Lecture 11 - Population Genetic Structure

Question 1

What processes affect differentiation?

Answer 1

-Gene flow reduces differentiation
-Genetic drift increases differentiation
-Selection could either:
Different selection pressures - increase
Universally adaptive genes - decrease

Question 2

How can we measure differentation?

Answer 2

The fixation index

Question 3

What is the Wahlund effect?

Answer 3

A subdivided population has fewer heterozygotes than expected from the overall population

Question 4

What is genetic differentiation?

Answer 4

Fst
-Reduction in heterozygosity due to subpopulation divergence

Where:
Ht = expected heterozygosity (He) from the overall p and q in the total population
Hsbar = average expected heteroxygosity from each of the sub populations

-always positive

Question 5

How does inbreeding affect heterozygosity?

Answer 5

Inbreeding leads to a reduction in heterozygosity

Question 6

What is the inbreeding coefficient?

Answer 6

Fis
-Average difference between observed and expected heterozygosity due to non-random mating

Where:
Hi = average observed heterozygosity
Hsbar = average expected He across each of the sub populations

-Can be positive (deficit of heterozygotes) or negative (excess of heterozygotes)

Question 7

What is the overall inbreeding coefficient?

Answer 7

-Includes inbreeding and population divergence

Where:
HT = expected heterozygosity from the overall p and q in the total population
Hi: average observed heterozygosity

-can be positive (deficit of heterozygotes) of negative (excess of heterozygotes)

Question 8

What equation relates Genetic differentiation to the Inbreeding coefficient?

Answer 8

Overall inbreeding coefficient

1 - Fit = (1 - Fis)(1 - Fst)

Question 9

What do you know about population divergence when Fst = 0?

Answer 9

No population divergence

-all allele frequencies are the same

Question 10

What do you know about population divergence when Fst > 0?

Answer 10

There is population divergence

-allele frequencies differ

Question 11

What do you know about the status of inbreeding when the actual heterozygosity is less than the expected heterozygosity, and Fis > 0?

Answer 11

Evidence for inbreeding

-reduction in heterozygosity

Question 12

What do you know about the status of inbreeding when the expected heterozygosity is equal to the actual, Fis = 0?

Answer 12

No evidence for inbreeding

Question 13

What are the features of isolation by distance?

Answer 13

There is greater differentiation between populations that are further away from each other due to less dispersal and gene flow

Question 14

What is the formula for the estimation of number of migrants per generation?

Answer 14

Wright’s F statistics, 1931
Fst = 1/(4Nem + 1)
Ne = effective population size
m = migration rate

Question 15

What are the limitations of Wright’s f-statistics for estimating the number of migrants per generation?

Answer 15

Makes a number of unrealistic assumptions

• no selection
• no mutation
• all populations are identical (with equal numbers of emigrants)
• no spacial structure
• populations are at equilibrium

Question 16

What are the features of the lake Erie garter snakes (selection)?

Answer 16

Melanistic forms unusually common near lake Erie
-selection for thermoregulation may outweigh selection for crypsis
Colour pattern is a one locus, two allele trait
-melanistic is recessive to striped
Morph frequencies highly variable amongst sites
-there may be an element of drift influencing colour pattern
Higher Fst (genetic differentiation) expected at loci under divergent selection compared to neutral or universally adaptive traits

Question 17

How is Fst expected at loci under divergent selection compared to neutral or universally adaptive traits?

Answer 17

Higher Fst expected at loci under divergent selection

-compared to neutral or universally adaptive traits

Question 18

What are Wright’s three F-statistics?

Answer 18

Genetic differentiation (Fst)
Inbreeding coefficient (Fis)
Overall inbreeding coefficient (Fit)

Question 19

Which of Wright’s three F-statistics are the most important?

Answer 19

Genetic differentiation (Fst)

Question 20

What can Genetic differentiation (Fst) be used to asses?

Answer 20

• population structure (i.e. is most genetic variation between or within populations)
• migration rates (but with caution)
• selection (which loci, and what kind of selection)