Selection and Breeding

A dominant theme in the history of agriculture has been crop improvement and yield advancement through selection and exploitation of genetic diversity within the species and its close relatives. And now, there is bioengineering where a gene can come from anywhere in the biological world (genetically modified crops). The earliest stages of domesticated crops were probably not much more productive than the wild progenitors, but the act of cultivation and saving the seed to replant was a radical break with the past. Human selection (artificial selection) was replacing natural selection in shaping the plant. Traits associated with the domestication process are seeds and fruits that remain attached to the plant (nonbrittle rachis and nondehiscent fruits) and do not self sow. Another trait is larger fruits and seeds and less nondigestible fiber in seed coats and woody fibers (cellulose) in the fruits. This increases the palatability of these structures but leaves the plant less protected to insect or rodent predation, so that humans had to take greater care in postharvest storage. When humans planted the seed, they set in motion many selection forces that characterize domesticated plants: simultaneous and immediate germination on being sown in the ground; rapid and uniform growth; and a trend toward annuality if biennial/ perennial. Additionally, a shortened vegetative phase often resulted in increased reproductive effort, thus increasing yield and uniform flowering and ripening. Most of these traits would be harmful for a wild plant.

Once domesticated plants began to travel through human migration and conquest beyond their local area of genetic adaptation, a large amount of genetic variation was released by chance hybridization of diverse forms or freedom from constraints (such as pests, pathogens, frost, and day length) of the old habitat. Citrus, for instance, was brought from East India to Spain by the Arabs, then taken to the West Indies by Europeans after Columbus. One mutant form gave rise to grapefruit, while a mutant orange in Brazil was the origin of the familiar navel orange.

The Columbian Exchange (New World plants to the Old World and vice versa) in the sixteenth century was the single most dramatic migration and acclimatization of crops throughout the world. Coupled with hybridization between dissimilar species, introductions of a tremendous number of new forms were generated. Examples are the potato from Peru, which conquered north Europe as a food plant displacing wheat/barley and turnips/peas; and tomatoes from Mexico, which were embraced in Italian cooking.

Recent yield improvement traces back to the rediscovery of Austrian botanist Gregor Mendel's (1822-1884) classic experiments on the heredity of garden peas. For the first time the plant breeding community had a set of principles by which to proceed with the crop improvement process. Products of this era are hybrid corn, changes in the photoperiod response of soybeans, and the dwarf-stature wheat from the International Center for the Improvement of Wheat and Corn (in Mexico) and rice from the International Rice Research Institute (in the Philippines). These late 1960s Green Revolution cereals and the genes they hold (dwarf stature and fertilizer responsive) now enter the food supply of three billion plus people and are directly responsible for feeding more than eight hundred million people by their increased yield alone. Never in world history had there been such a dramatic yield take-off as the Green Revolution. The hope is that the new and developing biotechnologies will have a comparable favorable outcome for global agriculture.

The irony of using elite improved varieties and commercial seed is that they have a tendency to eliminate the resources upon which they are based and from which they have been derived. Current elite varieties yield better than their parents and they displace them from farmers' fields. Once a displaced variety is no longer planted, its genes are lost to future generations unless it is conserved, usually in a seed bank collection or as a heirloom variety. The saving of old folk varieties, farmer landraces and garden seed passed down through a family, maintaining them in home gardens, has become increasingly widespread. Many of these heirloom varieties taste better, cook better, or possess other unique characteristics that set them apart, but they lack the productivity mechanized farming demands in modern agriculture. see also Agriculture, Modern; Agriculture, Organic; Agronomist; Green Revolution; Seed Preservation; Seeds; Vavilov, N. I.

Garrison Wilkes

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