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. When the process leads to undesirable outcome, it is called negative selection.
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 Dachshund to Wolfhound, shows the power of artificial selection through selective breeding.
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.
As James Baldwin pointed out, organisms can alter their environment, and thus alter selective pressures, which can thus affect the evolution of the organism's own species. Artificial selection in this sense refers to the conscious ways human beings alter the environments of organisms (including their own environment) so as to alter the evolution of these organism's species. The most common example is the creation of new breeds of animals through the control of their reproduction. The decision not to conceive, or to abort a fetus, based on knowledge of the traits of the offspring (e.g. that the child would have a congenital birth-defect) is an example of artificial selection acting on humans themselves.
Another example of artificial selection is the spread of various species of European grass in North America, which must be maintained by lawn care and pesticides. Another example is the extermination by human hunters of the Great Apes, who are large and thus easy to hunt for their "bushmeat". An even more prevalent example is dog breeding for specific extreme traits, e.g. the large size and eating habits of the Great Dane, e.g. the small size of the Chihuahua. Traits that would lead to thriving under natural selection, such as a gorilla's size, or to utter extinction, such as aggressive behavior in small yappy dogs, are very often the exact traits which lead to the opposite outcome under artificial selection pressure.
Controversially, profound examples of artificial selection are often 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 argue that sexual selection in all mammal species favors aggressive males, and that males of other primate species trade food for sex - and that aggression and food-gathering capacity are causally related to social power in any social species. This argues for a fuzzy line between ecological, artificial and sexual selection, especially in a highly cognitive and highly social species.
However, a more obvious artificial selection takes place when an unmarried human male is imprisoned for a crime - removed from the general human society and mating environment for a period of time - most likely for the aggression or valuables-gathering capabilities that natural selection must encourage.
Similarly, in many societies, such institutions as polygamy and the harem have served to permit some of the most successfully aggressive and logistically competent (and lucky) males to father drastically more offspring than others. Similar observations are made of harems of the Ottoman Empire and Siam, and of polygamy in Islam and among Mormons.
There is substantial controversy as to whether these are truly artificial selections, or examples of ordinary sexual selection, or even (when the economics predominate) natural selection, carried to extremes in human beings. Controversies of this sort lead some to make a careful distinction between ecological selection and sexual selection, i.e. to differentiate which of the two aspects of natural selection can be said to apply to humans in what circumstances, and what ";artificial" aspects are perhaps "environmental" but neither naturally ecological nor wholly sexual.
Another way to resolve the controversy is to suggest that when humans act on humans, as when other animals act to select any of their own species as mate or when members of their species are harmed as a part of the mating rituals, the entire process should be considered part of sexual selection, and that only man acting on non-human species is a true "artificial" selection.
In this view, even such complex matters as economics may be considered wholly "sexual" (by certain green economists) or even wholly "natural" (by some anthropologists using the neoclassical political economy). This begins to intersect with Social Darwinism and fascism to a degree, both of which held that dominant human societies optimized, but did not alter, the fundamental factors driving selection of individuals to propagate a species.
Predictably, such assumptions also conflict with Marxist or feminist views which hold rather that an imposed political economy, i.e. capitalism (classical and neoclassical economics) or patriarchy/feudalism/militarism (as in the harem case above), are creating artificial conditions drastically at odds with what is of highest survival utility for the human species as a whole. But, as other cases of runaway sexual selection demonstrate, individuals or cliques within a species do not in general act for species-wide utility, and individuals and even whole species are sometimes extincted due to excesses of their species' sexual selection process. Were a Marxist or feminist view to predominate, then their shared humanism would be artificial selection.
Some believe the term artificial selection should be employed only to make a distinction between the 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 moral selection or ethical selection, e.g. as in the prison case.
These terms, too, often recur in moral philosophy and in arguments regarding influences on the evolution of societies deemed "desirable". The central focus in many religious traditions of suppressing sexual desire seems to suggest that what is usually deemed desirable for the society as a whole may well be exactly the opposite of what is desirable for the individual under normal natural selection, e.g. a human male who learns he has Human Immunodeficiency Virus could attempt to impregnate the maximum number of females hoping to have one born without the virus, or could choose to forgo sexual activity entirely due to the suffering it could cause. Or, alternatively, could choose to reveal the deficiency to partners, exactly the type of acquired trait that males in natural sexual selection would conceal. Removing oneself from the mating pool or drastically limiting mating choices in these ways would seem to constitute both morally sound and reasonably ethical behavior, and also clearly exemplify artificial selection.
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.