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.
The Amount of Genetic Variation
In the 1960s there was considerable debate
regarding how much genetic variation actually
exists in populations. The common view was
that polymorphic loci are fairly rare. Then, the
development of the technique of gel
electrophoresis allowed biologists to examine
patterns of protein variation across
populations and to quantify genetic variation.
Biologists detected surprisingly large amounts
of genetic variation. In most vertebrate species,
for example, approximately 30 percent of genes
were found to be polymorphic. Studies in the
1970s in humans showed that genetic variation
occurs at approximately the same levels as in
other animal species. The studies in humans
also revealed, famously, that so-called human
races are not real biological groupings. It was
found that there is considerably more genetic
variation within races than between them.
Since then it has been the absence of genetic
variation that is considered anomalous.
Absence of genetic variation in populations
generally suggests that there was a population
bottleneck in the recent history of the group, a
time when the population size became very
small. The result of a population bottleneck is
that all members of the current population are
descended from a small number of individuals,
and therefore have only limited genetic
variation. Genetic variation is expected to build
up over time in these populations as new
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