What is the main requirement for basic plant breeding?

The most important factor for basic selective breeding is genetic variation in the characteristic of interest.

For example, a farmer who wants to select a plant with resistance to an insect pest will watch for the plants that survive an insect attack. Another farmer wanting larger fruits will save seeds from plants yielding the biggest fruits in the field.
For some traits, such as the colour of maize kernels, there is a great level of genetic variation in nature.