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How is sexual selection different from natural selection? Asexual or selfing population: Survive > grow > reproduce = fitness Outcrossing sexual population: Survive > grow > find a mate > reproduce = fitness Biological basis of sexual selection • Asymmetry of parental investment in offspring (female HI; male LO; anisogamy) • Conflict of genetic interests between male and female Biological basis of sexual selection Investment differential varies with mating/rearing strategies • Polygamy (polygyny, polyandry) • Monogamy • No parental care • Maternal care only • Biparental care • Paternal care only Biological basis of sexual selection For the most common mating/rearing systems: • Female fitness limited by egg production and/or capacity to rear young • Male fitness limited by mating opportunities • Bateman experiment (Evolutionary Analysis Fig. 9.4, p. 293) Angus John Bateman Bateman experiment Results of sexual selection For the most common mating/rearing systems: Females should be CHOOSY • Resources (e.g., food, territory) • Good genes • Sexy sons Males should be COMPETITIVE for access to females/eggs • Male-male combat • Sperm competition • Display (colors, vocalizations, ornaments, engineered structures) Male-male combat Males are: • Larger than females • Better armed and armored Ritual combat Examples: Sperm competition Bill Rice experiment What is the fate of a mutation that increases male competitiveness even if it is at the expense of long-term survival and reproduction of his mate? Sperm competition Bill Rice experiment: • Males and females have inherent conflicts of genetic interest • Genes that increase male fitness may be deleterious (even lethal) to their mates • The ‘battle of the sexes’ is a coevolutionary process Male display Ornamentation Lekking Looking for Mr. Goodbar or, How do females exercise choice? • Resources (e.g., food, territory) • Good genes • Sexy sons • Manipulation of offspring sex ratio • Extra-pair copulations in ‘monogamous’ females Direct acquisition of resources Male entices female with: Food Territory Good genes William Hamilton • Greater size, enhanced color, louder/more complex vocalization, larger ornaments reflect general health and vigor Amotz Zahavi • Ornament as handicap • ‘Harrison Bergeron effect’ Sexy sons R.A. Fisher’s ‘runaway sexual selection’ hypothesis • Genetic variation for male trait and female preference for that trait • Female increases her fitness (= no. of grandchildren) disproportionately by producing attractive sons • Male trait and female preference lead to assortative mating • A genetic correlation between the male trait and female preference is established -- how? • In each generation the female preference applies strong directional selection on the male trait to become more exaggerated, even if the trait has no adaptive value, and the correlated female preference also becomes exaggerated Gender bias in red deer Red deer (Cervus elaphus elk/wapiti) on the island of Rum in Scotland Studied continuously by Tim Clutton-Brock and his associates since 1971 Polygynous ‘harem’ mating system; stags (males) lek Dominance hierarchy among hinds (females) Gender bias in red deer Dominant hinds produce more sons than daughters. Since sex ratio is supposed to be under frequency-dependent selection with a 1 male:1 female stable optimum, what is going on? Extra-pair copulation in ‘monagamous’ females How might a ‘monogamous’ female benefit from extra-pair copulation?