Origin of Genetic Variation

1. Mutation

Change in the

• DNA Sequence
• Chromosome Structure
• Chromosome Number

Due to: Environmental Factors

Copying Errors

2. Recombination

• Reshuffling of chromosomes during sexual reproduction
• Crossing-over

Natural Selection Depletes Variation

Stabilising Selection: the mean is favoured over the tails, which leads to the loss of genetic variation in the population. Endangers the continuation of evolution. Therefore processes that maintain genetic variation in natural populations are needed. We discuss several of these processes.

Processes Maintaining Genetic Variation

1. Oscillating directional selection

• Directional selection: one tail is favoured over the rest
• Example: Darwin Finches
• Directional Selection on bill size changes over time in concert with El Niño
• (patterns of rain fall)

El Niño (see Campbell p. 422, fig. 22.7)

2. The Red Queen Hypothesis

Ongoing arms races between organisms exerts constant directional selection pressure

3. Heterogeneous environment

Different phenotypes are favoured in different sub-areas of the population Example: African Fire Finches (see Campbell p. 438, fig. 23.9)

4. Disruptive selection

Selection favours the two tails over the mean

5. Diploidy

Carry two copies of each gene. If heterozygote:

• Dominant allele expressed
• Recessive allele hidden

6. Heterozygote advantage

Heterozygote individuals have a higher fitness than either homozygous individuals

Example: Sickle-cell disease

7. Frequency-dependent selection

A mode of selection where a phenotype is only favoured when it is either rare or common.

Example: Swallowtail Butterfly (Papillo dardanus)

Females have different morphs that each resembles another, but noxious species.

Example: Left- and right-mouthed cichlid fish in Lake Tanganyika (see Campbell p. 439, fig. 23.10)