From Wikipedia,
the free encyclopedia.
Selective breeding in
domesticated
animals is the process of
developing a cultivated
breed over time.
In general, the owners of the
animals use three strategies to
refine local populations:
Selective Breeding Methods
- 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 Perce 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 Perce were observed to
have developed strong, hardy,
spotted horses.
It is not known if the Nez
Perce 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 Eurpose, 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 modern invention,
in fact some breeds of cats have
been in existence less than fifty
years and most do not have closed
studbooks.
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.
See also
