3.1 Origin domestication and geographical distribution

The cultivation of olive trees (Olea europaea subsp. europaea var. sativa) is concentrated between the latitudes of 30° and 45° in both the northern and southern hemispheres, primarily in regions with a Mediterranean climate, which is typified by hot, dry summers. In recent decades, the cultivation of olives has expanded beyond the Mediterranean climate zone. In fact, olive groves are currently found in tropical areas, where the climate is influenced by altitude, such as Brazil or the Canary Islands.

The cultivated olive is composed of a vast array of clonally propagated varieties. These cultivated varieties frequently coexist with their wild ancestor, the oleaster (Olea europaea subsp. europaea var. sylvestris), which is native to many humid and sub-humid regions of the thermo-Mediterranean zone where frost is rare. Despite their geographical proximity, the genetic relationship between cultivated and wild olives remains somewhat puzzling and not fully elucidated. Archaeological evidence suggests that olive cultivation originated in the eastern Mediterranean basin approximately 6,000 years ago. Further analysis of chloroplast DNA provides additional support for the hypothesis that the primary domestication site was along the Syrian-Turkish border. 

The introduction of clonal propagation techniques after domestication is likely to have accelerated the spread of olive cultivation across the Mediterranean. This method was also applied to other long-lived perennial crops, such as grapes and figs, thereby facilitating the consistent reproduction of desirable traits in new olive trees. 

The effectiveness of clonal propagation is evidenced by the fact that 90% of olive cultivars throughout the Mediterranean basin share the same "Eastern-like" chlorotype. This fact supports the hypothesis of a "single domestication" center, whereby the local wild olives potentially provided pollen for the primary domesticated varieties. This gene flow would have helped mitigate the negative effects of inbreeding and facilitated the development of cultivars better suited to local conditions. However, an alternative or maybe complementary hypothesis posits the existence of a secondary domestication center in the west-central Mediterranean basin. This hypothesis is supported by the presence of certain cultivars with unique chlorotypes, which are found only in the wild populations of those regions.

A distinctive feature of the olive tree is its remarkable longevity, with some specimens surviving for several centuries or even millennia. A considerable number of these ancient trees, frequently grafted onto wild olive rootstocks, can be found throughout the Mediterranean basin. These remarkable trees serve as living archaeological records, offering valuable insights into the historical genetic diversity of cultivated olives. They provide a crucial link that allows us to understand how olive cultivation and genetic selection have evolved over millennia.