Migration, Genetic Drift, and Molecular Evolution

What is gene flow?

Gene flow is the movement of alleles between populations through migration.

What is migration in population genetics?

Migration is the movement of genes or alleles from one population or subpopulation to another.

Is migration an evolutionary mechanism?

Yes, because it changes allele frequencies within populations.

What can immigration do to genetic variation?

It can introduce new alleles and increase genetic variation within a population.

What can emigration do to genetic variation?

It can remove alleles and reduce genetic variation within a population.

What is a metapopulation?

A metapopulation is a larger population made up of subdivided local populations connected by migration.

Why is migration described as movement of genes rather than just movement of organisms?

Because population genetics focuses on whether alleles move between populations, not just whether individuals physically travel.

What does m represent in migration models?

m is the proportion of migrants entering a local population.

What does (1 − m) represent in migration models?

It represents the proportion of alleles in the local population descended from residents.

What does pc represent in the migration equation?

pc is the allele frequency in the continental or source population.

What does pt represent in the migration equation?

pt is the allele frequency in the local population before migration.

What does p’t represent in the migration equation?

p’t is the allele frequency in the local population after migration.

What is the allele frequency after one generation of migration?

p’t = (1 − m)pt + mpc.

What is the change in allele frequency due to migration?

Δpt = −m(pt − pc).

When is the change in allele frequency due to migration greatest?

When the migrant proportion m is large and the difference between pt and pc is large.

When will migration stop changing allele frequencies?

When there are no new migrants (m = 0) or when pt = pc.

What is the overall long-term effect of migration alone on populations?

It homogenizes populations so they become more genetically similar.

How can migration reduce differences between populations?

By introducing alleles from one population into another until allele frequencies converge.

How can heavy migration disrupt Hardy-Weinberg equilibrium?

Strong migration can change genotype and allele frequencies, pushing a population away from equilibrium expectations.

What does FST measure?

FST measures the degree of genetic differentiation among populations.

What does a high FST value mean?

It means populations are more genetically different from one another.

What does a low FST value mean?

It means populations are genetically similar.

How can founder events affect population differentiation?

Founder events can increase genetic differences among populations by randomly changing allele frequencies.

What is migration-selection balance?

It is the balance between gene flow introducing alleles and natural selection favoring or removing alleles locally.

Why is gene flow sometimes compared to mutation?

Because both can introduce new alleles into a population and increase within-population genetic variation.

What are the three things selection can do to a new allele introduced by gene flow?

Selection can do nothing, favor it, or act against it.

How can migration oppose local adaptation?

By continually introducing alleles that are not optimal in the local environment.

When can migration override natural selection?

When migration is strong and local selection is weak.

What happened in the Northern Water Snake example?

Gene flow from mainland populations interacted with local selection on island populations, affecting banding patterns.

What trait was favored on the mainland in the Northern Water Snake example?

Banded snakes were favored on the mainland.

What trait was favored on islands in the Northern Water Snake example?

Unbanded snakes were favored on islands.

Why were islands closest to the mainland more similar to mainland snakes?

Because they experienced more gene flow from the mainland.

Why were islands farther from the mainland more different?

Because they had less migration and stronger local adaptation.

What are the main migration concepts to know?

Metapopulation, gene flow, migration effects on allele frequencies, and migration-selection balance.

What is genetic drift?

Genetic drift is random change in allele frequencies from one generation to the next due to chance.

Is genetic drift adaptive?

No, genetic drift is a nonadaptive evolutionary mechanism.

Why is genetic drift considered random?

Because allele frequencies change due to chance sampling error, not because one allele is better adapted.

Why did Darwin not focus on genetic drift?

Darwin mainly emphasized natural selection as the driver of evolution.

Why does Hardy-Weinberg require very large population size?

Because finite populations experience sampling error, which causes genetic drift.

When is genetic drift weakest?

In very large populations, because random effects average out.

When is genetic drift strongest?

In small populations, because chance events have a larger effect.

What is effective population size (Ne)?

Ne is the size of an idealized population that would experience the same amount of drift or inbreeding as the real population.

Why is effective population size often smaller than census size?

Because not all individuals reproduce equally or contribute genes to the next generation.

What is census population size?

The actual number of individuals in the population.

Why is Ne more important than census size in evolution?

Because only breeding individuals contributing alleles to the next generation affect evolutionary change.

What factors can reduce Ne?

Unequal sex ratio, variation in offspring number, overlapping generations, fluctuating population size, and nonrandom mating.

What is the equation for effective population size with unequal sex ratio?

Ne = 4NmNf / (Nm + Nf).

What does Nm mean in the Ne equation?

Nm is the number of breeding males.

What does Nf mean in the Ne equation?

Nf is the number of breeding females.

How does unequal sex ratio affect effective population size?

It lowers Ne.

What would happen to Ne if a population were completely inbred?

Ne could be as low as 1 even if census size is large.

Why are nonbreeding individuals considered evolutionary dead ends?

Because their alleles are not passed to the next generation.

What kinds of chance events matter in evolution even without selection?

Who leaves offspring, how many offspring they leave, and which offspring survive.

What is sampling error in population genetics?

Sampling error is random deviation from expected allele or genotype frequencies due to chance.

What is a simple family-level example of sampling error?

A Gb × Gb cross is expected to produce a 3:1 phenotype ratio, but by chance all offspring could be bb.

Why is random loss of an allele not natural selection?

Because no genotype had a fitness advantage; the allele was lost purely by chance.

What are two major consequences of genetic drift?

Random fixation or loss of alleles and reduced heterozygosity.

What is fixation?

Fixation is when an allele reaches a frequency of 1.0 in a population.

What happens to other alleles when one allele becomes fixed?

The other alleles are lost from the population.

Why are random fluctuations in allele frequency larger in small populations?

Because each sampling event has a proportionally bigger effect.

What is the probability of fixation of an allele under drift?

It equals the allele’s starting frequency.

If an allele starts at frequency 0.10, what is its fixation probability?

0.10 or 10%.

If an allele starts at frequency 0.20, what is its fixation probability?

0.20 or 20%.

If an allele starts at frequency 0.60, what is its fixation probability?

0.60 or 60%.

If there are 2N alleles in a diploid population, what is the fixation probability of one copy of an allele?

1 / 2N.

If there are X copies of an allele in a diploid population with 2N total alleles, what is its fixation probability?

X / 2N.

What is a founder effect?

A founder effect occurs when a new population is started by a few individuals, causing random shifts in allele frequencies.

What is a bottleneck effect?

A bottleneck effect is a sharp reduction in population size that reduces genetic variation by chance.

How do founder effects reduce variation?

Because a small founding group carries only a subset of the source population’s alleles.

How do bottlenecks reduce variation?

Because many alleles are lost when population size drops dramatically.

Why can a founder population not capture all alleles from the source population?

Because only a small sample of individuals contributes to the new population.

Why is allele loss especially likely in a new small population?

Because many alleles will be absent by chance from the founders.

What global human pattern supports drift during migration?

Genetic diversity is highest in Africa and decreases with distance from Africa.

What happened to CYP detoxification alleles outside Africa?

Allelic diversity decreased moving away from Africa.

What is heterozygosity?

Heterozygosity is the frequency or proportion of heterozygotes in a population.

Why is heterozygosity often used as an estimate of genetic variation?

Because higher heterozygosity usually reflects more underlying allelic diversity.

What is expected heterozygosity at a two-allele locus under Hardy-Weinberg?

2pq or 2p(1 − p).

How does genetic drift affect heterozygosity?

Genetic drift reduces heterozygosity over time as alleles are lost or fixed.

Why does loss of alleles reduce heterozygosity?

Fewer alleles mean fewer possible heterozygous combinations.

How does loss of allelic variation affect homozygosity?

It increases homozygosity.

Why does no genetic variation limit natural selection?

Because selection needs heritable genetic differences to act on.

In which populations does heterozygosity decline faster?

In small populations.

What did Buri’s Drosophila experiment demonstrate?

That small populations lose heterozygosity and allele diversity over generations due to random drift.

What was the setup of Buri’s Drosophila experiment?

He maintained many small populations of 16 flies, all initially heterozygous at an eye-color locus, for 19 generations.

What general outcome would you expect for allele frequencies over many generations in small fly populations?

They would fluctuate randomly, with some alleles becoming fixed and others lost.

How can genetic drift be detected in genomes?

By looking for random fluctuations in allele frequencies, especially in noncoding or nonfunctional regions.

Why are noncoding regions especially useful for detecting drift?

Because they are less affected by selection, so changes there more strongly reflect demographic history and drift.

What demographic factor most strongly influences drift?

Population size.

What are the major outcomes of stronger drift in small populations?

Random fixation of alleles, reduced heterozygosity, and increased inbreeding.

Why is loss of genetic diversity a major problem in conservation?

Because it is much easier to lose adaptive variation than to recreate it.

How does genetic drift often lead to inbreeding?

By reducing allele diversity and making mating among relatives more likely in small populations.

What is inbreeding?

Inbreeding is mating among genetic relatives.

What is one major genetic effect of inbreeding?

It increases homozygosity.

Why does inbreeding tend to happen in small populations?

Because individuals are more likely to mate with relatives when few mates are available.

What is the inbreeding coefficient F?

F is the probability that two alleles in an individual are identical by descent.

What does “identical by descent” mean?

It means both alleles came from the same ancestral allele.

What does F = 1 mean?

The individual is completely homozygous by descent at that locus.

What is the general formula concept for F based on pedigrees?

It sums the probabilities that both alleles trace back to the same common ancestor.

If half-siblings mate and the common ancestor is not inbred, what is Fx?

1/8.

Why is inbreeding often described as a consequence of drift?

Because drift reduces allele diversity, which increases the probability of mating among relatives and sharing alleles by descent.