Name: Benoit L.
Date: December 2002
In the Hardy-Weinberg theory, one of the important aspect
is that the frequency of the gene are always constant. Is there
situations or events where this frequency can be changed ?
Actually the Hardy-Weinberg theory models a population that is stable and not
evolving. For the population to be stable, the conditions of the theory need
to be met, ie. no immigration/emigration, a small population, etc. In a
stable population the gene frequencies don't change. If they DO change, then
the population is evolving. This is actually what the theory is designed to
The gene frequency can change in response to environmental, cultural (e.g.,
migration of populations) or catastrophic events (e.g., war). Say, for
instance, every person with blue eyes was sent overseas on a special
blue-eye scholarship, and the ship carrying the blue-eyed people was lost in
a storm. The remaining population would consist of the brown-eyed
homozygotes and brown-eyed heterozygotes only.
The point of Hardy-Weinberg is that, if the remnant of the population
continue to mate randomly with regard to eye color, a new equilibrium will
be established in just one generation.
In a war, let us say all the able-bodied men are sent to the front and many
of them die, and those with medical exemptions for various reasons stay
behind and get to marry the pretty girls. If there is a genetic basis for
the difference between those who go and those who stay, then we would expect
the frequencies of the genes causing physical strength to be decreased and
the frequencies of certain disease genes prevalent in the non-soldiers to be
increased in the next generation.
Let us say the people of Russia are oppressed by poverty and the Czar, and
many of the young people decide to emigrate to America. Perhaps those with
ADHD (attention deficit disorder) are more likely to emigrate, because they
have a more adventurous, restless streak. If this is true, and if there are
genes causing ADHD, then the gene frequency of those genes would tend to be
higher in the new American population and lower in those left behind.
Let us say a small Jewish village in Europe is invaded by Crusaders on their
way to "fight the Infidel" in the Holy Land. After a rampage of rape and
destruction the Crusaders leave and are never seen in the village again.
They leave a sizable percentage of women pregnant and so have altered the
gene frequencies. For instance, if every Crusader had blue eyes, then every
child born of these assaults will carry the gene for blue eyes. If some of
the attackers had blue eyes, but still more than the local population, then
the frequency of the blue-eyed gene will be increased in the population of
children born. When all the children of the village grow up and get married
and have children, IF the children of these attacks are equally accepted in
matings compared with children not born of violence, then the new generation
will attain new genotype frequencies in the very first generation of random
Sarina Kopinsky, MSc, HED, CGC
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