Population structure

Question 1

what is Wright’s F statistic?

Answer 1

• Fst = fixation index of subpops relative to the total population
• 1940
• Sewall Wright: Theory of Isolation by distance
• F = Wrights fixation index (not linear) between 1 and 0
• results in population structure –> reduction in average proportion of heterozygotes relative to expected under random mating
• we use this reduction as a measure for differentiation between populations / regions

Question 2

what is the Fst equation?

Answer 2

Fst = (HT - Hs) / HT or 1 - Hs / HT

Question 3

expected heterozygosity / diversity?

Answer 3

He = average over many loci (if HW and 2 alleles h/He = 2pq or 1 - p^2 - q^2, if more than 2 alleles h/He = 1 - Epi^2)

• Hs = within subpopulations
• HT = Total across all samples (species level)
• often the expected heterozygosity (He) doesn’t equal observed heterozygosity (Ho) (aka no HWE)
• often He > Ho - except for heterozygote advantage and disassortative mating

Question 4

why would we see a deficit of heterozygotes?

Answer 4

• selection
• non-random mating
• population structure –> subdivision in a pop

Question 5

why is population structure caused?

Answer 5

• random loss of alleles in each subpop, founder effects, different selection pressures
• if homogenising effect of migration is smaller than the diversifying effect of drift and selection