Populaton genetics 2

Question 1

What is natural selection?

Answer 1

Alleles that enhance fitness (survival and reproduction) will contribute disproportionately to the next generation.

Repetition of this process leads to the selection of beneficial alleles and, eventually, their fixation.

Natural selection acts on the whole organisms which indirectly selects ont he genotype.

Question 2

Single locus population genetics

Answer 2

It is useful to study the effect of selection on single loci even though few traits are affected by single loci.

You can study
- Positive selection
- Negative selection
- Balancing selection

Question 3

Fitness

Answer 3

Relative fitness is the average number of offspring produced by an individual with a particular genotype compared to an individual with a different genotype.

In population genetics fitness is denoted as a selection co-efficient S which represents the increase or decrease in fitness conferred by an allele.

0: no selection

1: 100% greater fitness and positive selection

-1: 100% lower fitness and negative selection

Question 4

Modelling selection in haploids

Answer 4

Simplest model as selection as each individual has 1 alleles so do not need to consider dominance

Delta q = spq

Change in allele frequency of q depends on the selection coefficient of q and the realtive frequencies of p and q alleles.

What to expect:

Positive s: q increases

Negative s: q decreases

P or Q are extreme: slow change in allele frequency

P and Q close to 0.5: fast change in allele frequency

Example: sigmoid shape of influenza strains moving to fixation

Question 5

Modelling selection in diploids

Answer 5

Selection in diploids is more complicated as each individual has 2 alleles, so we must consider dominance as well as fitness advantage.

Degree of dominance= h

Fitness when working out delta q:

PP= 1
PQ= 1 + hs
QQ= 1 + s

If the degree of dominance is low then selection/fitness for q in the heterozygote will be lower.

Equation:

Delta q = spq (ph + q (1-h))

Example: heights Assuming height is effected by 1 locus and there is a short and tall allele.

Dominance: No intermediate heights
No dominance: intermediate heights

Question 6

Selection in diploids

Answer 6

Selection acts different on dominant and recessive alleles.

Dominant:
- Rapid initial selection as dominant allele phenotype is expressed in homozygotes and heterozygotes
- Selection slow as reach fixation as it is hard to purge all recessive alleles.

Recessive
- Slow initial selection as recessive alleles are only expressed in homozygous recessive individuals
- Selection increases around fixation as homozygous indivudal are present by chance

Additive
- If there is no dominance and allele are additive then the initial selection is slower than dominant as heterozygotes have intermediate fitness.

Question 7

Balancing selection

Answer 7

Balancing selection occurs when multiple alleles are maintained in a population rather than 1 going to fixation.

When does this occur?

1) It often occurs when there is a specific advantage to being a heterozygote.

Example: heterozygous to Sickle cell anemia means immune to malaria and not a carrier of Sickle cell.

Example: MHC gene
- females have disasortative mating preference based on odours released from MHC-> greater MHC diversity increases offspring’s immunological resistance

2) Frequency-dependent selection:
Rare alleles are advatagous

Example: pathogen be host
Example: mimicry of warning colouration -> needs to pay for predator to believe the colour
Example: hybrid vigour

3) fluctuation selection: allele fitness depends on an aspect of the environment that is rapidly and constantly changing

Question 8

Quantatitive variation

Answer 8

Characteristics tend to be controlled by more than 1 gene and the environment, leading to quantitative effects.

Increasing the number of loci which effect characteristics leads to more phenotypic diversity

This allows for
- Directional selection
- disruptive selection
- Stabilizing selection

Question 8

Areas of application of population genetics

Answer 8

• Crops and animal breeding
• Ecology
• MEdical genetics
• Conservation genetics

Example: COP15 committed to report the genetic diversity of all species
-> Loss of genetically distinct populations
-> Population being too small
-> Number of species

Question 9

Lecture overview

Answer 9

Population genetics is important for studying natural selection.

We can model selection in single loci models to understand selection on different alleles.

• Haploids: simple and fast
• Diploids: must consider dominance

Selection varies depending on whether the allele is recessive, dominant or additive.

Single loci genetics is uncommon, leading to the study of quantitative genetics.

The more genes that contribute to a trait, the greater the phenotypic diversity of the trait allowing:
- Stabilising selection
- Directional selection
- Disruptive selection

Question 10

Evolutionary forces

Answer 10

Evolutionary forces lead to diversion from HW equilibrium (allele frequencies remaining constant)

Molecular mechanisms: Mutation and LD

Natural selection

Genetic drift

Gene flow (migration)

Non random mating (inbreeding/ assortative mating)