Genetic Drift

Natural Selection vs genetic drift

Natural selection is when we have subset of population that is reproducing that has high fitness to cause allele changes in population (NONRANDOM) 

Genetic drift is subset of population that is reproducing is RANDOM 

Why does genetic drift occur?

Because of probability rules / gamete sampling error

As size of population increases, sampled frequencies begin to

look like expected frequencies

Why is drift a stronger force in smaller populations?

Allele changes are more extreme because will effect larger proportion than large populations

Genetic drift through sampling errors will ultimately cause allele to

drift to fixation or lost (ultimate fate) 

Migration vs Genetic Drift 

Migration

• Increases diversity within a population

• Decrease differences among a population (homogenizing force) 

Genetic Drift

• Decreases diversity within a population

• Increased differences among populations 

Probability of fixation in drift

• If allele starts off as more common, more likely to drift toward fixation

• Probability than an allele will drift towards fixation is equal to initial frequency of the allele

• Ex. If A2 starts off in population as 50%, 50% going to become fixed and 50% going to become lost

Given that mutation introduces new alleles into a population, but they are usually at low frequencies, what is most likely the fate of an allele that is introduced to a population?

It will become lost

What is founder effect?

Small number of individuals find a new population

One time sampling error associated with population

Might not look like original population

Ex. Amish have high frequency of polydactyly as one of founders of Amish population had the gene, so now gene is at higher frequency than original population

What is population bottle neck? 

Founder effect without moving

Population undergoes constriction in size causing effects to magnify if smaller population 

Impact depends on size of population reduction and length of time in bottleneck

Ex. Overhunting can cause bottleneck 

What did Buri Experiment reveal?

Showcased genetic drift in lab setting

Population size of 16 and allele frequency of 0.5

Evolved population under drift for 19 generations by choosing 8 flies at random to be next generation

Populations lost heterozygosity (Hz) at a rate more rapidly then expected, revealed effective population size of flies was 9 not 16

Heterozygosity (Hz)

Ranges from 0 to 1

Frequency of heterozygotes in population

Genetic drift causes heterozygosity (and other measures of genetic diversity) to decline

Decreases much more rapidly in smaller populations

No more heterozygosity when one of allele is lost and other allele went to fixation to only have homozygotes in population

Effective Population Size (Ne)

size of an ideal theoretical population based on its genetic diversity

number of individuals contributing alleles to next generation

can be explained by sexual selection for certain mating behavior for bias for decrease Hz

Why is Ne in humans 10,000 when population size is 7 billion?

Due to sexual selection and bottleneck in past which has decreased Hz greatly 

Knowing what you do about the effects of genetic drift on Hz and how the strength of genetic drift varies with population size, what relationship would you expect between population size and Hz?

As population increases, expect to see population more genetically diverse so Hz increases

Drift coupled with selection

• Both can cause loss of genetic diversity

• Selection selects for beneficial alleles and eliminates deleterious ones

• Drift: random alleles that are being fixed

• Drift makes selection more noisy as will look less like idealized S shaped curve (selection alone) and expect more variation

If there is low enough effective size (Ne)..?

Effects of selection are neglible as drift becomes dominating mechanism of evolution

If there is large population size and Ne…?

selection dominates and effects of drift are neglible 

How can drift work against selection?

• Beneficial allele can be lost randomly

• Deleterious allele can be fixed randomly

What is crossing fitness landscapes?

• Theoretical idea on how drift can aid selection

• If only selection is acting in a population, then selection is going to tend to drive populations to fitness optimum. Fitness optimum might just be local optimum, not global optimum A population get stuck

• X axis: genetic variation Y axis: Fitness

• With genetic drift and selection, it introduces more noise so it can randomly cross fitness valleys and reach new fitness optimum