Biology Reference
In-Depth Information
the time of genetic differentiation within and between populations. When DNA
variation is measured directly, the statistical analyses assume 1) nucleotides are
randomly distributed in the genome, 2) variation arises by base substitution,
3) substitution rates are the same for all nucleotides, and 4) all relevant bands or
fragments can be detected and bands that comigrate but are different are not
scored as identical (
Hoelzel and Bancroft 1992
). Although the first three assump-
tions usually are not valid, it is thought that small deviations from them will not
alter the conclusions significantly.
The most commonly used method for analyzing genetic distance in popula-
tions by protein polymorphisms is that of
Nei (1972)
. In two populations, X and
Y, the probability that two randomly chosen genes at a single locus (
j
k
) are iden-
tical is determined as follows:
j
=
Σ
X
2
and
j
=
Σ
y
2
x
i
y
i
where
x
i
and
y
i
are the frequencies of the
i
ith alleles at a given locus in popu-
lations X and Y, respectively. If there are two alleles at this locus with frequen-
cies
p
and
q
, then
j pq
=+
2
2
The probability that a gene is identical at the same locus in populations X
and Y is
j
xy
=Σ
X
ii
The normalized identity (I) between populations X and Y for all loci is as
follows:
)
/
12
I
=
J
J J
XY XY
/(
where
J
XY
,
J
x
, and
J
y
are the arithmetic means of
j
xy
,
j
x
, and
j
y
, respectively,
over all loci.
Nei's standard genetic distance
(
D
) between populations X and Y is
then as follows:
D
=−ln(I)
where I is multiplied by the natural logarithm (ln) to give a value that is 0.0
for genotypes that are completely dissimilar. The relationship between D and
time (
t
) is as follows:
t
= 0.5
aD
where
a
is the average rate of detectable change per locus per year.
