Evolution in Finite Populations

What is genetic drift?

Random fluctuation in allele frequencies due to sampling effects in finite populations.

What are the three general consequences of genetic drift?

Random changes in allele frequency, reduction in genetic variation, and divergence between populations.

How does genetic drift affect allelic diversity?

It reduces diversity as alleles drift to fixation or loss.

What did Sewall Wright show about allele fixation?

That the probability an allele will fix is equal to its current frequency.

What is the formula for the probability of fixation for an allele with k copies in a population of N diploid individuals?

k / 2N

What effect does drift have on heterozygosity?

It causes heterozygosity to decrease over time.

How is observed heterozygosity measured?

1 minus the frequency of observed homozygotes in the population.

How is expected heterozygosity measured?

1 minus the expected frequency of homozygotes under Hardy-Weinberg equilibrium.

What is the effect of small population size on the rate of heterozygosity loss?

It causes a rapid decline in heterozygosity.

What does the equation Ht = (1 - 1/2N)^t model?

The decrease in heterozygosity over time in a population of size N.

What is the coefficient of inbreeding (F)?

A measure of autozygosity—homozygosity due to descent from a common ancestor.

How does inbreeding relate to drift?

Both lead to a decrease in heterozygosity and increase in homozygosity.

What is the founder effect?

Random change in allele frequencies when a new population is founded by a small number of individuals.

What is a population bottleneck?

A sharp reduction in population size that leads to random shifts in allele frequencies.

How is the probability of an allele frequency combination calculated?

Using the binomial expansion formula.

What did the Galápagos lava lizard study show about population size and genetic diversity?

Smaller island populations had fewer alleles and showed greater divergence due to drift.

What are microsatellites?

Short, repeating sequences in DNA used to assess genetic variation.

What does the neutral theory of molecular evolution propose?

Most molecular variation is selectively neutral and evolves due to genetic drift.

Who proposed the neutral theory?

Motoo Kimura.

What is a substitution in molecular evolution?

A mutation that becomes fixed in the population.

What does the neutral theory say about mutation vs. substitution?

Most mutations are deleterious and purged, but most substitutions are neutral.

What is a synonymous mutation?

A DNA change that does not alter the amino acid sequence (silent mutation).

What are pseudogenes and why are they important?

Nonfunctional genes that provide insight into neutral evolutionary processes.

What is the molecular clock?

The concept that neutral mutations accumulate at a constant rate, allowing estimation of divergence times.

How does population size affect the rate of neutral substitutions?

It does not—substitution rate is independent of population size.

What is the formula for the neutral substitution rate?

k * n, where k is number of loci and n is mutation rate.

What is the nearly neutral theory?

Suggests that slightly deleterious mutations can become fixed via drift in small populations.

What happens when selection is weak and population size is small?

Drift dominates allele frequency changes.

What is Haldane's estimate for the fixation probability of a new beneficial mutation with fitness advantage s?

Approximately 2s.

Why can beneficial alleles be lost from a population?

Drift can eliminate them, especially when initially rare.

What determines whether drift or selection dominates?

The relative strength of selection vs. population size.