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Sexual Selection I. Motivation Sexual Dimorphism is Frequent Figure 11-1 Figure 11-3 The sexual dimorphism in Scarlet Tanagers, male, female Wesern Tanagers, male female long-tailed widowbirds Human males and females differ in height Why this sexual dimorphism Selection on survivorship and reproduction Ability to obtain mates: Sexual Selection: Different reproductive success due to variation among individuals in success at getting mates If there is heritable variation in a trait that affects to ability to obtain mates, then variants with higher success at obtaining mates will become more common over time II. Limits to reproduction differ between the sexes Guess What? With few exceptions, females invest much more time and energy in the production of young Figure 11-6 Asymmetries in sexual reproduction in rough-skinned newts Asymmetries in sexual reproduction in rough-skinned newts males females Exception to the Rule?” Asymmetries in sexual reproduction in broad-nosed pipefish Male with Kids (in pouch) Males Females Consequences of Asymmetric Limits on Fitness • Members of the sex subject to strong sexual selection will be competitive (Male-male competition) • Members of the sex subject to weak sexual selection will be choosy (Female Choice) • Bateman’s rule, males limited by access to mates, females limited by resources III. Natural Selection Limits Sexual Selection e.g., marine iguanas Larger iguanas expend more energy on metabolism Male marine iguanas in combat Large territorial males Alternative mating strategies in coho salmon IV. Male Access to Females Sperm Competition Sperm competition in damselflies Sperm Swimming Speeds Sperm morphology in rodents All for one and one for All! Infanticide V. Evidence for Sexual Selection on Trait: Female Choice Long tail feathers are a ball-and-chain for male red-collared widowbirds Female red-collared widowbirds prefer long-tailed males VI. Fisher’s Run Away Sexual Selection An example from UMD How runaway selection works in theory VII. Good genes model of sexual selection Are male gray tree frogs that give long calls genetically superior to males that give short calls?? Courtship and mating in hangingflies VIII. Origin of Trait Selection Water mite waiting in ambush for a copepod, male has found her A phylogeny of the water mite Neumania papillator and several related species. The boxes above the tips of the branches indicate which species have net-stance and which species have male courtship trembling. A colored box indicates the trait is present; an open box indicates the trait is absent. The two versions of the phylogeny show the two most likely scenarios for the evolution of these two traits. Redrawn from Proctor (1992). Lilies from from SW Aust. IX. Sexual Selection in Plants X. SEXUAL SELECTION IN HUMANS Kipsigis men and women A. Preindustrial Finnish Population B. Pimbwe, a horitcultural society in Tanzania unokais (killers) versus non-unokais among Yanomamö men Figure 11-50 Human Stature Taller Polish Men Have More Children: Importance of physical attractiveness in mate choice versus parasite prevalence in 29 human cultures GENETIC DIVERSITY REVEALED IN HUMAN FACES Hanne C. Lie, Gillian Rhodes, and Leigh W. Simmons: Evolution 2008 XI. Conclusion • • • • • • Sexual Dimorphism Sexual Selection Males limited by access to mates Females limited by resources Exceptions Universal, aspects found in plants and animals • YOU too may be a target for sexual selection • Very Kewl
