investment by females consists mostly of producing eggs, and parental investment by males consists entirely of producing ejaculates • Read Example Combat of Iguanas Pg. 408-412 Sperm Competition • Male-male competition does not necessarily stop when copulation is over • The real determinant of a male’s mating success is not whether he copulates, but whether his sperm fertilize eggs  If an animal has internal fertilization, and if a female mates with two or more different males within a short period, then the sperm from the males will be in a race to the eggs • Male-male competition can take the form of sperm competition. If a female mates with two or more males, the male whose sperm wins the race to the eggs has higher reproductive success. • In sperm competition from two fathers, the more sperm an ejaculate contains, the more the father has chance of the offspring being his • In an experiment of mating medflies, two males were put in a cage with a male and in another cage a male and a female were placed. In the first cage, both males had their turn to inject their amount of ejaculates into the female. The female was dissected right after the process, and the numbers of sperms released by both males were counted. It appeared that males raised and mated in the presence of a potential rival ejaculated more than 2.5 times as many sperm as compared to only one male mating with a female. • Large ejaculates do contribute to victory in sperm competition and that male medflies dispense their sperm to balance the twin priorities of ensuring successful fertilization and conserving sperm • In addition to large ejaculates, sperm competition has apparently led to various other adaptations  Males may guard their mates, prolong copulation, deposit a copulatory plug, or apply pheromones that reduce the female’s attractiveness  During copulation in many species of damselflies, the male uses special structure on his penis to scoop out sperm left by the female’s previous mates • This strategy is proven to be highly effective Infanticide • In some species of mammals, competition between males continues even beyond conception o An example of this is lions: Male-male competiti
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