Plant breeders manipulate the genetic resources of a species, that is, its germplasm, to produce plants that have increased value to humanity. Plant breeding is human-directed evolution. All of our major food crops and their respective strains or cultivars were developed by this process. Although humans have successfully manipulated the genetic resources of plants for several thousand years, the science of genetics and breeding was not developed until the 20th century. Initially, plant genetics research was conducted on human food plants such as maize (Zea mays L.), wheat (Triticum aestivum L.), and pea (Pisum sativum L.)
