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Sexual Selection (I)
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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Explain This ?
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