Population Genetics

Question 1

Population Genetics

Answer 1

Mostly developed before we understood hereditary
As such, abstracted from biology
Still very important for our understanding of evolution

Question 2

Fisher-Wright Model

Answer 2

Classic model of population genetics

Simplest form - What happens at a single locus (site) under selection, crossover and mutation

Question 3

Locus

Answer 3

Location of a gene

Type of gene at site will differ between individuals

Question 4

Alleles

Answer 4

Different types of genes
Genes modified from allele to another during mutation
Alleles exchanged during sexual reproduction (crossover)

Question 5

Fitness

Answer 5

Measure of the expected number of offspring an individual produces
Measures potential to reproduce

Question 6

Epistasis in Fisher-Wright

Answer 6

Fitness of an allele will typically depend on alleles at other locations
Assumed none in Fisher-Wright model. More complex models can include this.

Question 7

Sexual Reproduction vs Asexual Reproduction

Answer 7

Ubiquitous in higher organisms
If females reproduced asexually, they breed at twice the rate of mixed couples (twice as fit)
Sexual reproduction must give a two fold advantage in fitness on a very short time scale to survive

Question 8

Linkage Equilibrium (Fisher Wright)

Answer 8

Assumes lots of crossover (compared to selection and mutation) - Often not unrealistic
Allows us to treat the loci independently and concentrate on a single locus
Only need to consider the proportion of the different alleles at the locus

Question 9

Fisher-Wright for asexual reproduction

Answer 9

Alleles at different sites are coupled

Currently unsolved

Question 10

Diploids

Answer 10

Carries two copies of DNA (father and mother)
Not clear what the advantage of this is (like sex)
Tolerates mutations which can make one set of genes dysfunctional
Also allows detrimental mutations to build up in a genome (Incest likely to combine two dysfunctional alleles creating nonviable offspring)

Question 11

Haploids (Fisher-Wright)

Answer 11

Assume that each individual only carries one set of genes

Generalisation to diploid organisms complex without substantially changing things

Question 12

Gender (Fisher-Wright)

Answer 12

Life tends to be gendered (finding a mate consumes energy, specialisation can save some energy)
Assumes monoecious population (1 gender, although it still mates)

Question 13

Fisher Wright Assumptions

Answer 13

Simplifying the model allowing us to concentrate on the role of selection, mutation and crossover

Question 14

3 stages of population changing from one generation to the next

Answer 14

Each individual produces large number of seeds proportional to its fitness
Seeds mutated according to the mutation probabilities
Select sample of P of the seeds independently to form the next generation

Question 15

Fisher Wright - Infinite Population

Answer 15

Selection rapidly amplifies the mutants if they are fitter (fitter individuals initial grow exponentially)
If mutation rate is small, may take a long time to appear in finite populations
Mutation always occurs in infinite populations so can have far more rapid take over times than large, finite populations

Assumes effect of selection and mutation is small

Question 16

Fisher-Wright - Finite Populations

Answer 16

Draw a sample of P individuals from the seeds
Probability of drawing a given amount of mutants is given a proportion of mutants is given with a binomial distributions
Can simulate population evolution by drawing random binomial deviate at each generation

Question 17

Markov Model

Answer 17

Population at time t depends only on population at time t-1
Conditionally independent of population at earlier times given population at t-1
Pn’(t) = transition probability * pn(t) (summed from n = 0 to P)

Question 18

Use of stochastic differential equations

Answer 18

When dynamics depend on rare events which might not happen. Involves small random kicks.
With larger populations, Markov analysis quickly becomes intractable