The adaptation of plants to agriculture was vital to the shift from hunter–gatherer to agrarian societies. It is generally considered that crops were domesticated 10,000 years ago in diverse places called ‘centers of origins’ (Harlan, 1971).
From the perspective of population–environment systems, we need to move away from the notion of the individual, which is the term used by naturalists to describe each element or living thing in front of them. Individuals are replaced by the concept of population, a fundamental component of ecological systems. A population is a set of individuals of the same species that coexist in the same given environment. The concept of population is particularly interesting (Tilman, 1996) because it is considered to be a system characterized by different state variables. These state variables include:
• number (or density);
• spatial distribution;
• age structure;
• genetic class structure (gene frequency);
• social organization.