Download Evolution of Sex

A Recap Regarding Senescence
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Selection is more powerful in the stages up
to the age of peak reproductive value
If a pleiotropic gene is introduced that gives
an benefit early in life, at the expense of a
cost much later on, than it will be selected
Senescence is the outcome of such negative
effects later in life
The Evolution of Sex
The Mathematics Urge
Sublimated
What Use is Sex?
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The Problem:
 Asexual: 100% copy in offspring
 Sexual: 50% (usually) copy in offspring
 So why do some species have sex?
Some answers:
 Genetic “proofreading”
 Certainty of environment
 Parasite resistance
The Genetic Manuscript
“We are no4 ready for winter”
 What is the correct letter?
 A second copy from another organism of
the same species may help
 After “proofreading,” some recombination
takes place
 New genotypes are produced
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The Certainty of the Environment
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Offspring produced in environments similar to
parents:
 Asexual reproduction
Offspring reproduced in environments
dissimilar to parents:
 Sexual reproduction
Gambling theories are not well supported by
the data
Parasite Resistance
Parasites are responsible for far more deaths
than physical threats
 Correlation between high parasite infestation
and amount of recombination
 Sex allows for new genetic “locks” to keep
out old parasite “keys”
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Hermaphroditism
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Simultaneous hermaphroditism
 When sexual functions serve both sexes
Sequential hermaphroditism
 Generally due to maturation and sizeadvantage
Separate sexes
 When sexual functions do not serve both
sexes
Eggs & Sperm
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“Cheating” organisms may produce a greater
number of smaller gametes
Disruptive selection:
 organisms producing many small gametes
or fewer large gametes vs. producing
average gametes
Minimum Investment
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Females in most species have a greater
minimum investment in offspring than males
 Ova
 Gestation
 Lactation…
Male & female “needs” differ due to minimum
investment
The Sex Ratio
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If males can inseminate several females, why
do we have 1:1 ratio?
Reproductive efficiency?
 Island with 90 men & 10 women vs. 10
men & 90 women
Frequency dependent selection
Sexual Dimorphism
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Intrasexual contests
 Lead to dimorphism of competing sex
relative to other sex
 No intrasexual contests leads to females
typically being larger
 Ova production
The Wrap-Up
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Evolution of sex
 Proofreading
 Gambling
 Parasite resistance
Sex ratio
Minimum investment
Sexual dimorphism
Things to Come
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Long-term mating strategies
 Female
 Male
Short-term mating strategies