Continuous populations
Structured populations
Low density of
individuals
Mainland
Stepping
Island
Stone
Some density of
individuals
model
Metapopulation
High density of
individuals
Low density of
individuals
Subpopulation
Isolation by distance: the genetic distance between
individuals is correlated with geographic distance
Population (Genetics)
A population is a group of
individuals that interbreed
and that is somewhat
isolated from other
populations.
Population versus Deme
Species
Population
Population
Population
A deme is a group of
individuals that interbreed
and that is somewhat
isolated from other
demes.
Assessing structure
Species
Population
Deme
Deme
Deme
F-statistics
HI = average heterozygosity in individuals over all populations
F-Statistics (Wright 1922+)
H S = average of expected heterozygosity over all subpopulations
HT = expected heterozygosity ignoring structure
Individuals I
Subpopulations (Demes) S
Total Population T
FIS
H S − HI
=
HS
measures whether there are fewer or more
heterozygous individuals than expected in
subpopulation
FIS > 0 → Fewer heterozygotes than average
FIS < 0 → More heterozygotes than average
Sewall Wright 1924
F-statistics
HI = average heterozygosity in individuals over all populations
Contribution to difference between
subpopulations
H S = average of expected heterozygosity over all subpopulations
HT = expected heterozygosity ignoring structure
Mutations
Genetic drift
Local adaptations
Degree of genetic differentiation among
subpopulations.
FST =
HT − H S
HT
FST =
!
0
1
Subpopulations are not differentiated
They do not share alleles with each other
Migration (Gene flow) reduces differentiation and equalizes
allele frequencies among subpopulation
Connection between migration and FST
0.44
0.2
FST
0.15
1
=
1 + 4Ne m
10080 Ne
m
0.1
60 40
0 .2
0.2
20
0.3
1
0.75
0.1
0.05
Immigration rate
20
FST
40
60
80
100
m
0.5
FST
0.25
0
Ne
FST
Summary of genetic effects
Low population size (N ): reduction of heterozygosity H due to
e
genetic drift, inbreeding
High migration (Nm): increase of H in subpopulation (it brings
foreign alleles), counteracts genetic drift and inbreeding,
homogenizes populations.
Low migration leads to independent evolutionary histories of the
subpopulations (high F ), local adaptations
ST