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Sexual Selection

Sexual Selection. Elaborate traits, songs, dances, fights. Sexual selection. Recall: Special case of natural selection Selection for enhanced ability to obtain mates Acts on phenotypes, so any genes that help produce the beneficial phenotype will increase in frequency in the population.

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Sexual Selection

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  1. Sexual Selection Elaborate traits, songs, dances, fights

  2. Sexual selection • Recall: • Special case of natural selection • Selection for enhanced ability to obtain mates • Acts on phenotypes, so any genes that help produce the beneficial phenotype will increase in frequency in the population

  3. Bateman-Trivers Hypothesis • Observation: Males have elaborate traits involved in courtship & mating, females do not • Hypothesis • Pattern: Females choose males based on traits; sexual selection acts more strongly on males than on females • Mechanism: “eggs are expensive, sperm are cheap”** Females invest lots, males invest little

  4. Fundamental Asymmetry of Sex • 2 consequences* • Because eggs are expensive, females produce relatively few young over lifetime. Fitness limited by access to resources. • Sperm is inexpensive, so males can sire limitless offspring. Fitness limited by # of females he can mate with.*

  5. Predictions? • Females invest lots, should choose mates carefully • Bigger, stronger, healthier, etc…why? • Males invest little, should compete with other males for females • Only limit to Reproductive Success (RS) are other males • Sexual selection is stronger on males to develop traits for attraction, courtship, or competition • Male RS should be more variable than female RS

  6. Support: Female choice • On what factors do female birds base their choice of mates? • Pretty colors! • Males have bright plumage or beaks due to ingestion of carotenoids • Carotenoids stimulate immune response & repair free radical damage • Must obtain carotenoids from diet

  7. Support: Female choice • Males have bright plumage or beaks due to ingestion of carotenoids • Suggests that the brightest beaks = best fed (good foragers or competitors) & healthiest • After stimulating immune system, enough carotenoids are “left over” to be used for producing color as well.

  8. Is that a realtrade-off? • Can a sickly, unhealthy bird have a bright beak? Is color an honest signal? • Ex: European blackbirds • Carotenoids produce beak color

  9. Is that a realtrade-off? • Experimental group - stimulate immune system by injecting RBC’s from sheep • Control group - inject with saline solution…why? • Results - experimentals dimmed, controls remained bright • Conclusion: Experimentals diverted carotenoids to boost immune system • beak color = honest signal of health

  10. Do they prefer brighter beaks? • Experimental - supplement with carotenoids • Control - no supplement; groups are brothers. Why? • Brothers are genetically, and therefore phenotypically, very similar • Allow females to associate with whomever

  11. Other traits females choose • Any display that suggests “I’m healthy & well-fed. I’ll provide advantageous alleles for your offspring” • Songs • Dances • Other courtship displays • Resource provisioning (nuptial gifts) • Protecting nest site

  12. Proof: Male-Male Competition • Male sexually selected traits are also those that enhance competitiveability • Giraffe necks? Bighorn sheep; wrestling snakes • Used in battles for females, or to defend territories with or without resources

  13. Elephant seals • Females haul out in large groups on small beaches • Males battle for territories on beaches • Males mate with females in their territories* • Actually fighting for females; winners monopolize matings

  14. Clicker Q • If males battle for territories, what size of males do you expect to win more battles? • Large • Intermediate • Small

  15. Clicker Q • If winners monopolize matings with more females, which males do you expect to have higher reproductive success? • Large • Intermediate • Small

  16. Proof: Asymmetry in R.S. • Few males leave lots of offspring • Most males leave none • What does this do to Ne? • Lowers Ne • Increase Ne • No effect

  17. Consequences ofsexual selection • In most animal species, ANY female that survives to adulthood will mate • Many males get NO mates at all • Sexual selection is more intense in males • Leads to sexual dimorphism • Any trait that differs between the sexes

  18. Examples of dimorphism

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