Gene Flow

1
Gene Flow

• Migration (or gene flow) refers to the movement
  of individuals among subpopulations.
• It sets a limit as to how much genetic divergence
  can occur.

2
To illustrate the homogenizing effects of
migration Ill use an island model of migration -
where there are several subpopulations dispersed
geographically
3
The equation is
Average allele freq. of all subpops.
gen.
t
pt p (po - p ) (1-m)
migration rate
Freq. of allele in generation t
Initial freq. of an allele in a subpop.
4
Example where initial frequencies of an allele A
in five subpopulations arepo 1, 0.75, 0.50,
0.25, 0 (pop. large so drift can be ignored)p
(1 0.75 0.50 0.25 0)/5 0.5m 0.10
(i.e., 10 of individuals in any subpop in any
generation are migrants)
5
Subpopulation t
Allele Frequency 0 1 0.75 0.50
0.25 0 10 0.67 0.59 0.50 0.41
0.33 20 0.56 0.53 0.50 0.47 0.44 50
0.50 0.50 0.50 0.50 0.50
6
Allele Freq. (pt)
0 10 20 50
t (generations)
7
In addition, very little migration is required to
prevent substantial genetic divergence among
subpopulations resulting from random genetic drift
8
This can be shown by the following equation
1
Fst
4Nm 1
Equilibrium fixation index
of migrants/generation
9
Fst can be thought of as the reduction in
heterozygosity within subpopulations (Hs) - i.e.,
subpopulations become genetically homogenous due
to drift divergence of subpops
Hs Ht
Fst 1 -
10
When Nm 0, Fst1Nm 0.25, Fst0.50 (one
migrant every fourth generation)Nm 0.50, Fst
0.33 (one migrant every second generation)
11
WhenNm 1.0, Fst 0.20Nm 2.0, Fst 0.11
12
Fst
Fixation Index
0 1 2 3 4 5 6 7 8 9 10
migrants/generation Nm
13
Nm 25
Nm Nm 10
Nm 10
Nm 5
Nm 2
Nm 1
Nm 0.5
14
OMPG provides a desirable balance between drift
and gene flow by preventing the loss of alleles
and minimizing the loss of heterozygosity within
subpopulations but allowing genetic divergence to
exist among subpopulations
15
Calculating Fst An Example

• Pgm-2 locus, 43 Subpopulations
• Data Subpop pi
• 1- 40 1
• 41 0.49
• 42 0.83
• 43 0.91

16
Hs expected proportion of heterozygotes within
subpops assuming random mating (2piqi) averaged
over all subpops
Hs ?2piqi
N Hs (40 X 0) 2(.49 X .51) 2 (.83 X .17)
2(.91 X.09) 0.022 43
17
Ht expected proportion of heterozygotes over
the entire metapopulation
P (40 X 1) .49 .83 .91 0.9821 1- p q
0.0179 Ht 2pq 2 X .9821 X 0.179
0.0352 Fst Ht Hs Nem 1 - 1 Ht
4Fst 4
0.38
0.41
18
Some Examples of migration (gene flow) as a
homogenizing evolutionary force

• Giles, B.E. and J. Goudet. 1997. Genetic
  differentiation in Silene dioica metapopulations
  estimation of spatiotemporal effects in a
  successional plant species. American Naturalist
  149507-526

19
Red bladder campion Silene dioica Skeppsvik
Archipelago, Sweden
20
Bouzat, J.L. et al. 1998. Genetic evaluation of a
demographic bottleneck in the greater prairie
chicken. Conservation Biology 12836-843
IL KS MN NE Fst
Rst 0.57 0.59 0.65 0.63 0.044
0.012 .068 .071 .086 .060
Mean H per locus SE of Mean
Nm 5.4 for Fst Nm 20.6 for Rst
21
Parker, R.W., K.N. Paige and A.L. DeVries. 2002.
Genetic variation among populations of the
Antarctic toothfish evolutionary insights and
implications for conservation. Polar Biology
25256-261.
RAPD Data

• - Fst 0.297, Nm 0.6
• - 82.0 - 88.6 band sharing
• - 3 fixed differences
• Mantel Test significant difference
  observations within groups are more similar
• than between groups