Domestication

Domestication

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).

 

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