Genetic Variation in a...
Genetic variation in a population describes the
existence in that population of different alleles,
or alternative forms, for a given gene. The
presence of genetic variation implies that
individuals of the population vary in the alleles
they possess, meaning that individuals differ in
genotype. Genetic loci for which there are
multiple alleles are described as polymorphic.
Humans, for example, are polymorphic for
traits such as eye color and blood type.
How Genetic Variation Is Maintained
The discovery of large amounts of genetic
variation in nearly all populations led to the
formulation of a different question: How is
genetic variation maintained? In many cases,
after all, natural selection removes genetic
variation by eliminating genotypes that are less
Many factors act to increase or maintain the
amount of genetic variation in a population.
One of these is mutation, which is in fact the
ultimate source of all variation. However,
mutations do not occur very frequently, only at
a rate of approximately one mutation per
100,000 to 1,000,000 genetic loci per generation.
This rate is too slow to account for most of the
polymorphisms seen in natural populations.
However, mutation probably does explain
some of the very rare phenotypes seen
occasionally, such as albinism in humans and
A second factor contributing to genetic
variation in natural populations is selective
neutrality. Selective neutrality describes
situations in which alternate alleles for a gene
differ little in fitness. Because small fitness
differences result in only weak natural
selection, selection may be overpowered by the
random force of genetic drift. Alleles whose
frequencies are governed by genetic drift
rather than by natural selection are said to be
selectively neutral. Under neutrality, allele
frequencies vary over time, increasing or
decreasing randomly. Over long periods of
time, random fluctuations in the relative
frequencies of different alleles may result in
some being eliminated from the population.
However, genetic polymorphisms are long-
lived, and novel neutral alleles may arise
continually through mutation.
Finally, several forms of natural selection act to
maintain genetic variation rather than to
eliminate it. These include balancing selection,
frequency-dependent selection, and changing
patterns of natural selection over time and
Balancing selection occurs when there is
heterozygote advantage at a locus, a situation
in which the heterozygous genotype (one
including two different alleles) has greater
fitness than either of the two homozygous
geno-types (one including two of the same
allele). Under heterozygote advantage, both
alleles involved will be maintained in a
A classic example of heterozygote advantage
concerns the allele for sickle-cell anemia.
Individuals who are homozygous for the sickle-
cell allele have sickle-cell anemia, which causes
the red blood cells to become sickle-shaped
when they release oxygen. These sickle-shaped
cells become caught in narrow blood vessels,
blocking blood flow. Prior to the development
of modern treatments, the disease was
associated with very low fitness, since
individuals usually died before reproductive
Heterozygotes, however, have normal, donut-
shaped blood cells and do not suffer from
sickle-cell anemia. In addition, they enjoy a
benefit of the sickle-cell allele, which offers
protection from malaria. Consequently,
heterozygous individuals have greater fitness
than individuals who have two copies of the
normal allele. Heterozygote advantage in this
system is believed to have played a critical role
in allowing a disease as harmful as sickle-cell
anemia to persist in human populations.
Evidence for this comes from an examination
of the distribution of the sickle-cell allele,
which is only found in places where malaria is
Another form of natural selection that
maintains genetic variation in populations is
frequency-dependent selection. Under
frequency-dependent selection, the fitness of a
genotype depends on its relative frequency
within the population, with less-common
genotypes being more fit than genotypes that
occur at high frequency.
Frequency-dependent selection is believed to
be fairly common in natural populations. For
example, in situations where there is
competition for resources, individuals with
rare preferences may enjoy greater fitness than
those who have more common preferences.
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