Artificial Selection
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[edit] Definition
Artificial selection is the practice of selectively breeding plants or animals for inherited traits favored by the stockbreeder.
[edit] Origins
The term "artificial selection" was coined by Charles Darwin in Chapter 4 or the Origin of Species, in contrast with the process of natural selection, in which variable traits are selected for or against by environmental pressures. It would seem that Darwin used the term only once in the whole book; but it has become a standard term among biologists.
The origins of the practice of artificial selection are lost in the prehistory of the Neolithic; there is still some dispute over the earliest date of domestication. [1]
[edit] Examples
Darwin wrote at length on the effect of artificial selection in producing markedly different varieties of pigeon, noting their variations in appearance, morphology, development, and habit (see the Appendix below). Modern textbooks often cite the many breeds of dog, which we now know (as Darwin did not) are descended from a single wild stock, Canis lupus.
But perhaps the prize for greatest plasticity of a single species at the hands of artificial selection must go to Brassica oleracea, the wild cabbage.
The domesticated varieties include: [2]
- The acephala group: kale, collards, flowering kale, flowering cabbage, cole and borecale.
- The alboglabra group: Chinese kale and Chinese broccoli.
- The botrytis group: broccoli and cauliflower.
- The capitata group: green, red and Savoy cabbage.
- The gemmifera group: Brussels sprouts.
- The gongylodes group: kohlrabi.
- The italica group: Italian broccoli, asparagus broccoli, sprouting broccoli, cape broccoli and purple cauliflower.
- The tronchuda group: tronchuda kale, tronchuda cabbage, Portuguese kale, Portugese cabbage, and braganza.
The photographs on the right of this page show a small sample of the diversity of Brassica oleracea: from top to bottom: the wild type; a cauliflower; ornamental kale; a cabbage; Romanesco broccoli; kohlrabi. All these types were produced from the wild type by artificial selection: by farmers and horticulturalists selecting and breeding for traits arising by chance which they happened to favor.
[edit] Differences between natural and artificial selection
In one repect the practitioners of artificial selection have a large advantage over nature: they can segregate animals or plants with desirable mutations from the wider gene pool of which they are a part. By this means, new characteristics arising by chance can be protected from the effects of genetic drift.
Natural selection, on the other hand, has a much greater time-span over which to operate. Humans have only been practicing artificial selection since the Neolithic[3].
Also, the practitioner of artificial selection, for most of its history, has been able only to select for very superficial traits: size, color, external form, and so forth.
These three facts together explain what at first seems paradoxical. Different forms of dog differ so much that a taxonomist knowing nothing of dogs and finding the bones of, let us say, a poodle and a St Bernard, would certainly not guess them to be the same species --- perhaps not even the same genus. Darwin made the same observation about pigeons (see the Appendix below); and we believe that the varieties of Brassica oleracea pictured to the right speak for themselves. Yet in none of these cases has speciation occurred; perhaps more speciation has been observed occurring naturally in the past hundred years than has been produced in the whole history of stockbreeding.
Why should this be so? Because the differences we have listed between artificial and sexual selection ensure that the breeder will be able to make rapid changes in superficial morphology over a comparatively short period of time, without enough time passing for the two varieties to accumulate the profounder but less obvious differences in biochemistry which lead to reproductive isolation. In the brief course of artificial selection, the morphological changes by which species might usually be distinguished have outrun other evolutionary changes. The result is a marked divergence of form, such as evolution in nature might take a million years to produce --- while the genetic divergence is no more than can be produced in a maximum of ten thousand years or so, since the time when our Neolithic ancestors first domesticated animals.
[edit] Appendix: Darwin on pigeons
Believing that it is always best to study some special group, I have, after deliberation, taken up domestic pigeons. I have kept every breed which I could purchase or obtain, and have been most kindly favoured with skins from several quarters of the world, more especially by the Hon. W. Elliot from India, and by the Hon. C. Murray from Persia. Many treatises in different languages have been published on pigeons, and some of them are very important, as being of considerable antiquity. I have associated with several eminent fanciers, and have been permitted to join two of the London Pigeon Clubs. The diversity of the breeds is something astonishing. Compare the English carrier and the short-faced tumbler, and see the wonderful difference in their beaks, entailing corresponding differences in their skulls. The carrier, more especially the male bird, is also remarkable from the wonderful development of the carunculated skin about the head, and this is accompanied by greatly elongated eyelids, very large external orifices to the nostrils, and a wide gape of mouth. The short-faced tumbler has a beak in outline almost like that of a finch; and the common tumbler has the singular inherited habit of flying at a great height in a compact flock, and tumbling in the air head over heels. The runt is a bird of great size, with long, massive beak and large feet; some of the sub-breeds of runts have very long necks, others very long wings and tails, others singularly short tails. The barb is allied to the carrier, but, instead of a long beak, has a very short and broad one. The pouter has a much elongated body, wings, and legs; and its enormously developed crop, which it glories in inflating, may well excite astonishment and even laughter. The turbit has a short and conical beak, with a line of reversed feathers down the breast; and it has the habit of continually expanding, slightly, the upper part of the oesophagus. The Jacobin has the feathers so much reversed along the back of the neck that they form a hood, and it has, proportionally to its size, elongated wing and tail feathers. The trumpeter and laugher, as their names express, utter a very different coo from the other breeds. The fantail has thirty or even forty tail-feathers, instead of twelve or fourteen, the normal number in all the members of the great pigeon family: these feathers are kept expanded and are carried so erect that in good birds the head and tail touch: the oil-gland is quite aborted. Several other less distinct breeds might be specified.
In the skeletons of the several breeds, the development of the bones of the face, in length and breadth and curvature, differs enormously. The shape, as well as the breadth and length of the ramus of the lower jaw, varies in a highly remarkable manner. The caudal and sacral vertebrae vary in number; as does the number of the ribs, together with their relative breadth and the presence of processes. The size and shape of the apertures in the sternum are highly variable; so is the degree of divergence and relative size of the two arms of the furcula. The proportional width of the gape of mouth, the proportional length of the eyelids, of the orifice of the nostrils, of the tongue (not always in strict correlation with the length of beak), the size of the crop and of the upper part of the oesophagus; the development and abortion of the oil-gland; the number of the primary wing and caudal feathers; the relative length of the wing and tail to each other and to the body; the relative length of the leg and foot; the number of scutellae on the toes, the development of skin between the toes, are all points of structure which are variable. The period at which the perfect plumage is acquired varies, as does the state of the down with which the nestling birds are clothed when hatched. The shape and size of the eggs vary. The manner of flight, and in some breeds the voice and disposition, differ remarkably. Lastly, in certain breeds, the males and females have come to differ in a slight degree from each other.
Altogether at least a score of pigeons might be chosen, which, if shown to an ornithologist, and he were told that they were wild birds, would certainly be ranked by him as well-defined species. Moreover, I do not believe that any ornithologist would in this case place the English carrier, the short-faced tumbler, the runt, the barb, pouter, and fantail in the same genus; more especially as in each of these breeds several truly-inherited sub-breeds, or species, as he would call them, could be shown him.
(Charles Darwin, On the Origin of Species, Chapter I [4])
