Population Selection, Structure, & Drift Flashcards
Selection:
Non-random change in the frequency of a genetic variant due to differential fitness
Mutation is…
The process by which variants are created for selection to act upon
Is the mutation rate large or small
Small, about 2x10^8 per bp
If the mutation rate is so small, how are they also so common?
The rate is small (2x10^8 per bp) but the genome is very large (3x10^9bp) so there are lots of opportunities for mutations
What is used to describe a variant’s effect on fitness?
Selection coefficient (s)
Fitness:
The number of offspring an individual has that survive to maturity
To calculate s we normalize the fitness of the background population to…
1
The equation for fitness of a homozygous mutant is…
1+2s
The equation for fitness of a heterozygous mutant is…
depends on dominance (h)
1+2hxs
h=1 dominant
h=0 recessive
h=0.5 additive variants
What is the s value for beneficial, deleterious, and neutral variants respectively
B: s>0
D: s<0
N: s=0
What are the types of selection (think bell curves)
Stabilizing
Directional
Diversifying
What does the fate of a new mutation depend on?
Selection
Effective population size
Drift
What factors need to be large enough in order for selection to overcome drift
s and Ne (effective population size)
Mutations are most commonly………..and least commonly………..
In terms of the effect they have
Neutral, beneficial
What is the equation for a nearly neutral mutation
|sxNe|<1
What percent of all new mutations are lost within one generation due to chance
30%
What affectes effective population size
Whether or not mating is random
If the size is stable/fixed
Sex ratio distortion (dairy cattle)
What is the loss of AB genotypes called
The Wahlund effect
Two populations can fix alternative alleles. This is represented by
Fst
AA: p^2 + Fstpq
AB: 2pq(1-Fst)
BB: q^2 + Fstpq
Selections is …………in populations with larger Ne
Stronger
Relative efficiency (time until fixation) for codominant v dominant v recessive mutations
Co-dominant - fastest
Dominant - middle, inefficient
Recessive - low, very slow
What is a selective sweep
When a positively selected variant drags other variants (regardless of desirability) along with it.because they are located close together. Called genetic hitchhiking or genetic draft.
This reduces diversity and heterozygosity in the region
Extreme Fst outliers are a sign of…
Recent adaptation
Why do we still have genetic diseases? Isn’t that the point of selection?
Hitchhiking - deleterious mutations linked with positive ones –> not weeded out
Pleiotropy - variants can have multiple phenotypic effects or have different effects in different environments
Drift - small Ne –> strong drift, gives deletrious mutations a chance to increase in frequency
