In the theory of evolution, artificial selection is the process of intentional or unintentional modification of a species through human actions which encourage the breeding of certain traits over others.
Charles Darwin originally coined the term in order to contrast this process from what he called natural selection. He noted that many domesticated animals and plants had special properties that were developed by intentionally encouraging the breeding potential of individuals who both possessed desirable characteristics, and discouraging the breeding of individuals who had less desirable characteristics.
He then postulated that a similar process occurs naturally; individuals in the wild who possess characteristics that enhance their prospects for having offspring would then undergo a similar process of change over time; although in this case "desirable" characteristics would be not those which specifically satisfy human needs, but those which enhance survivability. This natural process forms the basis of the theory of Darwinian evolution.
The most obvious examples of artificial selection can be found in the range of specialised body shapes and even personality types in domesticated dogs. The wide range of sizes and shapes, from dacshund to wolfhound, show the power of artificial selection.
In a more modern sense, human beings are seen as a more integrated part of the whole of our world's ecology, creating a somewhat fuzzier distinction between "natural" and "artificial" selection.
On the one hand, certain characteristics may unintentionally be encouraged while intentionally selecting for a desired result. For example, the domestic chicken has been bred to reach a large size relatively quickly (compared to its feral ancestors). The resulting changes in the chicken's gut have come at the expense of a reduced brain size and relatively smaller leg bones; these latter changes were not intentional artificial selections, but through a parallel process sometimes called "unconscious selection".
On the other hand, human activity in the larger sense creates a new set of unintentional environmental pressures which in turn act in concert with "natural" selection pressures, sometimes in unexpected or ultimately undesirable ways. For example, the general habitat changes caused by the expansion of human communities in North America has tended to select for so-called "weed species" such as the starling and the norwegian rat who thrive in disturbed habitats, while selecting against species such as the spotted owl whose strategies are attuned to a particular stable ecology.
Controversially, examples of artificial selection are sometimes said to be seen in humans themselves, who employ substantial cultural bias in mate selection, most obviously in the preference of human females for socially powerful mates - a factor which is not directly related to natural ecology or to simple secondary sexual characteristics. Critics of this view point out that selective pressures must be linked to inheritable traits; and that at this point in time, it is unknown which human social traits are inheritable, and which ones are primarily transferred through learning and culture - the so-called "Nature-Nurture" argument.
However, most biologists and evolutionary theorists employ the term artificial selection only to make a distinction between the intentionally human-chosen "artificial" factors and the combination of instinct and ecology that is "natural" without human choice. To describe more deliberate influences on human behavior they may refer to sociobiology, moral selection or ethical selection.