Origins of Artificial Selection


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One of the markers of human civilization is the development of agriculture, in which food products are cultivated deliberately instead of, or as a supplement to, gathering wild foods. Several wild grains have been domesticated, including wheat and rice. Domestication occurs by artificial selection of traits humans value such as larger or more abundant fruit. In The Cradle of Agriculture the authors discuss when and how domestication of cereals occurred and conclude that when may have been as early as 8900 B.C. for rye and pulses (legumes such as beans and peas), and "where" may have been in the Fertile Crescent, in what is now southeastern Turkey and northern Syria. This location is based in part on the distribution of wild progenitors of domestic crops. Knowing where plants grow wild allowed the scientists to determine where domestication must have occurred first.

In two recent articles, scientists looked at how long domestication may have taken and identified the molecular mechanism of domestication. Both papers use the concept of "shattering" in which ripe grains are shed by the plant. As a wild grass, dropping grains as they mature is a good way to spread seed. However, for a farmer, this is an undesirable trait because it reduces harvest yields. Early farmers would have chosen those plants that had the least amount of shattering in the field, but still shattered when threshed.

In "Rice Domestication by Reduced Shattering" scientists looked at the molecular mechanism of shattering. What they found was that a relatively simple mutation at one locus was responsible for the majority of the difference in shattering between wild and domesticated rice. The mutated gene appears to play a role early in development in the establishment of the layer of cells involved in releasing the grain, and possibly also in the actual release of the grain from the plant. The gene is expressed differently in varieties of rice, probably due to differences in regulation of expression. This is a common theme in "EvoDevo" which looks at evolution in development. The authors suggest that artificial selection by early farmers selected for grains that did not shatter easily in the field, but could shatter during threshing.

In "How Fast Was Wild Wheat Domesticated?", scientists examined wheat from the time and area where agriculture is thought to have originated and asked how quickly non-shattering forms became prevalent. Wheat "spiklets", the part of the plant bearing the seeds, from archeological sites were examined. Wild spiklets differ from domesticated spiklets in two ways. First, the "scar" from the grain’s release is visibly different, and second, there are more grains near the top of the ear in domesticated plants. The scientists concluded that it took at least 1,000 years for domesticated plants to become predominant. They suggest that artificial selection was not effective in this case because early farmers may have collected crops before shattering started to maximize yield, so they were not able to select for non-shattering crops. They also pointed out that in years of poor harvest, farmers might have to return to wild varieties for sustenance and the following year’s seed crop. Since agriculture was a new concept, the practice of improving crops may have taken some time to develop.

Artificial selection works in the same ways as natural selection. The raw materials are variation between individuals in a population. In natural selection, environmental pressures such as weather, competition within the population, and predation select the variants best suited for survival under those conditions. In artificial selection, humans select the variants they prefer, and will compensate for environmental pressures by sowing seeds, nurturing and protecting the crops. Both can yield new species, and artificial selection, when practiced rigorously, can do so very quickly.

How Fast Was Wheat Domesticated?
K. Tanno, G. Willcox, Science 311, 1996 (2006)

Rice Domestication by Reduced Shattering
C.Li, A. Zhou, T.Sang, Science 311, 1936-1939 (2006)

The Cradle of Agriculture
S. Lev-Yadun, A. Gopher, S. Abbo, Science 288, 1602-1603 (2000)