Lecture 6 - Genetic drift and small populations

Question 1

Define genetic drift

Answer 1

The change in allele frequencies by chance alone

Question 2

What is the importance of genetic drift?

Answer 2

• particularly important in small populations (population bottlenecks, founder effects)
• plays a role in speciation (especially allopatric speciation)
• describes the behaviour of neutral alleles in a population (why vairants are lost or get fixed)
• Interacts with other processes e.g. selection or mutation

Question 3

How does the average time to fixation change with population size?

Answer 3

the average time to fixation increases with population size

Question 4

How is genetic drift different in small populations?

Answer 4

Fluctuations from generation to generation are greater in small populations

Question 5

Why do allele frequencies fluctuate randomly?

Answer 5

• gametes are sampled randomly from one generation to the next
• unpredictable within a population but can calculate the probability that the next generation has a certain allele frequency

Question 6

How can the probability that a certain number of an allele are passed on?

Answer 6

Binomial formula
Where Pr(i) = probability that i alleles of A are drawn, 2N = number of gametes drawn, p = fruqency of allele A in generation t-1, q = frequency of allele a in generation t - 1

Question 7

What are the features of the binomial formula?

Answer 7

• can be useful to predict variance of allele frequences (σ^2 = pq)
• shows that the greatest variance at p =q = 0.5
• can predict variance between populations

Question 8

What are the features of variance predicted between populations based on the binomial formula?

Answer 8

• will increase with the number of generations

- increases faster in small population

Question 9

What does the probability of fixation equal?

Answer 9

the initial allele frequency

Question 10

Why is drift more important in small populations?

Answer 10

• random changes cause greater fluctuations of allele frequencies
• average time to fixation is shorter in smaller populations
• genetic diversity is lower in small populations
• newly deleterious mutations have a greater chance to become fixed before they are removed by selection

Question 11

What was Peter Buri’s (1956) experimental test with drosophila?

Answer 11

• initiation 107 populations, each with 16 bw^75/bw heterozygotes (bw allele = brown eyes)
• simulated using the Fisher-Wright model
• compared the experiment and the model results show:
• experimental data show faster fixation than the model
• populations are smaller than they appear

Question 12

What is the effective population size? (Frankham et al 2002)

Answer 12

The number of individuals that would give rise to the calculated loss of heterozygosity, inbreeding or variance in allele frequencies if they behaved in the manner of an idealised population

Question 13

What if the effective population size (Ne)?

Answer 13

the size of an idealised population with the same genetic properties as that observed in the real population

Question 14

What are the properties of an idealised population?

Answer 14

• equal sex ratio
• low variance in family size
• constant population size over generations

Question 15

How are the requirements of an idealised population never met?

Answer 15

the effective population size is nearly always smaller than the census population size

Question 16

What are the features of sex ratios in an effective population size?

Answer 16

If sexes are separate, then the number of males and females may deviate from 1:1
Ne = (4NfNm)/(Nf+Nm)
Nf = number of reproducing females
Nm = number of reproducing males

Question 17

What are the reproductive features of the African wild dog in the Selous Game Reserve? Example of a population size not representative of Ne

Answer 17

-127 adult females
-18 dominants, 5 subordinates bred
-effectively only 23 reproductive females
-170 adult males
-2 reproductice males for every breeding female
-effectively only 46 reproductive males
N=297 but Ne = 61

Question 18

What are the features of family size in an effective population size?

Answer 18

• even with a 1:1 sex ratio, some individuals leave more offspring than others
• Ne = N when offspring number follows a Poisson distribution with mean and variance of 2
• the effective population size is smaller than N
• if N = 1000 and v = 0, Ne = 1999 i.e. the effective population size is greater than the consensus population size

Question 19

What is v in family size formula?

Answer 19

v = variance in family size

Question 20

What are the features of fluctuations in the effective population size?

Answer 20

-the ideal population has a constant N but most populations fluctuate across generations

Question 21

What are the two types of causes of intermittent drift?

Answer 21

Bottleneck effect

• Original population -> bottlenecking event -> smaller surviving population
• Founder effect

Question 22

Give an example of Intermittent drift

Answer 22

Norther Elephant Seal

• was down to 20-30 individuals in 1884 (due to hunting), possible a single harem
• now protected and >200,000 individuals
• No allozyme diversity at 20 allozyme loci (northern in southern sp.)
• Two mtDNA variants (23 in southern sp.)
• Two MHC alleles (at least 8 in southern sp)
• therefore, loss of genetic diversity, possibly more prone to population decline dues to diseases or changes in the environment

Question 23

How does genetic drift affect heterozygosity?

Answer 23

loss of heterozygosity through drift

• use formula to calculate the proportion of heterozygosity lost in one generation of drift
• single generation bottlenecks do not cause a big loss in heterozygosity

Question 24

How much of the original heterozygosity is lost after a bottleneck?

Answer 24

1/(2N) of the original heterozygosity is lost after a bottleneck

• a bottleneck of N = 25 reduces heterozygosity by 2%
• a bottleneck of N = 100 reduces heterzygosity by 0.5%
• less loss of heterozygosity the larger the population size after bottlenecking event

Question 25

Give six examples of genetic drift affecting heterozygosity

Answer 25

• Colour blindness on Pingelap
• Dwarfism in US amish populations
• Tay-Sachs disease in Ashkenazi jews
• Diastrophic dystrophy in Finns
• Familial hyperchylomicroemia in Quebecios
• Huntingtons disease in Africaners

Question 26

What are the features of colour blindness on Pingelap?

Answer 26

• Pingelap: 1.8km^2 remote island in Mikronesia
• in 1775 a typhoon killed 90% of the population
• only about 20 people survived
• today 10% population suffer from colourblindedness
• 30% are expected to be heterozygous carriers
• Us 0.003% population are affected

Question 27

What are the features of Dwarfism in US amish populations?

Answer 27

• Six fingered dwarfism (Ellis-van Creveld syndrome)
• 82 individuals affected out of 13000
• all parents trace their ancestry back to two founders
• one of the two must have carried the recessive allele

Question 28

What are the three features of effective population size?

Answer 28

constant population size
constant variance in family size
equal sex ratio

Question 29

What does intermittent drift cause?

Answer 29

• loss of rare alleles
• small affect on heterozygosity
• reduces population fitness
• response to selection