Lecture 4 - Random genetic drift and NE

Question 1

Genetic Drift

Answer 1

Stochastic changes in allele frequency by random sampling of gametes to form offspring in finite population

Question 2

For a single locus with two alleles, random sampling follows what distribution?

Answer 2

Binomial distribution

Question 3

What are the changes in probabilities of allele frequencies in the next generation for different population sizes?

Answer 3

For smaller populations there is quite a high probability that it will change the frequency

In larger population, there is less variance

Question 4

What is Buri’s study of genetic drift

Answer 4

A study carried out in Drosophila

Actual data for 107 populations

Randomly selected 8 males and 8 females for the next generation for 19 consecutive generations

Calculated the frequency of bw75 allele (eye colour) and generated a frequency distribution for the 107 populations

Allele frequencies tend to become different overtime

Question 5

Rate of allele frequency change under genetic drift with population size?

Answer 5

Varies with population size

In larger population sizes allele frequencies have fluctuations with lower amplitude

Fixation takes longer in larger population size

Question 6

What does the initial allele frequency determine in genetic drift?

Answer 6

Determines the probability of fixation or loss under genetic drift

Question 7

What does genetic drift lead to?

Answer 7

Loss of variation

Question 8

What is genetic drift caused by?

Answer 8

Variation in a random sampling of gametes in finite populations

Question 9

Why can we not predict allele frequency change in genetic drift?

Answer 9

Random process (stochastic)

Unlike mutation pressure of natural selection which are deterministic

Question 10

How does genetic drift differ across replicate populations?

Answer 10

Average allele frequency does not change but the variation in allele frequency changes overtime

Question 11

Census population size (N)

Answer 11

A head count of the population

Question 12

What are the characteristics of an ideal population?

Answer 12

Mating is random

All individuals are equally likely to have offspring and the number of offspring they have does vary more than would be expected by chance alone

The number of breeding individuals remains the same each generation

There are equal numbers of males and females involved in breeding

Question 13

Effective population size

Answer 13

Number of individuals in an idealized population that undergo genetic drift at the same rate as the actual population under consideration

Question 14

What are reasons for the effective population size to be lower than the census population size?

Answer 14

Variation in reproductive success (that is greater than that expected by chance)

Fluctuation in population size overtime

Variation between the number males and females that contribute offspring to the next generation

Overlapping generations/age-structured populations

Question 15

What is the effective population size a function of?

Answer 15

The number of breeding males and females

Question 16

What might cause low genetic diversity?

Answer 16

Population bottleneck

Question 17

Cheetah population bottleneck

Answer 17

Migrated from north America through Asia to Africa around 100,000 years ago

Another bottle neck 10,000 years ago at the end of the last glaciation

Continued low population size due to recent human impact

Question 18

Population bottleneck

Answer 18

Extreme reduction in population size that increases the effect of genetic drift

Ne is closer to bottleneck population size

Question 19

Ne in population expansion

Answer 19

Ne is closer to the pre-expansion population size

Question 20

Northern elephant seals genetic bottleneck event

Answer 20

Human hunting in the 1890 reduced their population to 20 at the end of the 19th centuary

New population 30,000

But genetically they resemble a much smaller population

They have much less genetic variation than southern elephant seals that were not hunted

Question 21

Founder effect

Answer 21

The loss of genetic diversity when a new smaller population is founded from a larger population

Loss of diversity is due to the genetic drift associated with this type of bottlenecking event

Question 22

Founder effect on blood type

Answer 22

Distribution of O type blood allele in native populations of the world reflect original settlements

Question 23

Founder effect in the amish community

Answer 23

The amish community in eastern united states was founded by a small number of colonists (around 200 from Germany)

They continue to be genetically isolated

The founders had a recessive deleterious autosomal alles that underlies a form of dwarfism, Ellis,van Creveld (EvC) syndrome, that also involved polydactyly of the hands as well as short stature and congenital heart malformation

Heterozygotes now make up about 12.3% of the population