POPULATION STRUCTURE AND MIGRATION
POPULATION STRUCTURE
Population structure: the location in which individuals are observed (e.g. islands) matters in terms of genetic
variation.
Recall the island example in which isolated populations fixed different alleles by genetic drift
The resulting set of subpopulations represent a ‘structured population’ in which allele frequencies
vary across subpopulations
How does structure arise, what reduces structure and how can we describe structure?
ORIGINS OF POPULATION STRUCTURE
Genetic drift - isolated populations drift independently to different allele frequencies.
Selection in which different subpopulations experience selection
favouring different alleles
DRIFT
If nothing else happens (no migration/ selection), islands are
expected to become genetically different. Drift is a background
process.
An event separates populations into sub-populations.
MEASURING POPULATION STRUCTURE
We saw ‘F’ when we considered inbreeding
F = measure of distribution of alleles in a population into genotypes (proportion of homo/heterozygotes)
Wright created a series of F statistics to describe various deviations from Hardy-Weinberg expectations:
- H-W predicts that random mating leads to a certain distribution of genotype frequencies (p2, q2, 2pq)
For a set of subpopulations, compare the level of heterozygosity expected if they were a single random
mating population (HT) to what we actually observe (HS)
, - In metapopulations, different populations have different allele frequencies therefore H-W proportions are
not seen.
- F measures deviation from these H-W proportions: how different are allele frequencies from p2, q2, 2pq?
F AND INBREEDING:
We saw F is the proportional reduction in heterozygosity due to inbreeding (compared to that expected in a
random mating population) – alleles being sampled to make individuals
- Heterozygosity just means allelic variation/ genetic diversity ( H Exp H Obs )
- This is FIS:
F IS
H Exp
Heterozygosity expected if population was randomly mating – heterozygosity
observed when population is inbreeding / expected heterozygosity
F AND POPULATION STRUCTURE:
FST is the variance in allele frequencies across populations: how
much of the variation is within populations and how much is
between populations?
FST = heterozygosity expected if individuals were all part of the
same population – heterozygosity observed when separated into
sub-populations/ expected heterozygosity
If no heterozygosity is observed (HS = 0), this means that all the
variation is between H HS
populations rather FST T than var( p ) within, so FST = 1. This is a structured
population. H T
FST
FST as a reduction in heterozygosity:
pq
FST as a measure of differences (variation in allele
frequencies) between populations
POPULATION STRUCTURE
Population structure: the location in which individuals are observed (e.g. islands) matters in terms of genetic
variation.
Recall the island example in which isolated populations fixed different alleles by genetic drift
The resulting set of subpopulations represent a ‘structured population’ in which allele frequencies
vary across subpopulations
How does structure arise, what reduces structure and how can we describe structure?
ORIGINS OF POPULATION STRUCTURE
Genetic drift - isolated populations drift independently to different allele frequencies.
Selection in which different subpopulations experience selection
favouring different alleles
DRIFT
If nothing else happens (no migration/ selection), islands are
expected to become genetically different. Drift is a background
process.
An event separates populations into sub-populations.
MEASURING POPULATION STRUCTURE
We saw ‘F’ when we considered inbreeding
F = measure of distribution of alleles in a population into genotypes (proportion of homo/heterozygotes)
Wright created a series of F statistics to describe various deviations from Hardy-Weinberg expectations:
- H-W predicts that random mating leads to a certain distribution of genotype frequencies (p2, q2, 2pq)
For a set of subpopulations, compare the level of heterozygosity expected if they were a single random
mating population (HT) to what we actually observe (HS)
, - In metapopulations, different populations have different allele frequencies therefore H-W proportions are
not seen.
- F measures deviation from these H-W proportions: how different are allele frequencies from p2, q2, 2pq?
F AND INBREEDING:
We saw F is the proportional reduction in heterozygosity due to inbreeding (compared to that expected in a
random mating population) – alleles being sampled to make individuals
- Heterozygosity just means allelic variation/ genetic diversity ( H Exp H Obs )
- This is FIS:
F IS
H Exp
Heterozygosity expected if population was randomly mating – heterozygosity
observed when population is inbreeding / expected heterozygosity
F AND POPULATION STRUCTURE:
FST is the variance in allele frequencies across populations: how
much of the variation is within populations and how much is
between populations?
FST = heterozygosity expected if individuals were all part of the
same population – heterozygosity observed when separated into
sub-populations/ expected heterozygosity
If no heterozygosity is observed (HS = 0), this means that all the
variation is between H HS
populations rather FST T than var( p ) within, so FST = 1. This is a structured
population. H T
FST
FST as a reduction in heterozygosity:
pq
FST as a measure of differences (variation in allele
frequencies) between populations
