Lecture 17 - Genetic Drift and Natural Selection

Question 1

What is the Hardy-Weinberg Equilibrium (HWE) null model?

Answer 1

It is a model for a sexually reproducing, diploid population with random mating. It predicts three genotypes with expected frequencies:

Homozygous (AA): p²
Homozygous (TT): q²
Heterozygous (AT): 2pq

Question 2

What are the five assumptions of HWE?

Answer 2

Random mating (no assortative or disassortative mating, no inbreeding)
No mutation
No selection
No genetic drift
No migration

Question 3

What does random mating ensure in HWE?

Answer 3

Random mating ensures that genotype frequencies match the expected proportions (p², q², 2pq) after one generation.

Question 4

What is the difference between assortative and disassortative mating?

Answer 4

Assortative mating: Genotypically similar individuals mate, leading to more homozygotes.

Disassortative mating: Genotypically dissimilar individuals mate, leading to more heterozygotes.

Question 5

How does inbreeding affect HWE?

Answer 5

Inbreeding increases homozygosity for deleterious alleles and reduces heterozygosity, similar to assortative mating but with a focus on recessive alleles.

Question 6

What is inbreeding depression?

Answer 6

Inbreeding depression is reduced fitness due to increased homozygosity for deleterious recessive alleles, common in small or isolated populations.
Seen in highly inbred dog breeds.

Question 7

What is the formula for the inbreeding coefficient (F)?

Answer 7

F = 1 - (Hₒ / 2pq), where:

Hₒ = observed frequency of heterozygotes
2pq = expected frequency of heterozygotes

Question 8

What is genetic drift, and when is it most significant?

Answer 8

Genetic drift is the change in allele frequencies due to random sampling, most significant in small populations.

Question 9

What is the founder effect in genetic drift?

Answer 9

It occurs when a small group of individuals establishes a new population, leading to different allele frequencies compared to the original population.

Question 10

What are the three types of selection?

Answer 10

Directional selection: Shifts the mean phenotype (e.g., darker fur color).

Disruptive selection: Selects for extreme phenotypes over intermediates.

Stabilizing selection: Reduces variation by selecting for intermediate phenotypes.

Question 11

How is fitness measured in selection?

Answer 11

Fitness (W) measures survival/reproductive success. For a genotype A₁A₁:

W₁₁ = 1 (highest fitness, reference)
W₁₂ = 0.9
W₂₂ = 0.8
Selection coefficients:

S₁₁ = 1 - W₁₁ = 0
S₁₂ = 1 - W₁₂ = 0.1
S₂₂ = 1 - W₂₂ = 0.2

Question 12

How does mutation affect allele frequencies?

Answer 12

Mutation introduces new alleles, often starting at a frequency of 1/N in haploid populations or 1/2N in diploid populations.

Question 13

What is the effect of migration on allele frequencies?

Answer 13

Migration introduces new alleles into a population, with the impact depending on the migration rate and the difference in allele frequencies between populations.

Question 14

What happens when HWE assumptions are violated?
no drift
no selection
no mutation
no migration
mating

Answer 14

Drift: Increases fluctuations in allele frequencies, especially in small populations.

Selection: Alters allele frequencies based on fitness differences.

Mutation: Introduces new alleles.

Migration: Adds alleles from other populations.

Question 15

What is the founder effect in genetic drift?

Answer 15

A phenomenon where a small group of individuals establishes a new population, leading to reduced genetic diversity compared to the original population.

Question 16

How does population size affect genetic drift?

Answer 16

Smaller populations experience more significant genetic drift, leading to faster fixation or loss of alleles compared to larger populations.

Question 17

What is the role of selection coefficients (s)?

Answer 17

Selection coefficients measure the relative disadvantage of genotypes compared to the fittest genotype, influencing how quickly allele frequencies change.

Question 18

How does heterozygote advantage affect allele frequencies?

Answer 18

can cause allele frequencies to evolve toward a stable equilibrium, regardless of the initial allele frequency.

Question 19

What are the outcomes of directional, stabilizing, and disruptive selection?

Answer 19

Directional selection: Shifts the population’s mean phenotype toward one extreme.

Stabilizing selection: Reduces variation by selecting against extremes.

Disruptive selection: Favors both extremes, increasing phenotypic variation.

Question 20

How do mutation and migration differ in their effects on allele frequencies?

Answer 20

Mutation: Introduces new alleles, often at low frequencies.

Migration: Introduces or redistributes alleles between populations, affecting allele frequencies based on the migration rate and source population.

Question 21

How do you calculate the inbreeding coefficient (F)?

Answer 21

F = 1 - Hobserved/2pq, where Hobserved is the observed heterozygosity and 2pq is the expected heterozygosity.

Question 22

What is overdominance in fitness?

Answer 22

Overdominance occurs when heterozygotes have higher fitness than either homozygote, maintaining both alleles in the population.

Question 23

What is the relationship between migration rate and genetic diversity?

Answer 23

High migration rates increase genetic diversity by introducing new alleles, while low rates can lead to population divergence.

Question 24

What is the significance of neutral mutations?

Answer 24

Neutral mutations do not affect fitness and are subject to genetic drift, contributing to genetic variation over time.

Question 25

How can selection lead to allele fixation or loss?

Answer 25

Alleles with higher relative fitness increase in frequency over generations, leading to fixation, while less fit alleles decrease and may be lost.