Chapter 6: Genetic Drift

What needs to be violated in order for evolution to occur?

One or more of the Hardy-Weinberg assumptions

Genetic drift definition

Random change in allele frequencies in a population over time

Will allele frequencies be the exact same in random samples of different generations, even if no directional selection was acting?

No! The samples are random, and it is very unlikely that allele frequencies will remain the exact same. Allele frequencies are always changing, even if 2 alleles have the same fitness

Genetic drift will drive an allele to either ___ or ____

Fixation or extinction

True or false: genetic drift removes genetic variation from populations

True!

What determines the time until allele fixation occurs WITHOUT directional selection?

Population size!

What will be observed in allele frequency fluctuations between small and large populations?

Small populations will exhibit more drastic, jagged frequency shifts, while larger populations will exhibit less dramatic, smoother shifts. Thus, allele fixation will occur more quickly in smaller populations!

True or false: polymorphic loci (more than one allele per locus) is reduced in smaller populations. Namely, genetic variation is reduced in smaller populations

True

What is the result of a genetic bottleneck?

Quick reduction in genetic variation

What are some examples of genetic drift and the bottleneck effect?

When some humans migrated from north to South America, the A and B blood type alleles were randomly lost

With no directional selection, how often is an allele, A1, expected to go to fixation if it has no selection advantage over A2 and it has a starting frequency of 0.5? What if it had a starting frequency of 0.10?

1. It would go to fixation 50% of the simulation runs

2. It would go to fixation 10% of the simulation runs

If A1 has a frequency of 0.5 in a LARGE POPULATION and a selection advantage of 1%, how will the expected fixation rate be affected?

Now that the A1 allele is actually favored, so it will go to fixation much more than 50% of the time, usually 100% of the time.

Under what conditions is directional selection favored, and under what conditions is genetic drift favored?

Directional selection: when the favored allele is high in frequency and/ or in a large population

Genetic drift: if the favored allele is low in frequency AND population is small

How might a disfavored allele go to fixation?

If the population is small, and the disfavored allele starts out at a high frequency

How might genetic drift affect small populations in terms of conservation efforts?

• Deleterious alleles might go to fixation

• Genetic variation is heavily reduced

• Population is more susceptible to one disease

Generic load definition

The frequency of deleterious alleles that have accumulated in a population

True or false: sexual reproduction introduces new alleles into the population?

False! Sexual reproduction mixes up alleles, doesn’t generate new ones - mutations do that

When is the power of genetic drift greater than that of directional selection?

When populations are small

What evolutionary mechanism best explain the presence of fewer alleles per locus in small population than in big?

Genetic drift