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.
Curtis, Helena. Biology. New York: Worth
Futuyma, Douglas J. Evolutionary Biology.
Sunderland, MA: Sinauer Associates, 1998.
Gould, James L., and William T. Keeton, with
Carol Grant Gould. Biological Science,6th ed.
New York: W. W. Norton & Company, 1996.
Patterson, Colin. Evolution, 2nd ed. Ithaca, NY:
Ridley, Mark. Evolution. Boston: Blackwell
One well-studied example of genetic variation
in populations is that of Biston betularia, the
There are three color morphs in the peppered
moth: a light morph, a dark or melanistic
morph, and an intermediate morph. Before the
Industrial Revolution, the light morph was the
most common form, although melanistic moths
were also seen occasionally. However, by the
end of the nineteenth century, the melanistic
morph had become much more common, and
had practically replaced the light morph in
Biologists traced this shift to industrial
pollution in urban areas. Without camouflaged
resting places, the light moths became easy
targets for bird predators. This explained both
the prevalence of melanistic moths in polluted
urban environments, and of light moths in
comparatively pristine country habitats.
The puzzling aspect of the peppered moth
story is that genetic variation was not entirely
eliminated in populations. In urban areas, for
example, melanistic moths make up only from
90 to 100 percent of the total population,
despite very strong selection. Apparently there
are forces other than predation pressure at
work. It was hypothesized briefly that
heterozygote advantage might be the
explanation, but that theory was ultimately
rejected. It is now believed that gene flow
between country and urban areas, and
frequency-dependent selection are viable
alternatives. However, much work remains to
be done on this historic system
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