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