sexual selection
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Sexual Selection. Variance in reproductive success Intrasexual selection Intersexual (epigamic) selection Direct benefits Indirect benefits Fisher runaway process Good genes. Why are there ornaments?. Sexual selection history. - PowerPoint PPT PresentationTRANSCRIPT
Sexual Selection
• Variance in reproductive success
• Intrasexual selection
• Intersexual (epigamic) selection– Direct benefits– Indirect benefits
• Fisher runaway process
• Good genes
Why are there ornaments?
Sexual selection history
• 1871 - Darwin published “Sexual selection and the Descent of Man”
• 1930 - RA Fisher “Theory of Natural Selection” - proposed runaway selection
• 1948 - Bateman’s experiment
• 1972 - Trivers “Sexual selection and parental investment”
• 1981 - Lande “Speciation and sexual selection”
Bateman’s ExperimentMethods
3 males + 3 femalesper vial. Each fly hada unique dominant markerwhich allowed him toidentify the parent ofall offspring
Conclusions from Bateman’s experiment
• Males usually have higher potential reproductive success than females because egg production limits reproductive rate
• Males show greater variation in reproductive success than females
• Therefore, traits which influence male reproductive success experience selection
In mammals, VLRSmale is usually > VLRSfemale
Elephant seals Kipsigis
Human maxima: male paternity: female maternity:
888 69
Intrasexual selection: one sex (typically males) competes for access to and fertilization of the other
Sexual dimorphism reflects the intensity of selection on males
Elephantseal
Harborseal
Why are females choosy?
Intersexual (epigamic) selection: female choice
• Choice provides direct benefits to female– find fertile or compatible mate– improve immediate survival or fecundity
• Choice provides indirect (genetic) benefits– “Fisher process”
• choosy daughters produce ornamented sons
– “good genes”• male ornament indicates offspring viability
Direct benefits: Females choose resources
Pied flycatcher females prefer dominant males
Dark males providemore food to nestlingswhen mate is removedthan dull males
When males only provide sperm
• Females should select for “indirect” benefits, i.e. genetic characteristics that their sons and daughters will inherit
• Genes that influence male attractiveness produce “sexy sons” and lead to the Fisher process
• Genes that influence offspring viability are referred to as “good genes”
Hens prefer sperm from dominant males
Pizzari, T. and T.R. Birkhead 2000 Female feral fowl eject spermof subdominant males. Nature 405:787-789.
Fisher’s Runaway process
• If females exhibit preference for a male trait
• And selection does not act on females
• Then their sons and daughters will carry genes for both the preference and the trait
• This creates a genetic correlation between the preference and trait
• And leads to geometric increase until further increase in the male trait is opposed by natural selection
Lande’s model of Fisher’s runaway
Female preference exhibits a genetic correlation with male trait in sticklebacks
Natural selection opposes sexual selection
Sexual and natural selection on barn swallow tails
Males withlonger tailspair quicker
and fledgemore chicks
but growshorter tailsthe next year
Arbitrary traits in zebra finch or sensory bias?
Good genes models• Require a mechanism for maintaining
heritable variation in offspring viability– Recurrent deleterious mutations– Parasite-host coevolution maintains parasite
resistance
• Handicap models refer to male traits that can only be displayed by males in good condition. These can be “honest” indicators of male condition
Good genes models
For continued evolution of male trait and female preference,heritable variation in offspring viability must be present
Parasites indicate plumage brightness
Swallow tail feather length indicates ectoparasite resistance
Swallow chicks have fewer mites if their biological, but not foster, fathers had longer tail feathers
Cross-fostered chicksSame nest as father
Peahens prefer males with eyespots which have better offspring survival