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Information on crop evolution is vital in the current effort to understand and conserve biodiversity, and provides a basis for the improvement of plant species. Plant domestication has changed considerably over the course of human history. 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.

With the concept of population, it is easy to apply indicators of demographic processes (birth rate, mortality, emigration, immigration) that give a population its dynamics. Because these processes depend on both individual and environmental properties, we speak of the population–environment system. The concept of species diversity is based on the fact that an individual organism’s variable features are recorded in its genetic heritage. The set of characteristics and behaviours of living things, known as the phenotype, studied by naturalists when they are working in the field depends first on their genetic structure, or genotype. Therefore, it is the set of genes and the genetic modifications that take place on the genes and chromosomes during DNA replication that determines species diversity (Collins and Qualset, 1998; Jarvis et al., 2007).

With this short introduction, we investigate the relation between variability and evolution, and its uses for domestication. In this way, we will try to understand why agrobiodiversity is a human creation. We will focus here on the dynamic character of biodiversity and trace the development of cultivated diversity in different agroecosystems. At the first level of biodiversity, genetic diversity is a source of adaptability enabling farmers to respond to changes in the environment and, by allowing farmers to make selections, permits the production of new varieties that respond to new needs. It is thus due to genetic diversity that evolution within a species is possible and that farmers are able to match ecotypes and cultivars, corresponding to specific environments at a local scale (Bazile et al., 2008).

The first important point is that hunter-gatherers around the world possessed a thorough knowledge of plants – their survival depended on it (Diamond, 2002). They knew which plants growing near their camps may be harvested, how to transform and process bitter or poisonous plants, and had knowledge on the range of medicinal or alimentary uses of these plants. Early farmers quickly understood that there was no point sowing or maintaining plants that already grew in the wild close to their camps. This is the reason we now believe that the beginning of agriculture involved secondary plants that could not be found easily.

Hunter-gatherer societies disappeared during the Neolithic period, although gathering, hunting and fishing practices have continued to this day. Over time new activities – ones that essentially were linked to a different food strategy – developed, with agriculture being one of them. The birth of agriculture is entwined with the search for new food products to support demographic growth (Cauvin, 2008). This effort also included developing techniques that allowed the consumption of these products, such as grinding and cooking. Yet, at the same time, people also continued to feed themselves through gathering, particularly wild cereals (wheat and barley in the Middle East, rice in Asia, millet and sorghum in Africa, maize in the Americas) (Wood and Lenné, 1999; Kaihura and Stocking, 2003).

The shift from gathering to cultivation involved a new way of thinking that was radically different from the past, requiring precise knowledge regarding the selection of seeds, when to sow, how to prepare the land into fields, rotate and distribute species, fertilize with manure, irrigate, and store (granaries, silos) and cook products. This is why there were numerous intermediary stages in agriculture, particularly in the protection of useful plant species and the destruction of harmful species. Thus, we speak of selection, conscious or not, of a certain number of plant types.

We can understand how this domestication took place if we consider, for example, wild cereals, which reproduce more easily when their grains detach easily from the ear. Farmers, however, need grains that remain on a solid ear and stem in order to be able to harvest the maximum amount in the shortest period of time. The same is true for vegetable plants, whose role was essential in the beginning of agricultural practices. The selection of desired characteristics, which was made almost automatically, was certainly at the origins of agriculture. The three essential conditions for the birth of agriculture were:

• people were settled into villages;

• they knew how to sow and harvest; and

• they were specialized in the gathering of species that later would be domesticated.

The original societies in Highland of the Andes existed in this way, with settlements around Lake Titicaca and a situation emerged that helped develop agriculture from wild species (Maxted et al., 2012).

The initial stage of domestication was often determined entirely by unconscious selection. In fact, the phenotypic changes associated with domestication are likely to have arisen via unconscious selection occurring from automatic practices during harvest or unintentional practices that participated in the process of domestication. Generally, it is observed that the phenotypic changes associated with adaptation under domestication are substantial, and they are illustrative of the process and effects of natural selection combined with changes produced by human activities (Lenné and Wood, 2011).

Human societies have common features that explain the permanent domestication process. Farmers are looking for larger fruit or grain, reduced branching, gigantism, the loss or reduction of seed dispersal, the loss of seed dormancy, synchronized seed maturation, an increase in grain size, larger inflorescences, changes in photoperiod sensitivity, and the loss or reduction of toxic compounds. The phenotypic changes associated with domestication could be separated in two parts: characteristics such as colour or fruit size that were probably desired by humans and other traits resulting from unconscious selection that would have been difficult for early cultivators to notice or that would have changed without any direct effort. Finally, there is often a natural counterpart in the agroecosystems. Conscious or unconscious selection is not limited to the visible part of phenotypes. And much of the adaptation under domestication may have involved physiological or developmental changes corresponding to the new edaphic, photosynthetic, hydrological and competitive regimes associated with cultivation (Brookfield, 2001).

To summarize how farmers create diversity, there are three essential points to keep in mind:

1. Farmers domesticate wild plants. That is to say, they seek to adapt wild species to agricultural production. These species are ones that they had once obtained by gathering in the wild.

2. Farmers add to diversity by adapting crops to new ecosystems or changing needs. This may be in terms of human consumption or for other uses such as animal feed.

3. Farmers continuously discover new crops to cultivate, which means that diversity in agriculture is not fixed but is in a constant state of evolution.