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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.
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