Genetic Drift

Question 1

Define genetic drift.

Answer 1

Genetic drift is the random change in allele frequencies within a population from generation to generation.

Question 2

Why care about genetic drift?

Answer 2

• Genetic diversity is the raw material for adaptive evolution
• A loss of heterozygosity also entails an increase in homozygosity which often leads to reduced fitness

Question 3

What are two possible effects of genetic drift?

Answer 3

loss of heterozygosity
Random fixation of alleles

Question 4

When does genetic drift lead to random fixation of alleles?

Answer 4

When genetic drift is the only mechanism acting on alleles within a population.

Question 5

What are the four principles of genetic drift?

Answer 5

o Effects of genetic drift on allele frequencies are greater in small populations than in large populations
o Genetic drift causes every population to follow a unique evolutionary path
o Genetic drift can produce substantial changes in allele frequencies
o If genetic drift is the only evolutionary mechanism causing changes in allele frequencies at a locus, a single allele will eventually be fixed at each locus, given sufficient time

Question 6

What does this equation represent? Define all variables.

Answer 6

This equation is used to calculate the time until genetic drift fixes an allele.
N = population size
p = initial allele frequency

Question 7

What is the probability that a selectively neutral allele will eventually be fixed?

Answer 7

Equal to its initial frequency

Question 8

Define effective population size.

Answer 8

The size of an ideal random mating population that would lose heterozygosity at the same rate as an actual population of interest

Question 9

What does this equation represent? Define all variables.

Answer 9

This is the equation for effective population size (Ne).
NM = number of males
NF = number of females

Question 10

What type of changes is Ne susceptible to?

Answer 10

Changes in sex ratio

Question 11

How does Ne typically compare to actual population size?

Answer 11

Almost always smaller

Question 12

What are the 4 instances in which genetic drift is important?

Answer 12

When populations are small
When different alleles, or variation in traits, have equal fitness
During population bottlenecks
Founder events

Question 13

Define population bottleneck.

Answer 13

o An event in which a significant percentage of a population or species is killed or otherwise prevented from reproducing

Question 14

Define founder events.

Answer 14

o A new population is established by a small number of colonists

Question 15

What are the three allelic variations with no effect on fitness?

Answer 15

Silent/synonymous substitutions
Variation at loci
Psedogenes

Question 16

Differentiate between natural selection and genetic drift (3)

Answer 16

Selection: alleles of individuals with highest fitness pass on the most copies to next gen
Drift: Alleles passed onto next gen influenced by chance events

Selection: evolution is not random
Drift: evolution is random

Selection: evolution by natural selection leads to the evolution of adaptations
Drift: evolution cannot lead to the evolution of adaptations

Question 17

Who formulated the neutral theory of evolution?

Answer 17

Motoo Kimura

Question 18

Outline the neutral theory of evolution.

Answer 18

• Claims that most DNA mutations have no effect on fitness
• Molecular evolution is dominated by neutral evolution (=genetic drift)
• Genetic drift is acting on neutral alleles at a consistent rate

Question 19

Under mutation-selection balance, what is the equilibrium frequency of a lethal recessive allele? A lethal dominant allele?

Answer 19

Recessive: frequency equal to square root of mutation rate
Dominant: frequency equal to mutation rate