Lecture 20 : Mutation, Genetic Drift and Migration

Question 1

Explain Directional selection:

Answer 1

Directional selection is the basis for adaptive evolution but other types of selection can maintain genetic variation within populations.

Question 2

Explain mutations (2)

Answer 2

1. process that generates genetic variation within populations
2. Meiosis creates new combinations of alleles, but all genetic variants ultimately arise through mutation.

Question 3

The effect of recurrent mutations on allele frequency (2)

equation?

Answer 3

the amount G2 will change ( q) due to mutation depends on:

1. the rate of G1 to G2 mutation (u)
2. the frequency of G1 in the population (p)
3. The change in G2 as the result of mutation equals the rate of mutation times the allelic frequency

change in q = up

This formula shows that as p decreases due to mutation, the change in allele frequency due to mutation will become less and less.

Question 4

The effect of reverse mutations on allele frequency? (3)

Answer 4

1. Reverse G2 -G1 mutations also occur at a rate v, which will probably be different from u.
2. the rate of change due to reverse mutation equals the reverse mutation rate times the frequency G2 ( change in p = vq)
3. overall change in allele frequency is a balance between forward and reverse mutation:

change in q = up - vq

Question 5

Reaching equilibrium of allele frequency? = 3

Answer 5

1. Equilibrium occurs when the rate of genes undergoing forward mutation is counter balanced by the number of genes undergoing reverse mutation (gain = loss).
2. change in q = 0, in spite of the fact that mutations continue to occur.
3. the point at which there is NO CHANGE in the allelic frequency of a population is referred to as EQUILIBRIUM

Question 6

Reaching equilibrium of allele frequency (2) equations?

Answer 6

1. At equilibrium, the frequency of G2 (Q) will be:

q = u/(u + v)

2. This shows that the equilibrium frequency of G2 will be determined solely by the forward (u) and reverse (v) mutation rates.

Question 7

Mutation: summary of effects (3)

Answer 7

1. when the only evolutionary force acting on a population is a mutation, allele frequencies change because some alleles mutate into others.
2. allelic frequencies eventually reach equilibrium when (change in q = 0) which is determined only by the forward and reverse mutation rates.
3. Mutation rates are typically very low. therefore the change in allelic frequencies each generation is typically very small.

G1 <—> G2

Question 8

Explain Migration

effects are twofold why?

Answer 8

1. Migration is the movement of genes between populations. it is also called gene flow.
2. The overall effects of migration are twofold:

— it prevents divergence between populations, and

— it increases the genetic variation within populations

Question 9

Change in allelic frequency due to migration

• equation
• and explain it

Answer 9

Equation: change in q = m(q1- q2)

The equation above tells us:

1. the change in q is directly proportional to migration (m)
2. the magnitude of change is also affected by the differences in allele frequencies between populations (q1-q2)

Question 10

Explain the mIgrational Equilibrium: 2

Answer 10

1. With each generation of migration, the allele frequency in populations becomes more similar
2. equilibrium is reached when the allele frequency of the recipient population equals the source population.

Question 11

Migration: Summary of effects (4)

Answer 11

1. Migration causes chnages in the allelic frequency of a population by introducing alleles from other populations
2. the magnitude of change depends on the amount of migration and the difference in allelic frequencies between populations ( change in q = m(q1-q2)
3. Migration DECREASES genetic differences between populations and INCREASES genetic variation within the population
4. a MIGRATIONAL equilibrium is reached when there is no difference in allelic frequencies between populations

Question 12

Explain genetic drift : 2

Answer 12

1. HARDY-WEINBERG LAW assume the population are INFINITELY LARGE

however no population. is infinitely large and the gametes that unite to form the next generation are only a sample of the alleles present in the parental gene pool.

2. therefore …change can cause the composition of. sample to deviate from the rental gene pool.

Question 13

Predicting the magnitude of genetic drift
EQUATION and explanation

Answer 13

1. genetic drift expressed in terms of the variance in allele frequencies among the populations (S(subscript p) ^2)

S(subscript p) ^2 = pq/2N

2. The magnitude of genetic drift is determined by two parameters (1) the allelic frequencies ( p + q ) and (2) the population size (N)

Question 14

Effective population size: 3

Answer 14

1. population size is usually defined as the number of individuals in a group (Nc)
2. However, the evolution of the gene pool depends only on the individuals that contribute to the genes to the next generation
3. The effective population Size (Ne) is defined as the number of equivalent breeding adults in a population.

Question 15

Factors influencing the equivalent Ne (3)

Answer 15

1. Variation in reproductive success among individuals
2. fluctuations in population size
3. sex ratio

Question 16

Causes of genetic drift

Answer 16

1. Environmental stress leads to small population sizes for multiple generations
2. Founder effect - populations being established by a small number of individuals
3. Genetic bottleneck - develops when a population undergoes a drastic reduction in population size.

Question 17

Genetic drift: Summary of effects (3)

Answer 17

1. changes in allele frequency - because genetic drift is random, te frequency of an alle may increase or decrease and will vary with time
2. Reduction in genetic variation within populations: random changes can cause allele frequencies to reach 0 or 1, when all individuals are homozygous.
3. Gentic divergence between populations - because drift occurs,] randomly, allele frequencies in different populations do not always change the same way.

Question 18

SUMMARY OF THE EFFECTS OF EVOLUTIONARY FORCES

Answer 18

THE PUNNET SQUARE TABLE NEEDED TO BE DRAWN FROM PAGE 33 OF LECTURE 20.