Explanation
Domestication is the process whereby a population of living
organisms is changed at the genetic level, through generations of
selective breeding, to accentuate traits that ultimately benefit
the interests of humans. A usual by-product of domestication is
the creation of a dependency in the domesticated organisms, so
that they lose their ability to live in the wild. Through
domestication a change in the phenotypical expression and in the
genotype of the animal occurs over generations. A
domesticated species is defined as "a plant- or animal-species in
which the evolutionary process has been influenced by humans to
meet the needs of mankind". Therefore, an important
factor on domestication is Artificial Selection
by humans.
Humans have brought these populations under their control and
care for a wide range of reasons, such as: to produce food (such
as wheat, beans, milk) or valuable commodities (such as wool,
cotton, or silk); to do types of work (such as transportation,
protection, warfare); to use for scientific research; to enjoy as
companions or ornaments (e.g. from
plants).
Historical Frame
Charles Darwin was
the first to describe how domestication, selection and evolution
are interlinked, and based on natural heritable variation among
individual plants and animals. Today we know that such
natural variation is caused by mutations in genes coding for
these traits, and by new combinations of already existing genetic
variation, based on earlier mutations. Darwin described how the
process of domestication can involve both unconscious and
methodical elements. Routine human interactions with animals and
plants create selection pressures that cause adaptation to human
presence, use or cultivation. Deliberate selective breeding has
also been used to create desired changes, often after initial
domestication. These two forces, unconscious natural selection
and methodical selective breeding, may have both played roles in
the processes of domestication throughout history. Both have been
described from human perspective as processes of Artificial
Selection. also called Extrinsic Eugenics.
Examples
The domestication of wheat
Wild wheat plants fall to the ground to re-seed themselves, when
ripened. But domesticated wheat stays upright on the stem, for
easier harvesting by man. For a wild wheat plant, this
'uprightness' may not be a clever way of dispersing its seed. There
is evidence that this change was possible because of a random
mutation that happened in the wild populations at the beginning
of wheat cultivation. Wheat plants with this mutation (i.e. a long-lasting
erect stem) were harvested more frequently by humans, and thus
became the seed for the next crop. Therefore, without realizing,
early farmers selected for this mutation, which may otherwise
have died out. The result is domesticated wheat, which now relies
on farmers for its own reproduction and dissemination.
The domestication of dogs
It is speculated that thousands of years ago, certain wolves
which were tamer than the average wolf and less wary of humans,
selected themselves as dogs over many generations. Most animals
love their freedom or independence, and hunt for their own
food. Some wolves may be sick or crippled in a fight, and have to
find other ways to get their meal for the day. So they become
opportunistic. These wolves were able to thrive by following
humans to scavenge for food near camp fires and garbage dumps;
this behaviour gave them an advantage over more shy
individuals. Eventually a symbiotic relationship developed
between people and these 'proto-dogs'. The dogs fed on human food
scraps, and humans found that dogs could warn them of approaching
dangers, such as large predators or other intruders. Some dogs
could help with hunting, act as pets, provide warmth, or
supplement the food supply of humans (!). As this relationship
progressed, humans eventually began to keep these self-tamed
wolves and breed from them the types of dogs that we have
today.
Scientific research on artificial selection
In recent times, selective breeding may best explain how
continuing processes of domestication often work. Some of the
best-known evidence of the power of selective breeding comes from
the Farm-Fox Experiment by Russian scientist, Dmitri K. Belyaev, in
the 1950s. His team spent many years breeding the domesticated
silver fox (Vulpes vulpes) and selecting only those individuals
that showed the least fear of humans. Eventually, Belyaev's team
selected only those that showed the most positive response to
humans. He ended up with a population of grey-coloured foxes
whose behaviour and appearance was significantly changed. They no
longer showed any fear of humans and often wagged their tails and
licked their human caretakers to show affection. Their behaviour
was more 'childlike' as if they were mentally stuck in a
youngster-phase, but with an adult body (This is called
Pedomorphosis: the retention of juvenile characteristics in the
adult body). These foxes had floppy ears, smaller skulls,
rolled tails and other traits commonly found in dogs.
Domesticated foxes had less pronounced stress hormones
(cortisol, adrenalin) and higher serotonin levels. It
took Belyaev's team some 10 to 30 generations of artificially
selecting fox offspring, to wilfully 'steer' the evolution
of behaviour in their desired direction!
Negative aspects
Selection of animals for visible "desirable" traits may make them
unfit in other, unseen, ways. The consequences for the captive
and domesticated animals were reduction in size, piebald colour,
shorter faces with smaller and fewer teeth, diminished horns,
weak muscle ridges, and less genetic variability. Poor joint
definition, late fusion of the limb bone epiphyses with the
diaphyses, hair changes, greater fat accumulation, smaller
brains, simplified behaviour patterns, extended immaturity, and
more pathology are a few of the defects of domestic animals. All
of these changes have been documented in direct observations of
the rat in the 19th century, by archaeological evidence, and
confirmed by animal breeders in the 20th century.
Other negative aspects of domestication have been explored. For
example: Man substitutes controlled breeding for natural
selection; animals are selected for special traits like milk
production of passivity [e.g. child-friendly Golden Retriever
dog], at the expense of overall fitness and nature-wide
relationships. Though domestication broadens the diversity of
forms (that is: increases visible polymorphism, for
example, the many kinds of sizes and colours dogs
have today) it undermines the crisp demarcations that separate
wild species. And it cripples our (i.e. modern
citizens) recognition of the species as a group. Knowing
only domestic animals dulls our understanding of the way in which
unity and discontinuity occur as patterns in nature, and
substitutes an attention to individuals and breeds. The wide
variety of size, colour, shape, and form of domestic horses, for
example, blurs the distinction among different species of Equus
that once were constant and meaningfully adapted to natural
surroundings.
Linguistic derivation
The term Domestication is derived from the Latin word
domesticus meaning "of the home".
External sources
http://en.wikipedia.org/wiki/Domestication .
http://10e.devbio.com/article.php?ch=23&id=223 (Fox
breeding).