Week 2: Genetic Drift + Selection Drift

Genetic Drift

• Stochastic changes in allele frequencies

• affects neutral alleles

• overlays selective changes

Consequences of drift

1. Loss of genetic diversity within populations over time

2. Genetic differentiation between independent/ isolated populations→ diverges pop. freq

What happens to genetic drift as population size increases?

• Strength of drift decreases with increasing population size

• Drift increases with 1 / reproductive population size

• Drift is strong

  • if a randomly mating population is small

  • if only few members of a larger population reproduce

→

So what matters is not the population size but the individuals contributing to reproduction

How does drift affect isolated populations?

• lose diversity independently

• populations diverge genetically

• populations fix different alleles

The effective population size, N_e

Ne ~ number of individuals that contribute to reproduction

Ne of a real population

= size of an idealised, randomly mating population with identical intensity of drift (rate of diversity loss)

How do we tell between selection and genetic drift?

Compare observations to neutral expectation:

• theoretical values

• simulated data

• data from known/assumed neutral loci

We need to describe genetic diversity and differentiation

How can you quantify the consequences of drift?

• quantities that are determined are the genetic identities: probabilities that alleles are identical

• Complement of genetic variation/heterozygosity

How is genetic variation distributed between individuals?

• Contrast F (inbreeding coefficient) and θ (coancestry)

• Allows for inferences about Inbreeding, mating system→ is mating random or is it positive/ negative assortment mating (more genetically similar mating and vice versa)

How is genetic variation distributed within populations/ between sub-pop’s?

Contrast θ and α

How can we observe population structure?

Use of Genetic markers

1. Single Nucleotide Polymorphisms (SNPs)→ more commonly used

2. Microsatellites→ rare in coding sequences

1- F (inbreeding coefficient)

= observed heterozygosity

1- θ (Coancestry)

= expected heterozygosity

(diversity between individuals)

1-α

= total heterozygosity

(diversity across/ between populations)

What does adaptation imply?

• favourable alleles go to fixation

• deleterious alleles are eliminated

But this requires

• effective selection

• weak drift

What are the conditions needed for effective selection and weak drift?

Alleles represented by large numbers of individuals

• infinite populations

• common-ish alleles in large-ish finite populations

What is the fate of new mutations?

1. Be lost→ could be lost later → most likely outcome

2. Not be lost [long-term: fixation]

For neutral mutations, what proportion are lost immediately?

• over 1/3

• independent of population size

What happens to mutations with a selective advantage (positive selection coefficient)?

• As s gets higher, probability of losing new mutation goes down very slowly→ very few mutations that increases survival/ reproductive rate by a lot

• Strong selection has no big impact on initial fate of mutations

• Many (even strongly) beneficial mutations are immediately lost

What happens after the first generation?

• passed to more generations if not lost

Long term outcomes

• loss at a later stage

• fixation

[P(extinction)] = [1-P(fixation)]

At start:

Alleles rare: stochastic invasion

After a few generations

Alleles Common: ± deterministic fixation

Implications of drift for conservation biology

Endangered species have small, isolated populations

• genetic drift is strong, selection inefficient

• accumulation of deleterious alleles

• populations decline further

• Vicious circle → 'Mutational meltdown'

Solutions→

• You can move individuals around

• Avoid mating between related individuals