Fundamental difference between male and female gametes
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Sperm are...
Small, unlimited and 'cheap' to produce
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Eggs are...
Large, (relatively) scarce and cost a lot to produce
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What is 'Anisogamous' sexual reproduction?
Anisogamy refers to a form of sexual reproduction involving gametes of different sizes. The smaller gamete is considered to be male (sperm cell), whereas the larger gamete is regarded as female (egg cell).
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Females may also gestate and...
Feed their offspring
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There is an asymmetry of...
'Parental investment' and 'parental effort'
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What is 'parental investment'?
Effort put into rearing an individual offspring
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What is 'parental effort' (PE)
Effort put into rearing all offspring produced during lifetime.
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What does 'parental investment' and 'parental effort' (PE) lead to?
Sexual conflict. Differential PE leads to different ideal strategies to maximise reproductive success.
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Where male PE is lower...
Males should maximise reproductive success by ruthless promiscuity. Hence, harems... (but beware; culture!) Problem, male 'ardour'
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Females should be...
Choosy and coy (to offset high cost of reproduction).
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Females will do...
Choosing and males will compete for access to females
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Females will invest mainly in...
PE
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Males will invest mainly in...
'Mating effort' (ME; i.e. acquiring mates)
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Degree to which it is true that males will invest mainly in ME will depend on the...
Inequality of PE
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Degree of inequality depends on...
Mating system
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Monogamy =
M and F form exclusive pair bond
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Polygyny =
M mates with several F's
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Polyandry =
F mates with several M's
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Polygyny + polyandry =
Polygamy
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If PE equal (monogamous species) =
More equal ME
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If PE unequal (polygamous species) =
Less equal ME
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If females do choosing and males compete for access to females, will lead to...
Selection of traits which are solely concerned with increasing mating success.
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'Sexual selection'
One aspect of natural selection, identified by Darwin himself: 'The descent of man and selection in relation to sex'
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What is the first type of sexual selection?
Intrasexual selection
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Explain intrasexual selection
Where M's compete for access to F's, those best equipped to win will pass on their genes to next generation.
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What does intrasexual selection lead to?
Tusks, antlers, claws, large muscles and large teeth. Sexual dimorphism - M becomes a fighting machine
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(E.g.) Effects of intrasexual selection
Red deer stags. Compete for F's in autumn rut; strongest stags command largest harems. Competition has led to sexual dimorphism: M more bulky than F. M possesses antlers (employed in aggressive, interlocked 'pushing' fights)
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Another e.g. of intrasexual selection
Competition between M's does not necessarily stop after copulation. Black winged damselfly. F may mate with several M's prior to laying eggs. M developed ability to remove existing sperm from F sperm storage organs. M's 'penis' -lateral horns, spines
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What is the second type of sexual selection?
Intersexual selection
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Explain intersexual selection
The reluctant females will prefer males who can show clear evidence of material resources and/or 'better' genes
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Example of evidence of material resources
Hanging flies. F hanging flies will mate with a M only if he provides a large insect to eat during copulation. Allowed duration of copulation is related to size of insect.
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