IV. Sex and Evolution

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IV. Sex and Evolution A. Types of Sex
Determination B. The Costs and Benefits of
Sexual Reproduction C. Sex Ratios 1. Why two
sexes? 2. Why 5050 ratios? 3. Why not
5050? - if there is an absolute difference in
the reproductive value (c/b) of sons and
daughters, then the sex ratio might
differ.
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3. Why not 5050? - if there is an absolute
difference in the reproductive value (c/b) of
sons and daughters, then the sex ratio might
differ. - in some organisms, males do little
work bees. - in some organisms, most males
dont mate deer.
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3. Why not 5050? - if there is an absolute
difference in the reproductive value (c/b) of
sons and daughters, then the sex ratio might
differ. - mechanisms haplodiploidy

-so sisters are related to each other by 75, but
to their brothers by only 50, so its better to
help your sister because her hard work and her
genes help you.
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3. Why not 5050? - if there is an absolute
difference in the reproductive value (c/b) of
sons and daughters, then the sex ratio might
differ. - mechanisms selective abortion

Sons (bucks) will only be successful if they are
large, whereas daughters( does) will mate even if
they are small.
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3. Why not 5050? - if there is an absolute
difference in the reproductive value (c/b) of
sons and daughters, then the sex ratio might
differ. - mechanisms selective abortion

Sons (bucks) will only be successful if they are
large, whereas daughters( does) will mate even if
they are small. If a female has access to lots
of food, then she can bring a buck to term and it
might win a harem.
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3. Why not 5050? - if there is an absolute
difference in the reproductive value (c/b) of
sons and daughters, then the sex ratio might
differ. - mechanisms selective abortion

Sons (bucks) will only be successful if they are
large, whereas daughters( does) will mate even if
they are small. If a female has access to lots
of food, then she can bring a buck to term and it
might win a harem. But, if a female is starved,
she will selectively abort male embryos. This is
adaptive (selected for) because she wont waste
energy on an offspring that wont mate. Small
males are worth less, absolutely, than small
females.
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IV. Sex and Evolution A. Types of Sex
Determination B. The Costs and Benefits of
Sexual Reproduction C. Sex Ratios D. Mating
Systems
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IV. Sex and Evolution A. Types of Sex
Determination B. The Costs and Benefits of
Sexual Reproduction C. Sex Ratios D. Mating
Systems - Promiscuous no long terms bonds
multiple mates
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IV. Sex and Evolution A. Types of Sex
Determination B. The Costs and Benefits of
Sexual Reproduction C. Sex Ratios D. Mating
Systems - Promiscuous no long terms bonds
multiple mates This is most common, and favored
when the probability of mating varies more among
males than females, and is largely random with
respect to phenotypic traits. So, just get the
sperm out there and hope for the best the more
sperm out there the better.
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IV. Sex and Evolution A. Types of Sex
Determination B. The Costs and Benefits of
Sexual Reproduction C. Sex Ratios D. Mating
Systems - Promiscuous no long terms bonds
multiple mates - Polygamous long term bonds
with multiple mates Polygynous one male with
harem of females Polyandrous one female with
harem of males
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IV. Sex and Evolution A. Types of Sex
Determination B. The Costs and Benefits of
Sexual Reproduction C. Sex Ratios D. Mating
Systems - Promiscuous no long terms bonds
multiple mates - Polygamous long term bonds
with multiple mates Polygynous one male with
harem of females Polyandrous one female with
harem of males Favored when one sex can restrict
access to mates by other members of their own sex
(by defending mates or resources that attract
mates).
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IV. Sex and Evolution A. Types of Sex
Determination B. The Costs and Benefits of
Sexual Reproduction C. Sex Ratios D. Mating
Systems - Promiscuous no long terms bonds
multiple mates - Polygamous long term bonds
with multiple mates - Monogamous pair
bond for life or a breeding season
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IV. Sex and Evolution A. Types of Sex
Determination B. The Costs and Benefits of
Sexual Reproduction C. Sex Ratios D. Mating
Systems - Promiscuous no long terms bonds
multiple mates - Polygamous long term bonds
with multiple mates - Monogamous pair
bond for life or a breeding season Favored
when additional investment of energy by both
parents increases offspring survival
significantly. Rare in mammals - females carry
embryo and nurse, not males Common in birds
both parents can incubate eggs and feed the
chicks
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IV. Sex and Evolution A. Types of Sex
Determination B. The Costs and Benefits of
Sexual Reproduction C. Sex Ratios D. Mating
Systems - Resource Trade-offs The polygyny
threshold If male territories vary
dramatically in quality, then females face a
choice mate with the mated males and share
the resources in the territory with other females
(Polygyny), or pick a lower ranked unmated male
on a poorer territory and mate monogamous but get
all the resources. If territories dont vary
much, then selection should favor females that
choose unmated males (monogamy). (well
described in book)
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IV. Sex and Evolution A. Types of Sex
Determination B. The Costs and Benefits of
Sexual Reproduction C. Sex Ratios D. Mating
Systems E. Extra-Pair Matings even in
monogamous species, neighbors mate. Males
benefit because they have more offspring, and
they wont have to contribute to raising the
offspring if they are in a neighboring
territory. Females benefit if the neighbor is
higher quality, or maybe simply by increasing the
genetic variation among her offspring.
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Evolution and Population Genetics I. The Modern
Synthesis II. Beyond the Synthesis III. Life
History Adaptations IV. Sex and Evolution V.
Types of Selection A. Natural Selection
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V. Types of Selection A. Natural
Selection 1. Directional Selection
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V. Types of Selection A. Natural
Selection 2. Stabilizing Selection
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V. Types of Selection A. Natural
Selection 3. Disruptive Selection
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V. Types of Selection A. Natural
Selection 3. Disruptive Selection results in
polymorphic populations
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V. Types of Selection A. Natural
Selection 4. Frequency Dependent
Selection rare mate phenomenon