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Individual differences |
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- This article focuses on selective breeding in domesticated animals. For alternate uses, see artificial selection.
Selective breeding methodsEdit
In general, the owners of the animals use three strategies to refine local populations: linebreeding(one bloodline, or strain) or inbreeding (mating closely related individuals), to facilitate the weeding-out of undesired characteristics and the fixation of desired traits. Inbreeding and linebreeding are controversial aspects of artificial selection, but have been practiced 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, and the Spaniards of that era were known to have horses with spotted coats. By 1806 (when they are mentioned in journals kept by the Lewis and Clark expedition) the Nez Percé were observed to have developed strong, hardy, spotted horses.
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 was recorded in mid 18th century England, by Robert Bakewell and Thomas Coke.
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, even if the ancestor in question was many generations back.
In most cases, the studbooks of purebred dogs only remain open if the breed is under development.
Purebred cats, dogs, and the debate over 'breed purity'Edit
Most purebred cats and dogs of breeds recognized by all-breed club registries are controlled by "closed studbooks". In a number of modern breeds recognized by the kennel clubs, there are high incidences of specific genetic diseases or disorders and sometimes increased susceptibility to other diseases, reduced litter sizes, reduced lifespan, inability to conceive naturally, etc. This came about because:
- Many breeds have been established with too few foundation dogs or ones that were already too closely related, or both.
- There was artificial isolation: the registries (stud books) are closed for most breeds; therefore one cannot introduce diversity from outside the existing population.
- Most selective breeding practices have the effect of reducing the diversity further. In addition, in the world of conformation dog shows, breeding specimens are often selected on the basis of aesthetic criteria only, without regard for soundness.
- Even if the foundation dogs were sufficiently diverse genetically, almost no one knows how their genetic contributions are distributed among the present day population, consequently, breeding is done without regard to conserving these contributions, which may be of value to the general health and survival of the breed.
Similar problems affect purebred cats, however to a lesser extent since selective breeding in cats has not been practiced for nearly the length of time that it has been in dogs. The purebred cat is a relatively new creature; some breeds of cats have existed less than fifty years.
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. 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.
- ONLINE BOOK: “In situ conservation of livestock and poultry”, 1992, Food and Agriculture Organization of the United Nations and the United Nations Environment Programme
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