Measures of population differentiation

1
Measures of population differentiation

• Sewall Wright one of the founders of population
  genetics, born in Illinois, got a Masters degree
  at Univ. Illinois, and spent most of his career
  as a professor at the University of Chicago.
• Invented F Statistics to understand the genetic
  effects of population structure.
• Note These are not the same as the F ratio
  used in the statistical procedure called Analysis
  of Variance.

2
Measures of heterozygosity needed for F statistics

• HI observed heterozygosity (proportion
  heterozygotes) within a subpopulation.
• HS expected heterozygosity within a
  subpopulation. If there are i different alleles
  at a locus in a subpopulation, pi is the
  frequency of the ith allele
• HT expected heterozygosity if there was random
  mating across the entire metapopulation.
• average frequency of the ith allele
  across all subpopulations

3
F statistics

• FST is the statistic that tells us how
  differentiated the subpopulations are. Formally,
  FST tells us if there is a deficit of
  heterozygotes in the metapopulation, due to
  differentiation among subpopulations
• Bars mean that the values are the averages over
  all the subpopulations that we are considering.

4
F statistics

• FIS tells us if there is inbreeding within
  subpopulations by comparing HI and HS
• Bars mean that the values are the averages over
  all the subpopulations that we are considering.
• So FIS measures whether there is, on average, a
  deficit of heterozygotes within subpopulations.

5
F statistics

• FIT tells us how much population structure has
  affected the average heterozygosity of
  individuals within the population
• Also (1-FIS) (1-FST) (1-FIT).

6
Two Structured Populations
7
Intuitive meaning of FST

• The proportion of total genetic variation that is
  distributed among subpopulations, rather than
  within subpopulations.

8
Metapopulation structure Drift within
populations, migration between populations
p0.7 N15
m.07
m.02
p0.4 N70
p0.6 N50
m.01
p0.3 N10
p0.5 N150
p1.0 N20
9
Drift and migration have opposite effects

• Drift makes subpopulations differerent
• Migration homogenizes subpopulations

10
Population differentiation under migration and
drift

• If Ne and m are small, FST is large
• If Nem lt 1 then
• FST gt 0.2
• If there is gt 1 migrant per generation,
  populations do not diverge much.

11
Useful for estimating gene flow

• If you know FST and Ne, you can calculate m

12
Introduction to Natural Selection
13
Selection

• Although several different environmental forces
  can change the genetic structure of organisms and
  populations, natural selection is the only one
  that causes organisms to become well adapted to
  their environment.

14
Natural selection and adaptation misconceptions

• Natural selection (NS) and evolution are the same
  thing.
• NS is the same as evolution by natural selection.
• NS will always cause evolution.

15
Natural selection adaptation

• Natural selection (NS) is not the same as
  evolution.
• NS is not the same as evolution by natural
  selection.
• NS is variation in the average reproductive
  success (fitness) among different phenotypes. It
  is purely phenotypic!
• NS can only cause evolution if the differences
  among phenotypes is partly genetic.

16
Fitness

• Selection is
• Fitness is ...
• Therefore, selection is ...
• Phenotypes will be differentially affected by
  selection if ...

17
What is fitness?

• In the real world, fitness is estimated by
  measuring ______________________, or more
  complicated metrics that take into account both
  amount and timing of _____________.
• Components of fitness

18
24 22 20
Phenotype (body size)
a a
A A
Genotype
5 2 1
Fitness (no. offspring)
a a
A A
Genotype
19
Population genetic models of selection

• Usually assume that the phenotypes are completely
  determined by the genotype.
• Therefore typically assign fitness values
  directly to genotypes.
• Sophisticated pop. gen. models dont make this
  assumption, and neither do ______________________
  models.

20
Relationships between genotype, phenotype, and
fitness
21
Selection when favored allele is
dominant(selection against a recessive allele)
Fitness
22
Selection when favored allele is partially
dominant (disfavored allele is partially
recessive)
Partial dominance Additivity when h1/2
Fitness
a a
A A
23
Selection when favored allele is recessive
Fitness
a a
A A
24
Selection ignore drift
Dominant
Additive
Recessive
25
Selection when heterozygote is favored-overdominan
ce
Fitness
26
Overdominance-stable equilibrium
27
Overdominance-stable equilibrium determined by s,
t
s0.2 t0.5
s0.2 t0.3
s0.2 t0.2
s0.2 t0.1
28
Selection when heterozygote is disfavored-underdom
inance
Fitness
29
Underdominance-unstable equilibrium
30
Additivity
Dominance
Phenotype Fitness
a a
A A
a a
A A
Underdominance
Overdominance
a a
A A
a a
A A
31
Additivity
Dominance
Fitness
a a
A A
a a
A A
Heterozygote Disadvantage
Heterozygote Advantage
a a
A A
a a
A A