Download Sexual Selection

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 Experiment
Methods
3 males + 3 females
per vial. Each fly had
a unique dominant marker
which allowed him to
identify the parent of
all 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
Human maxima: male paternity:
888
female maternity: 69
Kipsigis
Intrasexual selection: one sex (typically males)
competes for access to and fertilization of the other
Sexual dimorphism reflects the intensity
of selection on males
Elephant
seal
Harbor
seal
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 provide
more food to nestlings
when mate is removed
than 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 sperm
of 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 with
longer tails
pair quicker
but grow
shorter tails
the next year
and fledge
more chicks
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
Same nest as father
Cross-fostered chicks
Peahens prefer males with eyespots
which have better offspring survival