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Selective breeding in domesticated animals is the process of developing a cultivated breed over time.
In general, the owners of the animals use two strategies to refine local populations: linebreeding (one bloodline, or strain) or inbreeding (mating closely related individuals), for a long period of time, sometimes for centuries.
The Appaloosa Horse, which was developed by the Nez Percé Indians in the Northwest United States, provides an example. The Spanish colonists had established horse breeding in what is now New Mexico by about 1600. By 1806, when they are mentioned in journals kept by the Lewis and Clark expedition, the Nez Percé were observed to have developed attractive and hardy horses, some though not all of which were spotted. By the time of the capture of the Nez Perce people by General Nelson A. Miles in 1877, most of their animals were spotted.
It is not known if the Nez Percé practiced inbreeding, but they were reputed to geld stallions judged unsuitable for breeding, and to trade away mares likewise unsuitable for breeding, which accomplishes the goals of isolation and artificial selection.
In Europe, the first use of this process for Thoroughbred horses was recorded in mid 18th century England, by Robert Bakewell and Thomas Coke. However, written pedigrees of horses, tracking purity of bloodline, was practiced as far back as the 13th century, when Carthusian monks were entrusted with the task of keeping accurate records of the Andalusian horse breed.
A studbook is the official registry of approved individuals of a given breed kept by a breed association. It is said to be "closed" if individuals can be added only if their parents were both registered. It is said to be "open" if individuals can be added without their parents being registered, such as by inspection.
Studbooks have been kept for centuries; the concept of the breed associations and clubs is more recent. Some "purebred" horses have open studbooks. For example, a "purebred" mare can be "examined" by the Trakehner authorities; if she is found acceptable, her offspring can be registered as Trakehner. By contrast, the Thoroughbred has had a closed studbook for over 200 years. The Arabian horse is another breed which not only has a closed studbook, but will sometimes de-register some animals found to have outside blood, including all of their descendants. One example of de-registration was Shagya, the foundation sire of the Shagya Arabian breed, who had countless offspring by the time the pedigree error was discovered.
In most cases, the studbooks of purebred dogs only remain open if the breed is under development.
The very idea of 'breed purity' often strikes an unpleasant chord with modern animal fanciers because it is reminiscent of nineteenth-century eugenics notions of the "superior strain" which were supposedly exemplified by human aristocracies and thoroughbred horses.[1] The application of theories of eugenics has had far-reaching consequences for human beings, and the observable phenomenon of hybrid vigor stands in sharp contrast.
The idea of the superior strain was that by "breeding the best to the best," employing sustained inbreeding and selection for "superior" qualities, one would develop a bloodline superior in every way to the unrefined, base stock which was the best that nature could produce. Naturally the purified line must then be preserved from dilution and debasement by base-born stock. This theory was never completely borne out. It can be said that when the ideal of the purified lineage or aesthetic type is seen as an end in itself, the breed suffers over time. The same issues are raised in the world of purebred cats.
Charles Darwin discussed how selective breeding had been successful in producing change over time in his book Origin of Species. The first chapter of the book discusses selective breeding and domestication of such animals as pigeons, dogs and cattle. Selective breeding is used as a springboard to introduce the theory of natural selection, and to support it.
The term backyard breeder is a general term used to describe people who allow their dogs or horses to procreate regardless of physical, genetic, and/or emotional health. While this term is often reserved for "home breeders", or those who breed for fun, it has also found recognition in the description of unreputable show and working breeders. The latter two groups often focus on one aspect of the dog (such as aesthetics), while ignoring the original function and temperament of the breed. In the process of careless breeding, many backyard breeders produce genetically weak animals that can be predisposed to debilitating physical deformities.
When such breeding is carried out on a large scale, the venue is called a puppy mill (especially in North America) or puppy farm. Due to the time and expense of feeding and caring for horses, who produce one foal per year, there are fewer horse breeders who produce animals en masse, though some individuals do engage in animal hoarding and breed far more animals than they can support.
Backyard breeding is popularly blamed for the proliferation of aggressive dogs for the sports of baiting and dog fighting. Dog fanciers generally believe that such ill-bred dogs are the reason for the bad reputation of some breeds in the public perception, and the resulting breed-specific legislation. In the horse world, overbreeding of grade animals that cannot be sold raises concerns that such animals will be slaughtered for horsemeat.
Selective breeding is also used in research to produce transgenic animals that breed true (i.e. they are homozygous) for the artificially inserted or deleted genes.