Selective Breeding
Dogs
Selective Breeding
Selective breeding in
domesticated
animals is the process of developing a cultivated
breed over time.
Selective Breeding Methods
In general, the owners of the animals use three strategies to refine local
populations:
- Isolation. There must be a period in which the members of the group are
relatively fixed, so that no new genetic material comes in. Without genetic
isolation of the group, the differentiation that creates a new breed cannot
take place.
- Artificial selection. Breeders must prevent random mating from
coming about, and limit mating to those individuals who exhibit desired
characteristics. One logical consequence of this isolation is the next
characteristic: inbreeding.
- Inbreeding. Ordinarily those who are controlling the artificial breeding
will find it necessary at some stage to employ a degree of linebreeding
(mating within 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
Closed vs. open studbook
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. Most of the "purebred horses" have open studbooks. For
example, a "purebred"
Arabian mare
can be "examined" by the
Trakehner authorities; if she is found acceptable, her offspring can be
registered as Trakehner. By contrast, the studbooks of purebred dogs only remain
open if the breed is under development or if there is deemed to be an
insufficient genepool.
Crossbreeding and backbreeding
In some
registries, breeders may apply for permission to
crossbreed
other breeds into the line to emphasize certain traits, to keep the breed from
extinction or to
alleviate problems caused in the breed by
inbreeding from a
limited set of animals. A related preservation method is backbreeding, used by
some equine and canine registries, in which crossbred individuals are mated back
to purebreds to eliminate undesirable traits acquired through the crossbreeding.
Some horse societies accept crossbreds who meet certain criteria onto the
breed registry.
Purebred Cats, Dogs and the Debate over 'Breed Purity'
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
show world,
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.
Purebreds
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.
His claim that selective breeding had been successful in producing change
over time was one of the key arguments proposed by
Charles Darwin
to support his theory of
natural
selection in his acclaimed yet controversial work
Origin of
Species. Here, the “selective” does not mean breeding selected in a humanly
controlled fashion.
See also
Home | Up | Backyard Breeder | Breed Club | Breed Registry | Breed Standard | Crossbreed | Dog Hybrids and Crossbreeds | Fault | Pedigree | Puppy Mills | Purebreds | Selective Breeding | Snuppy | Stud Master
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