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How is sexual selection
different from natural
Asexual or selfing population:
Survive > grow > reproduce =
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
• Female fitness limited by egg
production and/or capacity to rear
• Male fitness limited by mating
• Bateman experiment (Evolutionary
Analysis Fig. 9.4, p. 293)
Angus John Bateman
Bateman experiment
Results of sexual selection
For the most common mating/rearing
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 combat
Males are:
• Larger than females
• Better armed and armored
Ritual combat
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
Sperm competition
Bill Rice experiment:
• Males and females have
inherent conflicts of genetic
• Genes that increase male
fitness may be deleterious
(even lethal) to their mates
• The ‘battle of the sexes’ is
a coevolutionary process
Male display
Looking for Mr. Goodbar
How do females exercise
• Resources (e.g., food,
• Good genes
• Sexy sons
• Manipulation of offspring sex
• Extra-pair copulations in
‘monogamous’ females
Direct acquisition of
Male entices female with:
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 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
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