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Sexual Selection.docx

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Biology 3466B
Yolanda Morbey

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Sexual Selection Males vs. Females Sexual Selection The strength of sexual selection differs in males than in females. Ch. 11 401-441 When mother makes a larger investment: Sexual selection – differential reproductive success due to  Females potential success is small variation among individuals in mating success  Realized success is limited by number of eggs (pregnancies)  Males potential success is large Objectives  Realized success is limited by # of female mates  Predict whether traits will be subject to sexual selectiLimits of reproductive success:  Explain why the strengths of sexual selection differ between the sexes  In the sex with greater parental investment limit is time  Contrast intrasexual and intersexual selection and their and resources effects on mating behaviors (competition and mate  In sex with lower parental investment limit is mating choice) success Sexual Dimorphism and Sex Limits of Reproductive Success in Newts Mating system Sexual dimorphism – a difference between males and females of a species  Males arrive at breeding ponds early and stay long  Darwin realized that individuals vary not only in their  Females visit ponds to mate with males  Females lay 300 eggs over several weeks or months success at surviving and reproducing but also in their  No parental care success at persuading members of the opposite sex to mate Results:  Failing to mate is the same as dying young (no genetic contribution)  Sexual selection was more powerful force in the evolution of males  Heritable traits in males that are associated with failure to Theory of Evolution by Sexual Selection mate will disappear If there is heritable variation in a trait that affects the ability to Heritable traits in males linked to mating success will become common obtain mates, then variants conductive to success will become more common over time  For males more than females in many species fitness is determined by access to mates Asymmetries in Sexual Reproduction Limits of Reproductive Success in Pipefish (Sexual  For sexual selection to explain differences between sexeRoles Reversed) it must act on the sexes differently Mating system  In many animals, eggs (pregnancies) are more expensive than ejaculates  Female lays eggs in brood pouch of makes Parental investment - Energy and time expended in constructing  Males provide all parental care until eggs hatch and caring for an offspring (measure of fitness) Results:  Mothers typically make larger parental investment to each  Females invested less energy which resulted in more offspring than fathers females with eggs to lay than males with space to accept o Eggs more expensive than sperm  Increases the reproductive success of the offspring them  More females than males fail to mate receiving it  Reproductive success of females is more strongly  Decreases the reproductive success that investing parent may achieve in the future (additional offspring) dependent on mating success  Sexual selection is more important in evolution of  In most animal species neither parents care for young females  Heritable traits associated with limiting mating success large body size in males despite the high cost and suboptimal tends to disappear size for survival.  Heritable traits associated with mating success become common Alt. Reproductive Tactics in Males  Pre-ejaculation – small territorial males solve problem Consequences of Limiting Fitness of disrupted copulation (by larger males) by ejaculating When sexual selection is strong for one sex and weak for another: ahead of time  Guard behavior – either of the female or territory  Members of the sex subjected to strong sexual selection  Sneak fertilization –(Salmon) hide near a female, then will be competitive dart out and spawn over eggs.  Members of the sex subjected to weak sexual selection  I.e. Sneak into another guarded territory to get past males will be choosy guarding it This can play out in 2 ways: Male-Male Comp: Sperm Competition Intrasexual selection – involves the competition (combat) among members of one sex for:  Producing as many sperm as possible is a form of competition  Direct control of mates  If a female mates with 2+ males the male who wins the  Direct control of resources race to the eggs has higher reproductive success  Top rank in dominance hierarchy  In bats this would mean bats should evolve larger testes if they are in large groups and must produce more sperm Intersexual selection – involves display among members of one  Sperm comp. has lead to other adaptations: may guard sec and choice by the other sex mate, prolong copulation, deposit copulatory plug or apply pheromones to reduce female at
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