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If the five aforementioned conditions aren't met,
the population will change as a result of a change
in the allele frequency.
Charles Darwin
Photo courtesy of National
Library of Medicine
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Mutations, the first condition,
must not occur to maintain genetic equilibrium
(when a population doesn't change over time,
but remains the same). A mutation is simply
a change of genes, usually occurring during
the DNA replication process, but sometimes due
to radiation, electricity or toxic chemicals.
Mutations can be extremely harmful, but can
be beneficial as well; without mutations, new
species would never evolve. The British naturalist
Charles Darwin proposed that natural selection
(survival of the fittest) favors mutations that
make organisms stronger. Mutations, however,
are rare. |
If a population dwindles in size, the gene pool
will be greatly affected because larger populations
(with respectively larger and more diverse gene
pools) are stronger. Smaller gene pools are more
susceptible to alteration because of random events.
A prime example of such alteration is the founder
effect which occurs when a population is founded
by only a few individuals. This effect is responsible
for the high number of cases of Huntington's disease
in western Ohio and southern Indiana where one of
the original pioneering families carried the disease.
It's also responsible for the number of seven-toed
cats in New England, where one of the original population
members was a mutant with seven toes. Another danger
related to small populations is inbreeding. If two
relatives mate, no new genetic material is introduced
and the resulting offspring can suffer from physical
deformities, a weak immune system, and even retardation
among other things. For this reason, it is against
the law for close relatives (such as siblings, first
and second cousins, parents and children, etc.)
to marry in the United States.
Mating must be random for genetic equilibrium to
exist. This is usually impossible, though (especially
with humans!), because some mates are more desirable
than others. Courting rituals and appearance and
smell are major factors. If mating is non-random,
the allele frequency will change to reflect the
new patterns.
If organisms leave or enter a population, equilibrium
will be lost. Most animals display some sort of
migration which causes the size and diversity of
the population to constantly change.
No gene can have more significance over another
(not dominant/recessive alleles, but genes); otherwise,
this, too, will cause equilibrium damage. Sometimes,
a single gene can cause damage or even death to
an individual in a population. This gene can be
transmitted through the generations to inflict many
of the members. Hemophilia is a common damaging
gene. In theory, harmful genes should disappear
from the population over time because they are against
the laws of natural selection and are not favorable
to the organism.
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