1323440975_CHAPTER11_SPR

1323440975_CHAPTER11_SPR - EVOLUTION/LECTURE1 Evolution...

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EVOLUTION/LECTURE1 file:///E|/CH11-SEX-SELEC-SPRING-2008/CHAPTER11_SPR_2008.HTML[12/8/2011 2:54:36 PM] Evolution ( PCB 4674 ). Chapter 11. Sexual selection Main topics of lecture: 1.- Sexual dimorphism - Charles Darwin contribution 2.- Asymmetries in sexual reproduction 3.- Male-male competition: Intrasexual selection 3.1.- Combat 3.2.- Sperm competition 3.3.- Infanticide 4.- Female choice 4.1.- Gray tree frogs 4.2.- Choosy females may get better genes for their offspring 4.3.- Choosy females may benefit directly through the acquisition of resources 5.- Diversity of sex roles 6.- Sexual selection in plants 1.- Sexual dimorphism - Charles Darwin contribution 1.1.- Males and females are often strikingly different in size, appearance, and behavior. These differences between the males and females of a species is called sexual dimorphism. In this lecture we ask why sexual dimorphism occurs in such a great variety of organisms. It is a question Charles Darwin wrote half a book about!!! 1.2.- Differences between sexes are not adaptive. They do not help to improve the survival of fecundity of the individuals that possess them. For instance males of widow birds have long tails. If they can improve the survival and fecundity of a widow bird, then why do only the males have them? In many aspects we would expect that long tail feathers probably make male widow birds easier for predators to find and catch. In addition growing long tail feathers requires considerable energy. 1.3.- Sex provides a solution to the puzzle of sexual dimorphism. The two main challenges of organisms are to survive and to reproduce. For organisms with sexual reproduction this means that they have:
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EVOLUTION/LECTURE1 file:///E|/CH11-SEX-SELEC-SPRING-2008/CHAPTER11_SPR_2008.HTML[12/8/2011 2:54:36 PM] i.- To find a member of the opposite sex. ii.- To persuade this member to cooperate by mating and therefore to pass her/his genes to the next generation. 1.4.- Darwin recognized that individuals vary not only in their success at surviving but also in their success at persuading members of the opposite sex to mate. In its evolutionary consequences, failing to mate is equivalent to dying young . The victim makes no genetic contribution to future generations. 1.5.- Differences among individuals in success at getting mates were called by Darwin as sexual selection . We can use the same principles of natural selection and establish that if there is heritable variation in a trait that affects the ability to obtain mates, then variants with high mating success will become more common over time. 2.- Asymmetries in sexual reproduction 2.1.- Sexual reproduction creates different selection pressures for females versus males. Eggs (or pregnancy) are more expensive than ejaculates. In more general terms, females typically make a larger parental investment in each offspring than males. By parental investment we mean energy and time expending both in constructing an offspring and in caring for it. 2.2.- A good example to understand major differences in parental
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1323440975_CHAPTER11_SPR - EVOLUTION/LECTURE1 Evolution...

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