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Artificial selection is an artificial mechanism by which evolution can occur. It is most commonly seen with the careful breeding of plants or animals in order to promote traits that suit human preferences. In this case, it is synonymous with the more widely used phrase selective breeding. It contrasts to natural selection in that it is both intentional and guided.
Artificial selection's long record of effectiveness also constitutes further empirical evidence of evolution, since it demonstrates how an accumulation of naturally occurring mutations can alter the appearance and other traits of an organism given a selection criteria. If the creation of permanent inheritable mutations was not allowed (a core claim of anyone who rejects evolution) and the genetic code was not malleable, artificial selection and selective breeding would be impossible. Despite the many, many sightings of teacup poodles and seedless grapes that suggest otherwise.
 Compared to natural selection
There are a few ways to distinguish artificial and natural selection. Artificial selection refers to breeding where there is an intentional motive to produce certain breeds of animals and the desired traits are known in advance. A breeder will know the features that they want to be present in the next generation, breeding in timidness in domesticated animals for instance, and specifically select individuals from the current generation to achieve this. By contrast, in natural selection there is no intention or guidance, hence the term "unguided evolution" is inherently associated with natural selection. In natural selection, the results of future generations aren't really known in advance so the focus of the selection criteria is on the current generation.
Generally speaking, artificial selection can work at a much faster pace than natural selection. This is because natural selection produces a general trend, while artificial selection rigorously enforces the selection criteria. Because of this speed, artificial selection primarily works by gene recombination rather than mutation. However, the overall mechanism—favoring animals that match certain selection criteria to breed with each other—is the same in both natural and artificial selection.
Some people do get confused and think that artificial selection is just natural selection when the selection pressures are artificial, such as animals evolving to cope with the effects of man-made pollution or humans evolving to adapt to the presence of technologies. However, the dividing line between what constitutes "artificial" and "natural" is so blurred as to be useless in this case. This may manifest in the belief that human intervention can "end" natural selection and evolution but this isn't the case, all we can do is alter the selection pressures. So long as we reproduce with mutation and have a chance of dying before we reproduce, natural selection and evolution will continue.
That traits can be inherited from an organism's parents was not a new concept during the development of the theory of evolution by natural selection in the late 19th century. The process of artificial selection had been observed and indeed fully exploited for much, much longer. Virtually all of our foodstuffs, work and companion animals, and ornamental plants are the result of (in some cases) thousands of years of artificial selection.
Gregor Mendel was one of the first to connect artificial selection with the modern theory of genetics through a series of experiments with pea plants in his garden.
 The process
Artificial selection starts with the raw material found in nature (which has already experienced natural selection). People find some animal, plant, or fungus that is in some way delicious or useful (as in a strong but relatively docile animal), and first simply use it as it is. Then if it is observed that some specimens are tastier or more useful they are kept and bred together (the key step is to figure out ways to breed them in captivity). Otherwise, the result of "natural" selection - as we keep eating the tastier ones or working to death the more useful ones - would be a selective pressure reducing the "delicious" gene in the wild population. Once effective breeding of only specifically chosen individuals can be achieved and they can be held in captivity, a domestic - as opposed to wild - stock has essentially been produced. This stock can then be further selectively bred for the desired traits. Wild examples can still be brought into the captive gene pool if needed, say after a disease drops the number of domestic animals below a sensible population (selectively in-bred animals have a tendency to be vulnerable to such diseases, as they haven't been exposed to as much natural selection to develop a wide immunity).
 Line breeding
Often a trait will turn up in nature or captivity that is desirable but very rare (or even unique). In order to build a domestic population showing this trait, the critters will be bred with their own offspring, increasing the frequency of the genes responsible for the desired trait. Since after a few generations of this process, undesirable traits (such as those belonging to uncommon recessive genes that would otherwise never be seen in the phenotype) may begin to accumulate as well. In this case, wild, or at least relatively unrelated, individuals will be bred back into the stock in order to increase the gene pool and dilute the negative effects of inbreeding. After many generations of this process, sustainable populations in which the trait is "fixed" (all individuals are homozygous for it) will be created.
 Really obvious examples
- Bananas - these reproduce asexually only with the help of humans, see: banana fallacy.
- Broccoli, cauliflower, brussels sprouts, cabbage, kale, kohlrabi, and collard greens - are all variations of the species Brassica oleracea and have been bred for their different appearances and flavours. In recent years, sprouts have been bred to have a much sweeter flavour in order to make them appeal more to children.
- Corn - as bred for food, the plant cannot even produce viable seeds, and "seed corn" has to be specifically raised to produce "food corn".
- Dogs - dog breeds exhibit everything that artificial selection can achieve, taken to extremes in some cases. Most pedigreed dogs have some genetic abnormality from in-breeding, and most breeds couldn't survive in the wild.
- Cows - They were bred from much less tame - and much less tasty - wild species of bovidae.
- Heikegani crabs - these animals were raised as a possible example of perhaps unintentional "artificial selection" by Carl Sagan in Cosmos. Heikegani crabs are a species of crabs that carries randomized patterns on their back often resembling a samurai face. Due to tradition, fishermen were keen to throw back the crabs whose markings most resembled a human face, and as a result (since only human face like markings survived) resulting in a selection force where the more said crabs looked like a samurai warrior, the better the chances for survival (though this is just speculation and is frequently debated as to whether it is the real reason the crabs are like this).