Genetic Variation - The fuel of natural selection Campbell et al, chapter 23.
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Transcript of Genetic Variation - The fuel of natural selection Campbell et al, chapter 23.
Genetic Variation -The fuel of natural selection
Campbell et al, chapter 23
• Populations are polymorphic
• Nature vs. Nurture
• Source of variation
• Maintenance of variation
• Populations are polymorphic
• Nature vs. Nurture
• Source of variation
• Maintenance of variation
Polymorphic populations
Example: Darwin finches on Galapagos
Polymorphic populations
Example: Lazuli bunting
Polymorphic populations
Example: Swallowtail butterfly
• Populations are polymorphic
• Nature vs. Nurture
• Source of variation
• Maintenance of variation
Nature vs. Nurture
PhenotypicExpression
Protein
Genome(blueprint)
Nature
Environment
Nurture
I found the gene that makes us believe alltraits are based on genes
Nature vs. Nurture
GeneChromosome
Genome
Genepool
Nature vs. Nurture
Diploidy
There are 2 copies of each gene
Father Mother
Offspring
2 identical copies = homozygot
Father Mother
Offspring
2 different copies = heterozygot
Nature vs. Nurture
• Genotype is more variable than phenotype
• Only genetic variation counts for evolution
Nature vs. Nurture
How to separate the two?
Example: Altitudinal gradient Common Garden Exp.
Gene
Environment
Mixture of both
• Populations are polymorphic
• Nature vs. Nurture
• Source of variation Mutation Recombination
• Maintenance of variation
Source of Genetic Variation
1. Mutation
Change in: DNA sequence
Chromosome structure
Number of Chromosomes
Due to: Copying Errors
Environmental factors
Source of Genetic Variation
2. Recombination
• Reshuffling of chromosomes during reproduction
• Crossing over
A
B
C
A
B
C
A
B
C
A
B
C
• Populations are polymorphic
• Nature vs. Nurture
• Source of variation
• Maintenance of variation Selection Heterosis
Maintenance of Variation
1. Stabilising selection
Favours mean over tail
Loss of variation
Maintenance of Variation
1. Stabilising selection
Example: Darwin Finches
Bill size
For
agin
g ef
ficie
ncy
Maintenance of Variation
2. Directional selection
One tail is favoured overthe other
Trait is changing over time
Maintenance of Variation
2. Directional selection
Example: Heliconius butterfly on passion flower vines
Plant:Toxin
Insect:Enzymes
Plant:False eggs
Leads to an arms race between plant and insect
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’
Maintenance of Variation
2. Directional selection
Selection may change in timeExample: Darwin Finches on Galapagos
Time
Bill
siz
e
El Nino
wet drysmall large seeds
Maintenance of Variation
2. Directional selection
Selection may change in spaceExample: African Firefinches
Maintenance of Variation
3. Disruptive selection
Selection favours the two tails over the mean
There are two forms
Maintenance of Variation
3. Disruptive selectionExample: Lazuli buntings
Showy male
Average male
Drabmale
attracted Sneak in
Maintenance of Variation
4. Frequency-dependent selection
A mode of selection where a phenotype is onlyfavoured when it is either rare or common.
Maintenance of Variation
4. Frequency-dependent selection
Example: Swallowtail butterfly (Papillo dardanus)
Males
Toxic species
Only works if the cheats are rare
Swallowtail females mimicthese toxic species without being toxic themselves
Maintenance of Variation
5. Heterozygote advantage
Heterozygote individuals have higher fitness than eitherhomozygote individuals.
This is a common principle in plant and animal breeding
Maintenance of Variation
5. Heterozygote advantage
Example: Red blood cells - sickle cell disease
Homozygot (normal cells): vulnerable to malaria
Homozygote (sickled cells): lethal
Heterozygote: non-lethal & resistant to malaria
A large number of processescreate and maintain geneticvariation that is the base forevolution...
…but does this lead tonew species? - next here