Artificial directional selection in corn (Zea mays)
Starting with a single line of corn, different strains can be selected for either high or low content of oil (left) or protein (right). Note that decline of oil content ceases after about 20 generations of selection, whereas the high-line has increased nearly three-fold over the same period, and is still increasingly linearly after 50 generations (14-fold difference). [Evidently there is a lower limit of oil content compatible with survival]. In another strain, protein content has approximately doubled over the same period. [Agron Journ 44:61 (1952)].