Over the course of thousands of years, American indigenous peoples domesticated, bred and cultivated a large array of plant species. These species now constitute 50–60% of all crops in cultivation worldwide. In certain cases, the indigenous peoples developed entirely new species and strains through artificial selection, as was the case in the domestication and breeding of maize from wild teosinte grasses in the valleys of southern Mexico. Numerous such agricultural products retain their native names in the English and Spanish lexicons.