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