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

Biological basis of sexual selection

• Asymmetry of parental investment in offspring (female HI; male LO; anisogamy)

• Conflict of genetic interests between male and female

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

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

Bateman experiment

Results of sexual selectionFor 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)

Male-male combatMales are:• Larger than females• Better armed and armored

Ritual combat

Examples:

Sperm competitionBill 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?

Sperm competitionBill 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

Male displayOrnamentationLekking

Looking for Mr. Goodbaror,

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

Direct acquisition of resources

Male entices female with:

Food

Territory

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’

Sexy sonsR.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

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)

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?

Extra-pair copulation in ‘monagamous’ females

How might a ‘monogamous’ female benefit from extra-pair copulation?