1. Why study population genetic structure?
In general, provides perspective on adaptation and speciation.
Can reveal the recent demographic history of a population and the role of:
Gene flow
Genetic drift
Inbreeding
Natural selection
Population size
Can reveal the history of population structuring over deeper time.
e.g. Phylogeography

2. Why do we expect population genetic structures to vary within and among organisms?
1) Differences in mobility/dispersal ability
2) Differences in reproductive attributes/system
3) Differences in life history attributes
4) Differences in behavioral attributes
Differences in geographic distribution
Habitat patchiness or variability
Historical reasons
e.g. See Table 6.3 and 6.4

3. One of the first idealized models of a population
From J. Hey, 2003, Nature Reviews Genetics, 4:

4. Models of population structure that allow for migration (Gene Flow)
Idealized Population Models
a. Island model
b. Stepping stone
c. Isolation by distance
d. Metapopulation

5. Statistical Description of Population Structure
Wright’s F statistics. A method to summarize how genetic
variation is partitioned among different hierarchical levels.
Among population level:
FST = Vp / p (1 - p)
This is a measure of the observed variation in allele frequencies
among populations (regardless of how the variation arose).

6. Another way from Avise: FST = (ht - hs) / ht
hs = mean expected heterozygosity at a locus within subpops under H/W
ht = overall expected heterozygosity in total population (given allele freq & H/W)
FST :
Ranges from 1.0 to 0.0
subpopulations fixed for
alternate alleles
subpopulations have same
alleles frequencies
“structured”
“not structured”

7. From Selander (1970): An analysis of mouse population
structure within and among barns in Texas.
Estimated Number of Mean Allele Variance of
Population Size Pops Sampled Frequency Allele Frequency
Es-3b Hbb Es-3b Hbb
Small ~
Large ~
FST = Vp / p (1 - p)
FST = /(0.418)(0.582) = for small pops
FST = /(0.372)(0.628) = for large pops

8. Consider the joint effects of genetic drift
and gene flow on population structure
In the absence of migration, finite populations become more inbred
and diverge from one another at random (with respect to allele
frequencies) as a result of drift.
The probability of autozygosity (that an individual carries IBD alleles
at a locus) increases faster, the smaller the population.
FST provides a measure of divergence under drift. At some point
in time, as a population approaches FST = 1, the increase in
autozygosity will be balanced by the rate of migration
(and/or mutation also, in reality). An equilibrium is struck.

9. Migration rates (Gene Flow) can be estimated assuming
an equilibrium FST has been reached:
For neutral alleles in an island model, the equilibrium
value of FST : ~
FST = 1 / 1 + 4Nm
or, ~
Nm = (1 - FST ) / 4FST
This is interpreted as the absolute number of individuals
exchanged between populations.
As Nm increases,
FST
decreases.

10. Gene Flow is a powerful thing
If Nm = 1,
FST = 0.20.
Subpopulations are 20% more structured
(inbred) than if all subpopulations essentially
comprised a single, randomly mating
population

11. However, FST is not very precise
Measure. At best it can only
provide qualitative perspective.
1.0
0.8
06.
0.4
0.2
0.0
FST Nm

12. More recently, there has been the development of DNA sequence
variation approaches to the characterization of population structure.
However, all summary
statistic approaches live
and die by the assumed
demographic model.
The model specifies meaning
to the parameters and the
assumptions that underlie
them.
A summary statistic doesn’t
necessarily provide insight.