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areas in Texas, California, Montana, and Alberta where the ranges of white-tailed and

mule deer (Odocoileus hemionus) overlap. Examination of male deer in Texas shows

that although they look like mule deer their mtDNA resembles that of white-tailed

deer. This suggests that at some point in the past these populations were derived

from matings between male mule deer and female white-tailed deer: the female mtDNA

was retained although the other characters are those of mule deer (Carr et al. 1986;

Derr 1991). The process of taking some feature of one species into the genome of

another is called introgression. In other areas of overlap, such as in Montana, there

is little introgression (Cronin et al. 1988). Similarly, mtDNA demonstrated introgression

in a hybrid zone of indigenous red deer (Cervus elaphus) and exotic Japanese sika

deer (C.nippon) in Scotland (Goodman et al. 1999).

Mitochondrial DNA analysis of brown bears (Ursus arctos) in North America

revealed four phylogeographic clades that do not correlate with present taxonomic

classifications based on morphology (Waits et al. 1998). The four clades probably

evolved before migration of this species into North America. They provide managers

with more reliable locations for the conservation of evolutionary units.

Population genetics studies are interested in the genetic structure of populations,

the genetic divergence between populations in different areas, and the gene flow

between populations. A simple approach assumes that individuals live in separated

clumps or demes(the whole being a metapopulation). Differences between these local

subpopulations can be detected using statistical tests of divergence in allele frequencies,

such as heterogeneity tests or estimates of Fst, the standardized variance in allele

frequency. The Fststatistic is based on the “island model” (see below): it is the

variance in allele frequencies between populations, σp^2 , standardized by the mean allele

frequency (p) at that locus. Thus,

Fst=σp^2 /[p(1 −p)]

and this can be measured relatively easily by sampling allele frequencies in the field.

Fstis a good measure of genetic differentiation among populations, which is essential

to understanding evolutionary change.

However, Fsthas also been used to measure the number of migrants between

populations because of the relationship

Fst=1/(1 + 4 Nem)

where Neis the effective population size (see Chapter 17 for an explanation of this)

and m is the migration rate. There are a number of assumptions, including (i) the

mutation rate is low; (ii) the populations are at equilibrium between migration and

genetic drift; (iii) there is random mating; and (iv) all individuals have the same poten-

tial to migrate. Given these then one can obtain an estimate of the average number

of migrants among populations (Nem). We should be aware that the estimate of Nem

is based on a large number of assumptions that are unlikely to be true. Hence

estimates of Nemtend to be unrealistic. Therefore, it is wise to restrict the use of Fst

to measures of genetic differentiation and avoid its use for measures of genetic migra-

tion. Instead, direct observations of migrants should be employed (Whitlock and

McCauley 1999).

30 Chapter 3

3.7.2Genetic
divergence between
geographic regions