Balancing Selection and Genetic Variation in Subdivided Populations

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Balancing Selection and Genetic Variation in
Subdivided Populations

• John Brookfield
• Nottingham

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Model of a Population

• Constant Ne, ?N, infinite sites model
• Equilibrium predictions
• Heterozygosity
• Distribution of Allele Frequencies
• Number of variable sites
• Deviations from Expectations used to test for
  Selection

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Potential consequence of a balanced polymorphism
Dgt0
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Heterozygote Advantage

• bb bbs bsbs
• Frequency p2 2pq q2
• Fitness 1-s 1 1-t
• Fitness of b p(1-s) q
• Fitness of bs p q(1-t)
• When allelic fitnesses equal, pt/(st)
• If s0.2, t1, p0.833

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Frequency-Dependent Selection

• Balanced Polymorphism when genotypes increase
  their fitness when rare
• Why?
• Predators- Apostatic Selection
• Batesian Mimicry
• Host-parasite interactions
• Heterogeneous enviroments and niche
  diversification

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Polymorphism in Cepaea nemoralis

• Colour and banding morphs affected by
  frequency-dependent predation

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Dgt0
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North
North
North
South
South
South
South
South
South
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Population Substructure

• Wahlund Effect-heterozygote deficiency
• Linkage Disequilibrium
• Unpredictable Allele Frequency spectrum

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FST- Variance in allele frequencies

• FST Vp/(p(1-p))

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Equilibrium Between Drift and Gene Flow

• FST 1/(14Nem)
• m proportion migrating
• Ne Effective subpopulation size
• ?Nem is a number, not a proportion

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What is a sub-population?

• Subpopulation structure creates Wahlund effect
  and linkage disequilibrium
• Population I 80A, 20a
• 60B, 40b
• Population II 35A, 65a
• 25B, 75b
• Excess homozygotes
• Linkage disequilibrium an association between A
  and B and between a and b

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STRUCTUREPritchard et al. (2000) Genetics
155945Falush et el. (2003) Genetics 164 1567

• Identifies the number of subpopulations from
  which a sample has been drawn
• Allows all individuals to be assigned to
  populations, with differing probabilities
• Allows individuals to have mixed ancestry
• http//pritch.bsd.uchicago.edu/structure.html

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Selection inferred from FST

• Examine a range of FST values across loci
• Unusually high FST may indicate selection
  differing among populations
• Unusually low FST may indicate balancing
  selection

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Table of FST Values
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Interpretation 1- Gene Flow Inferred Migration
Rates, Nm
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Interpretation 2- Ancestral Population
Splitting UPGMA Tree Of Population Ancestry
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Mutational Meltdown