Selective breeding

This article focuses on selective breeding in domesticated animals. For alternate uses, see artificial selection.

Selective breeding in domesticated animals is the process of a breeder developing a cultivated breed over time, and selecting qualities within individuals of the breed that will be best to pass on to the next generation. Breeding techniques such as inbreeding, linebreeding and outcrossing are utilized by breeders in the maintenance and improvement of their chosen breeds.

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 was used by Darwin used as a springboard to introduce the theory of natural selection, and to support it.

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Breeding stock

"Breeding stock" is a term used to describe a group of animals used for purpose of planned breeding. When an individual is looking to breed animals, he or she is looking for certain valuable traits in purebred stock for a certain purpose, or may intend to use some type of crossbreeding to produce a new type of stock with different, and presumably superior abilities in a given area of endeavor.

For example, to breed chickens, a typical breeder intends to receive eggs, meat, and new, young birds for further reproduction. Thus the breeder has to study different breeds and types of chickens and analyze what can be expected from a certain set of characteristics before he or she starts breeding them. Accordingly, when purchasing initial breeding stock, the breeder seeks a group of birds that will most closely fit the purpose intended.

Purebred breeding

See also: Purebred

The idea of breed purity may strike an unpleasant chord with some individuals because it is reminiscent of nineteenth-century eugenics notions of a "superior strain" in humans, supposedly exemplified by members of an aristocratic social class or of certain races.[1] The application of theories of eugenics had far-reaching consequences for human beings, particularly in cases such as The Holocaust.

However, the idea of a superior strain in animals, such as that incorporated in the breeding of Thoroughbred race horses, was less controversial. By "breeding the best to the best," employing a certain degree of careful inbreeding, considerable culling, and selection for "superior" qualities, one could develop a bloodline or "breed" superior in certain respects to the original base stock or landrace which had been produced primarily by natural selection.

This process forms the basis for a "true-breeding" line of animals, which today is generally called a "purebred" line. Once created, a purified line is "closed" and preserved from dilution and possible "debasement" by unrelated stock. When such animals are recorded with a breed registry, the closing of a bloodline group is sometimes referred to as having a closed stud book.

However, this theory is not without some negative consequences. When the ideal of purified lineage or purely aesthetic type is seen as an end in itself, without consideration of usefulness or vigor, a breed can suffer over time. Also, if a breed registry is closed with too small a gene pool, over time, degradation due to inbreeding depression may occur. For many purebred breeders, the need for a periodic outcross is recognized and incorporated into an overall plan. Some breed registries thus maintain an "open" stud book for certain types of approved outcrossed or crossbred animals.

The observable phenomenon of hybrid vigor stands in contrast to the notion of breed purity. However, on the other hand, indiscriminate breeding of crossbred or hybrid animals may also result in degradation of quality, particularly when animals are mass-produced without concern for overall health or longevity.

Backyard breeding

The term backyard breeder is a general term, sometimes considered derisive, used to describe people who are "home breeders", and breed only a few animals for personal use or occasional sale. However, while some backyard breeders pay strict attention to quality, the term usually describes those who allow animals, particularly dogs or horses, to procreate regardless of physical, genetic, and/or emotional health. It is also used to describe unreputable commercial breeders. In the process of careless breeding, many backyard breeders produce genetically weak animals that can be predisposed to genetic disorders or debilitating physical deformities, particularly those that result from inbreeding.

In dog breeding, these individuals focus on only a limited aspect of the dog (such as outward aesthetics), while ignoring the original function and temperament of the breed. When such breeding is carried out on a large scale, the venue is called a puppy mill (especially in North America) or puppy farm. Backyard breeding is popularly blamed for the proliferation of aggressive dogs for the sports of dog baiting and dog fighting. Dog fanciers generally believe that such ill-bred dogs are the reason for the bad public reputation of certain dog breeds, and the increasingly frequent enactment of breed-specific legislation.

Because of the time and expense of feeding and caring for horses, which 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. In the horse world, overbreeding of grade animals that cannot be sold except for very low prices raises concerns that such animals will be slaughtered for horsemeat. Some, though not all, color breeders make errors similar to puppy mill breeders by focusing on only one characteristic (in this case, coat color of the horse) to the detriment of health, soundness, and quality conformation. In addition, industrial breeders, such as those who produce pregnant mare urine for the manufacture of premarin, allow many mares to be bred indiscriminately, simply so that they are able to produce the urine laced with reproductive hormones that forms the base for the drug. Their foals are generally considered an unwanted byproduct, particularly males (colrs), and sold off, frequently to the slaughter market.

Problems with inbreeding and loss of vigor is also a concern in the world of purebred show cats.

Scientific research

Selective breeding is also used in research to produce transgenic animals that breed "true" (i.e. are homozygous) for artificially inserted or deleted genes.

See also

• Marker assisted selection
• Artificial selection
• Breed registry
• Breeding
• Quantitative genetics
• Natural selection
• Smart breeding

References

• Darwin, Charles (2004). The Origin of Species. London: CRW Publishing Limited. ISBN 1904633781.