Sexual SelectionDifferential = Survival + ReproductionReproductionDifferential = Survival + ReproductionReproductionWhy such exaggerated and costly traits?Gradual selection, stepwise accumulations,that provide fitness advantage.Advantage in mating above and beyondcost to survival.Darwin’s Other Big IdeaThree postulatesMallards – Linnaeus - Different species!Male FemaleDarwin’s Other Big Idea• Elaborate males, cryptic femalesElephant SealsMales fightingDarwin’s Other Big Idea• Elaborate males, cryptic females• Males appear to compete for femalesDarwin’s Other Big Idea• Elaborate males, cryptic females• Males appear to compete for females• Females appear to choose malesSexual SelectionStepwise preferenceand elaboration of traitsThe male birds in the video have elaborate ornaments because1. more-ornamented males survive longer than less-ornamented males.2. bright ornaments distract predators from offspring, increasing the reproductive success of ornamented males.3. female birds prefer to mate with more-ornamented males, increasing the reproductive success of ornamented males.4. more-ornamented males are more likely to win contests among males.5. Not enough information to distinguish among the above. Females are more likely than males to be choosy about their choice of mate because1. in most species, males outnumber females.2. females must protect their larger parental investment.3. males devote so much energy to competition with other males that little is left over for mate choice.4. the health and strength of a male’s mate do not affect his reproductive success.5. reproductive success is more important for females than for males.6. all of the above.Sexual selection -- a special type of natural selection:• Selection for enhanced ability to obtain mates.• Occurs when individuals in a population differ in ability to attract mates.• First described by Darwin; theory elaborated by Bateman (1948) and Trivers (1972).Sexual selection: pattern and process• pattern: seems to act more strongly on males; traits that respond to sexual selection are more elaborate in males. Sexual dimorphism• process: consequence of differences between the sexes in parental investment. Testing the theory: elephant sealsTesting the theory: elephant seals• extreme sexual dimorphism in size.– males average 4 times heavier than females (weigh up to 3 tons)• move onto land to breed• males fight over ownership of patches of beach where females congregateIs sexual selection stronger on male elephant seals? • Le Boeuf et al marked individuals in a population of seals and tracked lifetime reproductive success of males and females.If sexual selection is indeed stronger on male than on female elephant seals, we would predict that1. variation in reproductive success among males is lower than variation among females.2. variation in reproductive success among males is higher than variation among females.3. compared to males, a higher proportion of females dies without reproducing.4. the most successful reproducers among females produce more offspring than the most successful reproducers among males.5. More than one of the above is true. Number of young permale versus femaleFemales little variationMostly dudsMostly dudsFew studsDo female elephant seals make a substantially larger parental investment?• LeBoeuf et al found that– average female weighs 650 kg, gives birth to a 50 kg pup.– pups gain 100 kg in first five weeks, all from mother’s milk– typical female lost 200 kg during pregnancy and nursing. – females had to regain lost weight to breed again the following yearConnection between male dominance and male reproductive success is found in many species. Result: sexual selection on males for qualities (such as large size) that enhance fighting ability. BUT in many species, subordinate males may secure matings by alternate means.– example: satellite behavior in toads. Alternative Strategies Satellite MalesSatellite behavior• Some male toads (and frogs, insects, fish, birds) wait silently near another male that is calling to attract females.• Try to intercept and mate with females that are moving toward calling male.• Both behaviors persist in population, even though satellites have less reproductive success.Rove BeetlesAlternative male reproductive behaviors: the rove beetle.• males defend territories around bits of mammal dung or animal carcasses.• females are drawn to territories to feed on flies.• large males have a big advantage in acquiring and defending territories; they attack and chase away smaller males.Alternative male reproductive behaviors: the rove beetle.• some small males behave differently• when approached by dominant male, turn around and present tip of abdomen (which is what females do during courtship)• the dominant male responds by courting, rather than attacking• but the small male moves slowly awayRove beetle, continued• the small, deceptive male can thus remain present on the territory, and may encounter and mate with a female.The ‘imitate female’ behavior is most likely to increase in male rove beetles1. when a majority of male beetles exhibit the ‘aggressive defense’ behavior.2. when a majority of male beetles exhibit the ‘imitate female’ behavior.3. when all males exhibit the ‘imitate female’ behavior.4. regardless of the type of behavior that other males exhibit.Frequency-dependent selection: the phenotype that yields the greatest fitness depends on how common each alternative phenotype is.Many traits have evolved because they help males win the competition for access to females.But what about females?• Theory predicts females should be choosy.• Do females make beneficial, adaptive choices? • Easiest to tell when male provides female with direct material benefit.– e.g. food or parental careBlack-tipped Hanging FlyFemale mate choice: the black-tipped hanging fly.• males bring females a nutritious “nuptial gift” (a dead insect)• female won’t copulate without a gift (or if gift is unpalatable)• if male has gift, female allows copulation, which lasts as long as it takes to eat the gift.• copulation must last ~20 min for successful fertilization.Female mate choice: the black-tipped hanging fly.• Conclusion: female mate choice selects for males that bring large gifts.• Female gets clear benefit, choice behavior is adaptive. Sexual SuicideFor female