Plant breeding began when early humans saved seeds and planted them. The cultural change from living as nomadic hunter-gatherers, to living in more settled communities, depended on the ability to cultivate plants for food. Present knowledge indicates that this transition occurred in several different parts of the world, about 10,000 years ago.

Today, there are literally thousands of different cultivated varieties (cultivars) of individual species of crop plants. As examples, there are more than 4,000 different peas (Pisum sativum), and more than 5,000 grape cultivars, adapted to a wide variety of soils and climates.

The methods by which this diversity of crops was achieved were little changed for many centuries, basically requiring observation, selection, and cultivation. However, for the past three centuries most new varieties have been generated by deliberate cross-pollination, followed by observation and further selection. The science of genetics has provided...