Crop Plants: Evolution

Abstract

Agriculture arose independently in several locations across the world, beginning approximately 13 000 years ago in the Near East. All crop plants originated from wild plant populations. At the beginning of the domestication process, a number of changes began to appear in the genetic and physiological makeup of many crop species. Some of these changes were due to conscious selection such as increases in palatability and colour, but many of the others were the unconscious byproduct of planting and harvesting. Changes in crop species during domestication were gradual, likely taking thousands of years. Many of the traits associated with plant domestication are regulated by only a few genes, and in many cases these genes have pleiotropic effects, where they affect a number of traits simultaneously. Many of the genes associated with domestication are also clustered close together on the same chromosome. The process of domestication severely limited the amount of genetic variation in crop species. Key Concepts: The most potent force influencing variation in cultivated populations is artificial selection, a specialised case of directional selection with humans acting as the agents of selection. The morphology and physiology of crop species were gradually changed through both conscious and unconscious selection by humans. Many of the traits associated with plant domestication are regulated by only a few genes that are close together on chromosomes. A high percentage of crop species are polyploid, with more than one set of chromosomes. Agriculture probably first emerged in the hills above the Tigris River on the western edge of what is now called Iran approximately 13 000 years ago, but it also appeared independently in Mesoamerica, South America, China, Africa, Asia and Eastern North America. Until recently, it was thought that once humans began cultivation, selection for the domestication genes would be rapid; however, new archaeological evidence indicates that predomestication cultivation may have actually taken 1000 years. During the process of crop domestication, genetic variability was greatly restricted by the selection of a limited number of desirable individuals.