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Crop Plant Diversity
Crops are domesticated organisms that have arisen by human and natural selection from wild plant species. In the early 20th century, the Russian scientist Nikolai Vavilov, a geneticist and plant geographer, studied extensive collections of cultivated plants in the less developed parts of the world where the indigenous crop varieties had not been replaced by cultivars selected by plant breeders. He arrived at a theory on the centers of genetic diversity of different crops, which he believed were their centers of origin. All of them were located in mountainous regions with an ancient agricultural civilization.
Although subsequent scientists found that Vavilov's centers of crop diversity were not necessarily the crops' centers of origin, the concept of regions of genetic diversity has proved of great significance in the study of the evolution of crop species and in efforts to preserve food-plant diversity.
As crops were moved by humans, species were modified by the environment and by cultural methods; gene pools were diversified; and secondary centers of diversity evolved. The dispersal of genes was accompanied by selection pressures that made the crops resistant to local pests and pathogens and tolerant of different environmental conditions. These distinctive races that became locally adapted to their new environments are known as landraces. In the past century or so, "advanced cultivars" that are the products of scientific plant breeding have been developed and increasingly have replaced the myriad, locally adapted landraces.
The diversity of many crop plants reached its peak around the end of the 19th century. This diversity is now at risk, and the gene pool of many crops is eroding, mainly due to the development of highly uniform cultivars. This uniformity has led to epidemic outbreaks of diseases and the need for constant development of new, disease-resistant varieties. The search for new varieties obliges breeders to search outside the narrow gene pool of modern varieties to older varieties and wild relatives of the crop. Nearly all modern crop varieties contain genetic material recently incorporated from wild species or from more primitive genetic stocks still used and maintained by traditional agricultural peoples.