Genetic drift

Question 1

‘The differential survival of genotypes drives evolution’. What does this say about alleles?

Answer 1

Frequency is key to diversity and population structure.

Question 2

‘1-s’ is the selection coefficient. What does it say about a particular genotype?

Answer 2

That it will survive by proportion less well than others by proportion s.

Question 3

Is it possible for an unfit genotype to survive purely down to luck?

Answer 3

Yes, ‘there are always expected deviations from fitness values’.

Question 4

Loci can be either selectively advantageous, disadvantageous or neutral. When they are neutral, what changes in allelic frequency do we expect to see?

Answer 4

No changes between generations.

Question 5

Define genetic drift.

Answer 5

The chance disappearance of alleles in a small population caused when individuals die or fail to reproduce.

Question 6

Genetic drift drives evolution by creating variation. True or false?

Answer 6

True.

Question 7

Why does genetic drift have a bigger effect in small populations?

Answer 7

It leads to a loss of diversity or a fixation of genotypes.

Question 8

Consider a single, self-fertilising organism, the smallest population size possible. Explain why small population size leads to a loss of diversity.

Answer 8

1. If the organism is a heterozygote, it will produce 2 different types of gamete.
2. When the gametes come together, 1/2 will be heterozygous, 1/4 will be homozygous dominant and homozygous recessive.
3. Thus in one generation, 1/2 of all offspring (the homozygotes) have already lost an allele.

Question 9

If there is no variation there is nothing for selection to act on. True or false?

Answer 9

True.

Question 10

What happens when genetic drift acts on large populations? How does this occur?

Answer 10

It creates divergence which leads to speciation. Isolated sub-sets of an original population are all exposed to different environments, meaning selection acts on them independently and produces different results.

Question 11

What do genetic drift models assume about mating?

Answer 11

It is random.

Question 12

What 2 factors increase genetic drift in a population?

Answer 12

1. Small population size

2. Fewer individuals contributing to reproduction, i.e. species in which males have harems so not all males get to mate

Question 13

What 2 factors reduce drift in a population?

Answer 13

1. Large population size

2. Monogamy, every male gets to mate with a female

Question 14

Is it important to understand the mating systems of organisms when calculating genetic drift?

Answer 14

Yes.

Question 15

What is Ne?

Answer 15

The ‘effective population size’ coefficient. It refers to the number of organisms in a population that are reproductively active.

Question 16

Ne values are an estimate and can be misleading. Why?

Answer 16

Because the Ne values for a small, monogamous population might be the same as a large, random-mating population. Both monogamy and large size reduce drift but small size and random-mating increase it.

Question 17

What is genetic identity?

Answer 17

The probability that alleles are identical.

Question 18

What is ‘F’?

Answer 18

The inbreeding coefficient, it is a measure of homozygosity WITHIN an individual.

Question 19

What is ‘F’ used to assess?

Answer 19

‘Cosanguity’ or the degree of inbreeding in an individual.

Question 20

What is ‘θ’?

Answer 20

A measure of coancestry between individuals.

Question 21

What is ‘α’?

Answer 21

A measure of coancestry between populations.

Question 22

Fixation indices are used to compare heterozygosity. What is F(IS)?

Answer 22

The fixation indice WITHIN a population. F(IS) gives information about inbreeding and assortative mating.

Question 23

How do you calculate F(IS)?

Answer 23

F(IS) = [(1- θ) – (1 – F)]/(1- θ)

Question 24

Fixation indices are used to compare heterozygosity. What is F(ST)?

Answer 24

The fixation indice BETWEEN populations. F(ST) gives looks at whether there has been differentiation from an orginal population.

Question 25

What does it mean if F(IS) > 0?

Answer 25

Homozygosity is greater than expected, there has been inbreeding or assortative mating.

Question 26

What is meant by assortative mating?

Answer 26

When individuals preferentially mate with others of a particular genotype.

Question 27

How do you calculate F(ST)?

Answer 27

F(ST) = [(1- α) – (1 – θ )]/(1- α)

Question 28

What does it mean if F(ST) > 0?

Answer 28

There has been differentiation from a single, original population.

Question 29

What does ‘1 - F’ mean?

Answer 29

Observed heterozygosity.

Question 30

What does ‘1 – θ ‘ mean?

Answer 30

The average expected heterozygosity within a population.

Question 31

What does ‘1 – α’ mean?

Answer 31

The average expected heterozygosity between populations.

Question 32

What can we look at to observe variation within populations? Give 3 examples

Answer 32

1. SNPs
2. Genetic markers
3. Mutations in microsatellite regions

Question 33

How can the principles of genetic drift be used to track migration?

Answer 33

When a sub-set of the original population leaves and migrates to a new location, diversity is greatly reduced as they carry a low percentage of the total alleles. With each migration there is a reduction in diversity. The least genetically diverse populations are the most recent ones.