Sexual Selection

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 • 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”

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 2000 Female feral fowl eject sperm of subdominant males. Nature 405:787-789.

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