1. How is sexual selection different from natural selection?
Asexual or selfing population:
Survive > grow > reproduce = fitness
Outcrossing sexual population:
Survive > grow > find a mate > reproduce = fitness

2. Biological basis of sexual selection
Asymmetry of parental investment in offspring (female HI; male LO; anisogamy)
Conflict of genetic interests between male and female

3. Biological basis of sexual selection
Investment differential varies with mating/rearing strategies
Polygamy (polygyny, polyandry)
Monogamy
No parental care
Maternal care only
Biparental care
Paternal care only

4. Biological basis of sexual selection
For the most common mating/rearing systems:
Female fitness limited by egg production and/or capacity to rear young
Male fitness limited by mating opportunities
Bateman experiment (Evolutionary Analysis Fig. 9.4, p. 293)
Angus John Bateman

5. Bateman experiment

6. Results of sexual selection
For the most common mating/rearing systems:
Females should be CHOOSY
Resources (e.g., food, territory)
Good genes
Sexy sons
Males should be COMPETITIVE for access to females/eggs
Male-male combat
Sperm competition
Display (colors, vocalizations, ornaments, engineered structures)

7. Male-male combat Males are: Larger than females
Better armed and armored
Ritual combat
Examples:

8. Sperm competition Bill Rice experiment
What is the fate of a mutation that increases male competitiveness even if it is at the expense of long-term survival and reproduction of his mate?

10. Sperm competition Bill Rice experiment:
Males and females have inherent conflicts of genetic interest
Genes that increase male fitness may be deleterious (even lethal) to their mates
The ‘battle of the sexes’ is a coevolutionary process

11. Male display
Ornamentation
Lekking

12. Looking for Mr. Goodbar or, How do females exercise choice?
Resources (e.g., food, territory)
Good genes
Sexy sons
Manipulation of offspring sex ratio
Extra-pair copulations in ‘monogamous’ females

13. Direct acquisition of resources
Male entices female with:
Food
Territory

14. Good genes William Hamilton
Greater size, enhanced color, louder/more complex vocalization, larger ornaments reflect general health and vigor
Amotz Zahavi
Ornament as handicap
‘Harrison Bergeron effect’

15. Sexy sons R.A. Fisher’s ‘runaway sexual selection’ hypothesis
Genetic variation for male trait and female preference for that trait
Female increases her fitness (= no. of grandchildren) disproportionately by producing attractive sons
Male trait and female preference lead to assortative mating
A genetic correlation between the male trait and female preference is established -- how?
In each generation the female preference applies strong directional selection on the male trait to become more exaggerated, even if the trait has no adaptive value, and the correlated female preference also becomes exaggerated

16. Gender bias in red deer
Red deer (Cervus elaphus - elk/wapiti) on the island of Rum in Scotland
Studied continuously by Tim Clutton-Brock and his associates since 1971
Polygynous ‘harem’ mating system; stags (males) lek
Dominance hierarchy among hinds (females)

17. Gender bias in red deer
Dominant hinds produce more sons than daughters. Since sex ratio is supposed to be under frequency-dependent selection with a 1 male:1 female stable optimum, what is going on?

18. Extra-pair copulation in ‘monagamous’ females
How might a ‘monogamous’ female benefit from extra-pair copulation?