Sexual Selection

natural selection

competing successfully for resources, territory, etc. 

sexual selection

competing successfully for mating opportunities

types of sexual selection

intrasexual and intersexual

intrasexual selection

competition within sexes

intersexual selection

competition between sexes

fisher’s runaway hypothesis

the female-preferred trait will become more exaggerated over time, e.g., male tails becoming longer and longer (Andersson 1982)

sexy sons

female preference for trait + male trait = sexy son with trait and daughter with preference for trait

stalk-eyed flies

showed that trait exaggeration and female preference trait are influenced by the same genetic factor (covariance)

true/false: sometimes sexual selection works against natural selection

true; some ‘sexy’ traits make it harder to survive

direct benefits

non-genetic, e.g. male providing food to female in exchange for sex

indirect benefits

genetic, ‘good genes’

zahavi’s handicap principle

costly traits may be a handicap and indicate the quality of the male who has them (indicator mechanism)

qualifying handicap

males survive (good) or die (bad) as a result of the trait

conditional handicap

only the best males develop the trait

revealing handicap

the trait reveals things about the male, like good health

strategic choice handicap

the male advertises his traits at his discretion 

example of handicapped traits

longer tails on swallows are preferred by females and have less mites, BUT make it harder to forage

hamilton-zuk hypothesis

secondary sex characteristics can reflect resistance to disease, e.g., male peacocks with more eyespots have better immune systems

immunocompetence handicap hypothesis

some secondary sex characteristics come at a cost to immune function, e.g., testosterone in males is immunodepressive