What are four different types of selection pressure that could have an effect on a gene pool?

Selection pressure is usually divided into two
subcategories - biotic, which arises from interactions with living organisms, and
abiotic, which comes from nonliving factors such as sunlight or soil. By definition
selection pressures impact either the reproduction of a species or the frequency of
alleles in a species.

Artificial selection is an example of
Directional selection that most people are familiar with. This is the method through
which most modern breeds of domestic animals and crop plants were created. In artificial
selection humans choose specific traits to either retain in, or eliminate from, the gene
pool of a breeding population. Because this process is closely monitored, changes in the
population can be seen quickly, sometimes in just a few generations. Directional
selection can also be seen in the wild, where some environmental factor favors one
extreme of a genetic continuum and eliminates the other. An example of this would be the
fact that mammalian species  on islands tend to become smaller than their mainland
counterparts.

Sexual selection is another biotic pressure
that has been well studied. When members of a species choose their mates based in
specific criteria rather than mating at random, this selection process can lead to
changes in a species, such as sexual dimorphism.The tail of the male peacock is often
cited as an extreme example of this.

Stabilizing selection
is a form of selection where extreme traits tend to be eliminated, keeping the
population in a static situation genetically. Disruptive selection is the exact opposite
of this, eliminating the "average" individuals and favoring those at the extremes of a
genetic continuum.