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Lecture 7. Sexual Selection
Costs of sex
• Cost of meiosis
• Cost of producing
males
• Cost of courtship and
mating
Benefits of sex
• Protection against
mutations (Muller’s
ratchet)
• Protection against
environmental changes
(Raffle hypothesis)
• Protection against
biotic fluctuations
(Red Queen
hypothesis)
The Red Queen hypothesis
”Now here, you see, it takes all the running
you can do to keep in the same place.”
Why do male and female gametes
differ in size?
Geoffrey Parker et al.:
divergent evolutionary selection
favoured two types of gametes:
small and mobile (sperm)
sedentary and packed with
nutrients (eggs)
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Darwin (1871, p256):
“We are, however, here
concerned only with that
kind of selection, which I
have called sexual
selection. This depends on
the advantage which certain
individuals have over other
individuals of the same sex
and species, in exclusive
relation to reproduction.”
Sexual selection
1. More individuals are produced than manage to reproduce
2. Individuals differ in their ability to compete with others
for mates or to attract members of the opposite sex
 Result: the evolution of traits that enhance reproductive
success while decreasing survivorship
What about these
Inrasexual Selection
Members of one sex
compete among themselves
for access to members of
the other sex
Competition before mating
Competition after mating
Competition following conception
Bruce effect in rodents
The pregnancy of a recently inseminated female is terminated
upon exposure to an unfamiliar male
Male infanticide in langur monkeys.
Male infanticide
in lions
Explain This ?
Intersexual Selection
Members of one sex
(mostly males) ‘advertise’
that they are worthy of an
investment.
Members of the other sex
(usually females) choose
among them.
Sexually selected “ornaments” of males
Darwin argued that sexual selection via female choice was responsible
for the evolution of male displays and plumage
Sexual selection for tail length in long-tailed
widowbirds
The widowbird (Euplectes progne),
a species the size of a blackbird
that is endowed with a half-meter long tail
(Andersson 1982)
Mate-choice copying in the guppy (Poecilia reticulata)
One female’s choice of a mate affects another female’s choice
Male zebra finch
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Sailfin Molly (Poecilia velifera)
Fluctuating asymmetry: The extent of asymmetry in
bilaterally symmetrical traits
Normal
High
Perfect