Sexual Dimorphism and SEX.docx

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Department
Biomedical Engineering
Course
BMEN 515
Professor
William Huddleston
Semester
Fall

Description
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 sele
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