In our metapopulation example above, on the left all demes have p = 0.5 and the allele
frequency for the entire array is also p = 0.5. Right away you can se that there in no
differentiation among
demes. To quantify this we use Fst which is calculated as
Fst =
(Ht 
mean
Hs) / Ht
, where Ht = 2 (pooled p) (pooled q) [note that pooled q = (1pooled
p)], and
mean
Hs = the average of H values for each of the individual demes (i.e., 2 p q
for each deme, averaged across all demes). For the lefthand metapopulation Ht =
2(0.5)(0.5) = 0.5, and
mean
Hs = 0.5 also. Thus , Fst = (0.5  0.5) / 0.5 = 0.0. What does
this mean? In English this says that none
of the variation in this system of demes
lies between
demes; the population structure is zero. Since there is variation (i.e., p _
0) all
of the variation lies within
demes.
Now consider the metapopulation system on the right above. The pooled
allele
frequency is 0.5 because half of the demes are fixed for the A allele (p = 1.0) and half
are fixed for the a allele (p = 0.0). Hence the Ht value is 2 (pooled p) (pooled q) =
2(0.5)(0.5) = 0.5. The
m
e
an
Hs value is very different. Each deme has a heterozygosity
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 Fall '10
 JessicaDigirolamo
 Microbiology, Genetics, Population Ecology, Population Genetics, population size, demes

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