1. Sexual Selection Variance in reproductive success
Intrasexual selection
Intersexual (epigamic) selection
Direct benefits
Indirect benefits
Fisher runaway process
Good genes

2. Why are there ornaments?

3. Sexual selection history
Darwin published “Sexual selection and the Descent of Man”
RA Fisher “Theory of Natural Selection” - proposed runaway selection
Bateman’s experiment
Trivers “Sexual selection and parental investment”
Lande “Speciation and sexual selection”

4. 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

5. 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

6. In mammals, VLRSmale is usually > VLRSfemale
Elephant seals
Kipsigis
Human maxima: male paternity:
female maternity:
888 69

7. Intrasexual selection: one sex (typically males) competes for access to and fertilization of the other

8. Sexual dimorphism reflects the intensity of selection on males
Elephant
seal
Harbor
seal

9. Why are females choosy?

10. 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

11. Direct benefits: Females choose resources

12. Pied flycatcher females prefer dominant males
Dark males provide
more food to nestlings
when mate is removed
than dull males

13. 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”

14. Hens prefer sperm from dominant males
Pizzari, T. and T.R. Birkhead Female feral fowl eject sperm
of subdominant males. Nature 405:

15. 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

16. Lande’s model of Fisher’s runaway

17. Female preference exhibits a genetic correlation with male trait in sticklebacks

18. Natural selection opposes sexual selection

19. Sexual and natural selection on barn swallow tails
Males with
longer tails
pair quicker
and fledge
more chicks
but grow
shorter tails
the next year

20. Arbitrary traits in zebra finch or sensory bias?

21. 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

22. Good genes models
For continued evolution of male trait and female preference,
heritable variation in offspring viability must be present

23. Parasites indicate plumage brightness

24. 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

25. Peahens prefer males with eyespots which have better offspring survival