What are the causes for genetic variations within the general population?

There are three major sources of genetic diversity within
a population. Gene flow requires the participation of a second population; it is based
on new genes coming into the population via immigration of new gametes. In animals with
internal fertilization, this must be accomplished by migration of animals. In animals
with external fertilization, either individual animals can migrate, or their gametes
alone can move into a new area, bringing new genes to the population they arrive in. For
plants, pollen or seeds can be blown or carried to new areas by a huge variety of
dispersers.

The second source of genetic diversity is
mutation. Random mutations can be caused by a number of factors; wear and tear,
radiation such as cosmic rays or ultraviolet light, and a lengthy list of chemicals all
can cause mutation. Many of these mutations are deleterious to the individual, but some
are passed into the population. Evolution is believed to be dependent on the
accumulation of numbers of such mutations over long periods of
time.

The third contributor to genetic diversity is
procreation. Specifically, it's the crossing-over of chromosomes that occurs when cells
undergo meiosis to produce gametes. While this does not actually bring any new genes
into existence, it does shuffle the existing genes and produce new combinations, which
can work differently in the individual who inherits
them.

Another fascinating new science is epigenetics, which
is the study of the non-gene parts of inheritance. The protein structure of chromosomes
contributes heavily to the action of the genes within, and this effect, which is highly
responsive to environmental influence, is another source of variation with a
population.