P1247676907NuKeW

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Genetic Variation - The fuel of natural selection
Campbell et al, chapter 23
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• Populations are polymorphic

• Nature vs. Nurture

• Source of variation

• Maintenance of variation

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• Populations are polymorphic

• Nature vs. Nurture

• Source of variation

• Maintenance of variation

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Polymorphic populations
Example Darwin finches on Galapagos
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Polymorphic populations
Example Lazuli bunting
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Polymorphic populations
Example Swallowtail butterfly
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• Populations are polymorphic

• Nature vs. Nurture

• Source of variation

• Maintenance of variation

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Nature vs. Nurture
Phenotypic Expression
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Nature vs. Nurture
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Nature vs. Nurture
Diploidy
There are 2 copies of each gene
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Nature vs. Nurture

• Genotype is more variable than phenotype

• Only genetic variation counts for evolution

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Nature vs. Nurture
How to separate the two?
Common Garden Exp.
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• Populations are polymorphic

• Nature vs. Nurture

• Source of variation
• Mutation
• Recombination

• Maintenance of variation

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Source of Genetic Variation
1. Mutation
Change in DNA sequence
Chromosome structure Number of
Chromosomes
Due to Copying Errors Environmental
factors
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Source of Genetic Variation
2. Recombination

• Reshuffling of chromosomes during reproduction

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• Populations are polymorphic

• Nature vs. Nurture

• Source of variation

• Maintenance of variation
• Selection
• Heterosis

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Maintenance of Variation
1. Stabilising selection
Favours mean over tail
Loss of variation
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Maintenance of Variation
1. Stabilising selection
Example Darwin Finches
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Maintenance of Variation
2. Directional selection
One tail is favoured over the other
Trait is changing over time
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Maintenance of Variation
2. Directional selection
Example Heliconius butterfly on passion flower
vines
Leads to an arms race between plant and insect
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Maintenance of Variation
2. Directional selection
The arms race idea lead to a more general
hypothesis Red Queen Hypothesis
It takes you all the running to stay in place
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Maintenance of Variation
2. Directional selection
Selection may change in time Example Darwin
Finches on Galapagos
El Nino
Bill size
wet dry small large seeds
Time
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Maintenance of Variation
2. Directional selection
Selection may change in space Example African
Firefinches
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Maintenance of Variation
3. Disruptive selection
Selection favours the two tails over the mean
There are two forms
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Maintenance of Variation
3. Disruptive selection
Example Lazuli buntings
Showy male
Average male
Drab male
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Maintenance of Variation
4. Frequency-dependent selection
A mode of selection where a phenotype is
only favoured when it is either rare or common.
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Maintenance of Variation
4. Frequency-dependent selection
Example Swallowtail butterfly (Papillo dardanus)
Males
Only works if the cheats are rare
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Maintenance of Variation
5. Heterozygote advantage
Heterozygote individuals have higher fitness than
either homozygote individuals. This is a common
principle in plant and animal breeding
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Maintenance of Variation
5. Heterozygote advantage
Example Red blood cells - sickle cell disease
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A large number of processes create and maintain
genetic variation that is the base
for evolution...
but does this lead to new species? - next here