How do selection pressure affect the isolated gene pool in terms of allele frequencies?

An isolated population is more likely to undergo
allopatric speciation. The isolated group may be under different environmental pressures
than its parent population, and over time an isolated group will be subject to random
mutations which will create unique attributes.

If the
isolated population is small in numbers, then it is subject to additional pressures. The
first of these is the founder effect. If the frequency of any allele is different
between the isolated group and the parent population at the time the isolation occurs,
then the isolated group experiences genetic changes from a different starting point than
the parent population, and is likely to follow a different path over time, resulting in
different allele frequencies.

Very small populations are
also prone to the bottleneck effect, which results in a loss of genetic diversity. Small
populations are also prone to inbreeding, which again may reduce the diversity of
alleles.