BIO 15-Spiders&GiraffesWEB

BIO 15-Spiders&GiraffesWEB - Outline Why do...

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1 10/29/2008 1 15 15 -Spiders & Spiders & Giraffes Giraffes Sexual Selection Sexual Selection Continued Continued October 29 October 29 10/29/2008 2 Outline Outline • Why do giraffes have long necks? –Natural selection versus sexual selection –Science as a way of knowing— comparative observational studies • Jumping spiders—colouration and dance –Speciation via natural selection or sexual selection 3 10/29/2008 Why do giraffes have long necks? 4 10/29/2008 Giraffes’ Long necks food competition hypothesis : Giraffes compete for food. When food is scarce, giraffes with longer necks can out compete other animals (including other giraffes with shorter necks) natural selection favours giraffes with the longest necks
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2 5 10/29/2008 Giraffes’ Long necks ± What is a Lamarckian explanation for the giraffes’ long necks? 6 10/29/2008 Predictions (1) Neck length is variable. (2) Neck length is heritable. (?) (3) Giraffes feed high in the trees stretching their necks. 10/29/2008 7 Actual feeding habits of giraffes— never observed, just assumed! 10/29/2008 8
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3 9 10/29/2008 Giraffes’ Long necks sexual competition Alternative hypothesis: Male-male competition for mates Longer-neck males should win more competitions AND mate with more females Longer necks (reproductive advantage; heritable, variable trait) 10 10/29/2008 Sexual dimorphism--yes ± Male giraffes are around 15–17 feet tall at the horn tips, and weigh 1700–4200 lb. ± Females are one to two feet shorter and weigh
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BIO 15-Spiders&GiraffesWEB - Outline Why do...

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