Transcript Sexual Selection & Investment

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Why do males usually evolve the ‘sexy’ traits?
Why do females usually do the choosing?
What do we mean by ‘investment’?
Is all this relevant to human behaviour?
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Sexual Selection & Investment
• Reproduction requires an investment of
resources (energy)
• It is poor evolutionary sense to waste resources;
there must be a ‘return’ in terms of survival
and/or reproductive success
• The greater the amount of resources invested,
the harder an organism will try to protect its
investment
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Investments and Returns
• Sexually dimporphic species are typically
anisogamous – they produce gametes
(reproductive cells) of different sizes
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Anisogamy
Sperm – very small
& numerous; short
lived; rapidly
renewed
Ovum – very large
& energy rich; long
lived; limited in
number
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Anisogamy
• In anisogamous species, the female invests
more energy in reproduction than the male
• Females adopt a quality strategy (selective)
• Males adopt a quantity strategy (indiscriminate)
• Consequently:
• Females choose from the males on offer
• Males compete with each other to be chosen
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Anisogamy
Can produce
offspring with
many women
in a short time.
Can produce
offspring with
one man at a
time
Low energy
investment in
each gamete
High energy
investment in
gamete
Does not carry
offspring
Carries
offspring for 9
months
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Women & Men
• Both women & men are trying to reproduce
successfully
• Because of the physiological differences
between them they adopt different strategies
• So they are looking for different things in a
potential reproductive partner
• Consequently, evolution predicts that they will
find different characteristics attractive
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Women & Men