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Evolution of Sexual Selection: An Exploration of Reproductive Strategies in the Animal Kingdom

Dive into the fascinating world of sexual selection and reproductive strategies in various species. Explore the evolution of anisogamy, intra-sexual selection, and inter-sexual selection, shedding light on mating behaviors and evolutionary arms races.

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Evolution of Sexual Selection: An Exploration of Reproductive Strategies in the Animal Kingdom

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  1. You could study… • Birds in Costa Rica • Bats in Israel • Squirrels in Canada • Monkeys in Ethiopia • Pronghorn in Montana • www.animalbehavior.org

  2. Sexual selection

  3. Sex (the basics) • Asexual and sexual organisms • Sex and gender • Sexual selection • Selection affecting access to gametes http://www.micrographia.com/specbiol/cnidari/hydrozo/hydr0100/hydra-01.htm http://flickr.com/photos/billwalker/2521121709/

  4. Sex is a mixed bag • Costs of sex • Cost of meiosis • Breakup gene combinations • Mate acquisition • STDs • Benefits of sex • Adapt to changing environment • Abiotic • Biotic http://porpax.bio.miami.edu/~cmallery/150/life/meiosis.pics.jpg

  5. Hermaphroditism • Pathological in tetrapods • Simultaneous • Earthworms, gastropods, hamlet fish • Self fertilization is rare • Costs: Double the sex organs • Benefits: When optimal sex for relative size of mates changes • Sequential • Teleosts • Usually a one way ticket • Female  male in reef fish • When harem male dies • Only large males can defend a territory • Male  female in sea bass • Female reproductive output increases with size http://www.fsbio-hannover.de/oftheweek/196/800px-Mating_earthworms.jpg http://throughthestatic.wordpress.com/category/idiots/

  6. Anisogamy • The evolution of Anisogamy • Females and males • Consequences of anisogamy • Unequal parental investment • Unequal variance in fitness • Ornamented males • Choosy females • Easy males http://www.microscopy-uk.org.uk/thedark/eggbigx.jpg

  7. Kinds of sexual selection • Intra-sexual selection • Interference competition • Armaments (displays) • Size • Badges of status • Inter-sexual selection • Scramble competition • Locomotion and mate seeking • Mate choice • Ornamentation (displays) • Alternative tactics (e.g, coercion, female mimicry) • Sperm competition

  8. Intra-sexual selection

  9. The evolution of size dimorphism • Given interference competition • Directional selection for male size, armaments • Decrease in males’ mean viability • Evolutionary “arms race” • Distribution of male size as costs balance benefits • Mean investment is proportional to number of defended females (reproductive variance) http://www.innatmorrobay.com/elephant_seals_color.jpg

  10. Caveats on size dimorphism • Male size vs. male trait size • Shrinking females • Alternative strategies • Small male advantage http://crawford.tardigrade.net/journal/album/lillivatia.jpg From Legrand and Morse 2000 Biol. J. Lin. Soc. 71:643-664

  11. Precopulatory guarding • Amplexus • Female receptivity is limited, cyclic, asynchronous • Males are available to reproduce almost continuously • Mate guarding in birds http://www.treknature.com/gallery/Middle_East/Turkey/photo164737.htm From Birkhead 1979 Anim. Behav. 27:866-874

  12. Inter-sexual selection

  13. Which sex chooses? • The importance of the question • The operational sex ratio (OSR) • The potential reproductive rate (PRR) • The differential cost of reproduction • Covaries with OSR and PRR From Amundsen & Forsgren 2001 PNAS 98:13155-13160

  14. Direct benefits • Definition • Kinds • Fertilization success • Protection • Resources • Territories • Parental care • Nuptial gifts • Nutritious ejaculates • Sexual cannibalism http://www.biology.au.dk/trine.bilde.htm

  15. Indirect benefits Photograph by Medford Taylor

  16. Fisher’s runaway model Benefit of trait Male cost Different genes promoting trait / preference Genetic linkage Trait size Female preference Generations

  17. Quantifying the runaway model • Lande’s version • Quantitative genetic • Runaway when B / G > (v2 / w2 ) + 1 • In other words, when CVA (B) is high relative to VA of trait (G) with low variance in female preference (v2), weak utilitarian selection on trait (high w2) • Requires initial preference! • Natural selection • Drift • Sensory exploitation

  18. Testing for runaway • P1: Additive genetic variance for trait and preference • Generally supported • Traits are influenced by several loci • P2: Genetic co-variation between trait and preference • Method 1: Select for trait, see if preference also changes • Generally supportive • Method 2: Is there covariation among traits in brothers and preferences in sisters? • Supported in sticklebacks and crickets

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