Artificial selection
In 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. It was originally defined by Charles Darwin in contrast to the process of natural selection, in which the differential reproduction of organisms with certain traits is attributed to improved survival and reproductive ability in the natural habitat of the organism. Artificial selection that produces an undesirable outcome from a human perspective is sometimes called negative selection (but note that this term has a better-established meaning as a type of natural selection).
Historical development
Charles Darwin originally coined the term as an illustration of his proposed wider process of 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.
Contrast to natural selection
The difference between natural and artificial selection centers on the difference in environment among organisms subject to the two processes. Essentially, in artificial selection, the fitness which is the amount of offsping an individual contributes to a population relative to other individuals in that same population of an organism is defined in part by its display of the traits being selected for by humans. Since humans either intentionally or unintentionally exert control over which organisms in a population reproduce or how many offspring they produce, the distribution of traits in the organisms' population will change.
It should be emphasized that there is no real difference in the genetic processes underlying artificial and natural selection, and that the concept of artificial selection was first introduced as an illustration of the wider process of natural selection. The selection process is termed "artificial" when human preferences or influences have a significant effect on the evolution of a particular population or species.
Examples of artificial selection
Most examples of artificial selection fall into the category of selective breeding, in which particular individuals are selected for breeding because they possess desired characteristics or excluded from breeding because their traits are undesirable. Both processes have contributed to the domestication of animals and plants by humans.
The most obvious examples of artificial selection can be found in the range of specialised body shapes and even personality types in bred in domesticated dogs. The wide range of sizes and shapes, from Dachshund to Wolfhound, shows the power of artificial selection through selective breeding. Systematic selective breeding has led to extreme traits such as the large size and eating habits of the Great Dane versus the small size of the Chihuahua. It is possible for traits to be selected for under artificial selection - for example, aggressive behavior in small dogs - that would be selected against in the natural environment absent human influence. An even more illustrative example is the domestication of corn, which has been bred so that it no longer disperses its seeds, instead relying on human intervention to disseminate them. Because both organisms derive significant benefits from the other, this could be termed a symbiotic relationship.
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
It is also possible for humans to exert artificial selection pressures on our own species. Eugenics efforts, in which those with "undesirable" characteristics are prevented from reproducing and those with "desirable" characteristics are encouraged to reproduce, form the most extreme such example.