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Selective Breeding
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Selective breeding (also called artificial selection) is the process by which
humans breed other animals and plants for particular traits. Typically, strains
that are selectively bred are domesticated, and the breeding is normally done by
a professional breeder. Bred animals are known as breeds, while bred plants are
known as varieties, cultigens, or cultivars. The offspring of two purebreed
animals but of different breeds is called a crossbreed, and crossbred plants are
called hybrids.
There are two approaches or types of artificial selection, or selective breeding.
First is the traditional "breeder’s approach" in which the breeder or
experimenter applies "a known amount of selection to a single phenotypic trait"
by examining the chosen trait and choosing to breed only those that exhibit
higher or "extreme values" of that trait. The second is called "controlled natural
selection," which is essentially natural selection in a controlled environment. In
this, the breeder does not choose which individuals being tested "survive or
reproduce," as he or she could in the traditional approach. There are also
"selection experiments," which is a third approach and these are conducted in
order to determine the "strength of natural selection in the wild." However, this
is more often an observational approach as opposed to an experimental
approach.
In animal breeding, techniques such as inbreeding, linebreeding, and
outcrossing are utilized. In plant breeding, similar methods are used. Charles
Darwin discussed how selective breeding had been successful in producing
change over time in his book, On the Origin of Species. The first chapter of the
book discusses selective breeding and domestication of such animals as
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pigeons, cats, cattle, and dogs. Selective breeding was used by Darwin as a
springboard to introduce the theory of natural selection, and to support it.
The deliberate exploitation of selective breeding to produce desired results has
become very common in agriculture and experimental biology.
Selective breeding can be unintentional, e.g., resulting from the process of
human cultivation; and it may also produce unintended – desirable or
undesirable – results. For example, in some grains, an increase in seed size may
have resulted from certain ploughing practices rather than from the intentional
selection of larger seeds. Most likely, there has been an interdependence
between natural and artificial factors that have resulted in plant domestication.
History
Selective breeding of both plants and animals has been practiced since early
prehistory; key species such as wheat, rice, and dogs have been significantly
different from their wild ancestors for millennia, and maize, which required
especially large changes from teosinte, its wild form, was selectively bred in
Mesoamerica. Selective breeding was practiced by the Romans. Treatises as
much as 2,000 years old give advice on selecting animals for different purposes,
and these ancient works cite still older authorities, such as Mago the
Carthaginian. The notion of selective breeding was later expressed by the
Persian Muslim polymath Abu Rayhan Biruni in the 11th century. He noted the
idea in his book titled India, and gave various examples.
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The agriculturist selects his corn, letting grow as much as he requires, and
tearing out the remainder. The forester leaves those branches which he
perceives to be excellent, whilst he cuts away all others. The bees kill those of
their kind who only eat, but do not work in their beehive.
Selective breeding was established as a scientific practice by Robert Bakewell
during the British Agricultural Revolution in the 18th century. Arguably, his
most important breeding program was with sheep. Using native stock, he was
able to quickly select for large, yet fine-boned sheep, with long, lustrous wool.
The Lincoln Longwool was improved by Bakewell, and in turn the Lincoln was
used to develop the subsequent breed, named the New (or Dishley) Leicester. It
was hornless and had a square, meaty body with straight top lines.
These sheep were exported widely, including to Australia and North America,
and have contributed to numerous modern breeds, despite that fact that they fell
quickly out of favor as market preferences in meat and textiles changed.
Bloodlines of these original New Leicesters survive today as the English
Leicester (or Leicester Longwool), which is primarily kept for wool production.
Bakewell was also the first to breed cattle to be used primarily for beef.
Previously, cattle were first and foremost kept for pulling ploughs as
oxen[citation needed], but he crossed long-horned heifers and a Westmoreland
bull to eventually create the Dishley Longhorn. As more and more farmers
followed his lead, farm animals increased dramatically in size and quality. In
1700, the average weight of a bull sold for slaughter was 370 pounds (168 kg).
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By 1786, that weight had more than doubled to 840 pounds (381 kg). However,
after his death, the Dishley Longhorn was replaced with short-horn versions.
He also bred the Improved Black Cart horse, which later became the Shire
horse.
Charles Darwin coined the term 'selective breeding'; he was interested in the
process as an illustration of his proposed wider process of natural selection.
Darwin noted that many domesticated animals and plants had special properties
that were developed by intentional animal and plant breeding from individuals
that showed desirable characteristics, and discouraging the breeding of
individuals with less desirable characteristics.
Darwin used the term "artificial selection" twice in the 1859 first edition of his
work On the Origin of Species, in Chapter IV: Natural Selection, and in Chapter
VI: Difficulties on Theory –
Slow though the process of selection may be, if feeble man can do much by his
powers of artificial selection, I can see no limit to the amount of change, to the
beauty and infinite complexity of the co-adaptations between all organic beings,
one with another and with their physical conditions of life, which may be
effected in the long course of time by nature's power of selection.
We are profoundly ignorant of the causes producing slight and unimportant
variations; and we are immediately made conscious of this by reflecting on the
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differences in the breeds of our domesticated animals in different countries,—
more especially in the less civilized countries where there has been but little
artificial selection.
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