Name the key features of selection (5)
- Non-random
- Directional
- Determined by gene pool and environment
- Dependent on heterozygous effect and allele frequency
- Increases, decreases or stabillizes the frequency of a specific allele
What does selection drive / affect?
- Drives adaptation
- Affects specific loci/gene sequences - (said to be under selection pressure)
What does selection act upon?
- Probability of survival until adulthood
- Number of offspring - (lifetime reproductive success)
What is selection?
Process that selects specific alleles on individuals to change the inheritance pattern of genes
What different stages of the life cycle can selection occur?
- Viability selection: from fertilization to reproduction
- Sexual selection: choosing mates - selection in both sexes
- Fertility and gamete selection: ability to fertilize
- Fecundity selection: number of offspring
How is selection measured?
‘fitness’ - differences among genotypes
What are absolute and relative fitness?
- Absolute fitness: average number of offspring in next generation per individual or specified genotype born in this generation
- Relative fitness (W): absolute fitness of specified genotype / absolute fitness of reference genotype
What is the selection coefficient (s)?
The relative reproductive disadvantage a genotype has against the most fit genotype
- 1-relative fitness
What are the different types of selection, how do they affect alleles and how common are they?
- Positive selection: Increases the frequency of a given beneficial allele (adaptation) - less common
- Balancing selection: Maintains allele frequencies at an equilibrium - rare
- Negative (purifying) selection: Decreases the frequency of a given deleterious allele - common
What factors does the intensity of selection relate to?
- Presence of other alleles
- Environmental conditions
- Heterozygous effect
- Frequency of the allele
- Effective population size / genetic drift
- Linkage to other loci - Hill-Robertson effect
How can you model selection at one/multiple loci and what theories were adopted?
- Simplest model - one locus, two alleles, fitness difference between them
- More complicated - multiple alleles, multiple loci etc
- Use selection coefficient (s) - defined so that lowest fitness genotype has fitness 1-s
- Diploids: viability selection acts on different genotypes, and outcomes will depend on relative fitness differences between the diff homo/heterozygous genotypes
- Theory developed by Wright, Fisher and Haldane
How is the mean fitness of a population calculated and what is its symbol?
- p^2 and q^2 = homozygous, 2pq = heterozygous
- p^2 + 2pq + q^2 = 1
- w = omega
- w11 + w12 + w22 = mean w
- mean w is mean fitness of pop
What is relative fitness?
Relative fitness is the absolute fitness normalized - e.g., absolute fitness of each genotype divided by the absolute fitness of the fittest genotype
s = selection coefficient (0<s<1)
h = heterozygous effect
- A1A1 = 1
- A1A2 = 1 - hs
- A2A2 = 1-s
What is the heterozygous effect?
The measure of fitness of the heterozygous relative to the selective difference between the two homozygotes
What happens during positive selection and how does it differ for dominant / recessive alleles?
- Fittest allele pushed to fixation (reaches frequency of 1)
- Dominance and magnitude of fitness difference affects the speed of fixation of a beneficial allele
- Deterministic model - i.e. no drift
- Common
- ‘A’ - Dominant : selection acts straight away - pushed to fixation
- Intermediate dominance - not straight away but still quite quickly
- ‘A’ - Recessive : selection doesnt act straight away but increases quickly later on
What happens during purifying (negative) selection and how does it affect alleles?
- Without drift (deterministic model) - new deleterious alleles under purifying selection drop out immediately
- But amount of deleterious variation in a population will approach mutation-selection equilibrium as the loss of deleterious alleles due to selection is balanced by the input of new mutations
- With drift - dominance affects the probability of loss of a deleterious allele
- Recessive/partially recessive deleterious alleles can spread to an appreciable frequency in the population by drift - e.g., Tay-Sachs disease in some Ashkenazi Jewish enclaves)
- Estimated between 2 and 20 deleterious mutations in human per generation - many accumulated deleterious mutations - high mutational loads which is exposed when inbreeding occurs
How do positive and negative selection affect fitness difference and genetic variation?
- Both remove fitness differences - reduce genetic variation
- Makes two alleles the same in the end - no removes fitness difference
How does balancing selection work?
- Frequency of fittest allele maintained at an equilibrium rather than going to fixation under positive selection
- Is due to heterozygous advantage (heterozygote has highest fitness)
- Two or more alleles maintained in the population]
- Is rare
- Occurs when h<0 - over dominance
What does the equilibrium frequency of the genotypes and time taken to reach equilibrium depend on?
The fitness difference between the heterozygote and homozygous genotypes
What is balanced variation?
When selection actively maintains variation: Some types of balancing selection are:
- Heterozygote advantage - e.g., sickle cell anaemia, maybe CF
- Negative Frequency dependent selection - fitness of allele depends on its frequency in population - allele has its highest fitness when its rare in the population - as frequency increases - fitness decreases - e.g., host-parasite interactions - MHC - cyclicle shape
Give an example of negative frequency dependent selection
E.g., coevolution of hosts and parasites:
- Parasite wants to be adapted to most common genotype in population - to infect most hosts
- So hosts genotypes that are infrequent will become more and more selected for but the parasite will be constantly trying to catch up with the hosts change in selection
- See graph in lecture
How are malaria and HbS alleles distributed worldwide?
- Sickle cell anaemia allele - HbS
- Homozygous for HbS allele have very low fitness - poor health outcomes - mortality at early age - exacerbated by malaria - allow homozygous HbS allele to persist
- Heterozygous genotypes offer protection against malaria, so HbS maintained in malarial areas
- Selection against HbS outside malarial areas
- Papers in lecture
What is disruptive selection?
This form of selection causes allele to go to fixation or to be lost from population - depending on starting frequency of allele
- Rare - but important for sympatric selection
- Direction of selection depends on the initial frequency of the allele - if low freq = lost, if high freq = fixation
- Occurss in h>1, underdominance - heterozygotes have lowest fitness
How can environment effect selection? Give an example
- One allele is fitter in one environment and deleterious in another, then selection may maintain genetic variation across different habitat patches, or allele frequencies may change through time if environmental conditions change
- Selection coefs are therefore always relared to a given population within some specific environmental conditions
- Example: Peppered moth - Biston betularia
- Here: light moths are cryptic (cant be seen) on lichen covered trees but easily observed on trees where lichens have disappeared due to pollution, and vise versa for dark melanistic moth
- Observability effects predation rates by birds - so light forms are rarer in areas where trees have lost lichen cover
