Selection and drift combined

Question 1

Is there any drift in an infinite population?

Answer 1

No.

Question 2

Define deterministic genetics.

Answer 2

Genetics that can be completely described with a model and future values predicted.

Question 3

Brand new mutations are only ever represented by one individual. True or false?

Answer 3

True.

Question 4

What is the frequency of a brand new mutation in a haploid population?

Answer 4

1/N, where N is the frequency of the allele in an individual.

Question 5

What is the frequency of a brand new mutation in a diploid population?

Answer 5

1/2N, where N is the frequency of the allele in an individual.

Question 6

If a mutation is neutral what effect does it have on fitness?

Answer 6

None.

Question 7

What percentage of brand new mutations (beneficial and detrimental) are lost immediately?

Answer 7

Over a third.

Question 8

What is the probability of losing a brand new mutation by drift in a) a haploid population and b) a diploid population?

Answer 8

a) P = (1-1/N)N

b) P = (1-1/2N)2N

Question 9

What is the probability of losing a brand new mutation by selection? Explain what each term is.

N.B. probably don’t need to know this for exam, just need to recognise it.

Answer 9

P[k] = (ƛk/k!)e-ƛ

p = probability
k = no. of observed mutant offspring
ƛ = expected no. of offspring (or 1+s)
s = selection coefficient

Question 10

What distribution do expected frequencies fall under?

Answer 10

The poisson distribution.

Question 11

The better the mutant, the smaller the possibility of losing it straight away. True or false?

Answer 11

True.

Question 12

What percentage of beneficial brand new mutations are lost immediately?

Answer 12

Over a third.

Question 13

What is the purpose of modelling selection and drift together?

Answer 13

To assess the likelihood that particular alleles will survive to become fixed in populations.

Question 14

Define the ‘cumulative extinction probability’.

Answer 14

How likely an allele is to go extinct over time.

Question 15

Define the ‘stochastic phase of invasion’.

Answer 15

The very first generations to possess a new muation

Question 16

Define ‘deterministic fixation’.

Answer 16

When a mutant allele becomes fixed within a population.

Question 17

When is there a high probability of extinction for a new mutation and why?

Answer 17

At the stochastic phase of invasion because there are very few copies of the allele within the population.

Question 18

Why is there less chance of an allele being eliminated at deterministic fixation?

Answer 18

There are more copies of it within the population, thus loss is less damaging.

Question 19

What is the equation for the probability of fixation in a haploid population?

Answer 19

P ~ 2s

s = the selection coefficient

Question 20

What is the equation for the probability of fixation in a diploid population? Why does this differ to that for a haploid population?

Answer 20

P ~ 2(1-h)s

h = the dominance coefficient
s = the selection coefficient

Diploid populations must consider the dominance coefficient as there are 2 copies of the allele present.

Question 21

Chance plays a role in evolution: ‘the invasion of beneficial mutations in stochastic’ - what does this mean?

Answer 21

That beneficial mutations do not always survive.

Question 22

Chance plays a role in evolution: ‘the fixation of deleterious mutations is random’ - what does this mean?

Answer 22

Deleterious mutations sometimes survive.

Question 23

There is an equation that combines both drift and selection. What is it used for?

Answer 23

Determining the probability of fixation of an allele.

Question 24

Give the equation for the probability of fixation for a) a haploid and b) a diploid population.

Answer 24

a) Ne x s
b) 2Ne x s

Ne = effective population size
s = selection coefficient

Question 25

For the probability of fixation, what does it mean if Ne x s >1?

Answer 25

Selection is stronger than drift.

Question 26

For the probability of fixation, what does it mean if Ne x s <1?

Answer 26

Drift is stronger than selection.

Question 27

For the probability of fixation, how are selection and drift brought together?

Answer 27

s is the selection coefficient, representing selection. Ne is the effective population size used in drift.

Question 28

Are mutations under selection in large or small populations? Why?

Answer 28

Large because there is more diversity. If there is no variation, as in small populations, selection has nothing to act on.

Question 29

In which type of population is it harder to lose mutations. Give 2 examples.

Answer 29

1. Small populations

2. Those with assortative/selective mating e.g. humans

Question 30

Define ‘ineffective selection’.

Answer 30

The accumulation of mutations.

Question 31

When drift is extremely strong, selection against deleterious mutations is still effective. True or false?

Answer 31

False - selection has little effect to remove mutations when there is strong genetic drift.

Question 32

The combination of selection and drift is useful in conservation biology. Endangered populations are small and isolated, why is this a problem?

Answer 32

The small size/isolation creates much genetic drift within the population. Selection becomes ineffective at removing mutations. Thus individuals are of reduced fitness and die, which reduces population size further, a vicious cycle.

Question 33

What does ‘time to genetic unviability’ mean?

Answer 33

The smaller the population, the faster it will erode and become unviable.

Question 34

Decline in small populations is exacerbated by inbreeding depression. Why is this?

Answer 34

Homozygotes have reduced fitness.

Question 35

In large population, mutation and selection are unbalanced. True or false?

Answer 35

False - in large populations the (rate of) alleles generated is equal to the (rate of) alleles lost.

Question 36

In large populations, how likely is it that recessive carriers will come together to form homozygote offspring?

Answer 36

Rare.

Question 37

In small populations there is a high risk of recessive carriers mating to produce homozygous offspring. What can further exacerbate the reduced fitness of a homozygote.

Answer 37

Challenging environments.

Question 38

What is the equation that describes the proportion of homozygous recessives produced from random mating? Explain the terms.

Answer 38

q2N

q2 = the frequency of homozygous 
N = the total population size (NOT the effective size)

Question 39

If the total population size is small, is there a higher or lower risk of mating among relatives?

Answer 39

Higher.

Question 40

There can be a loss of beneficial mutations and a fixation of deleterious ones when drift is strong. True or false? Explain why.

Answer 40

True - when drift is strong diversity is reduced, i.e. beneficial mutations can be lost. If there is no variation selection becomes ineffective as it has nothing to act on, deleterious mutations can accumulate.