Although some propagation occurs through shoots rising from the stem and roots of the original most plant life continues mainly by creating seeds that either fall to the ground or are carried by the wind, birds or other animal life. .  Most new olive trees come from the seeds (olives) from the wild trees. The DNA in a newly formed tree comes from the olive fruit, normally from the original wild tree, which produces small black and very bitter olives. The fruit that is available for both table olives and oil does not come from the growth of the wild trees but has been grafted into the wild form from chosen stock.   Previous generations of farmers discovered choice forms of olive and learned that a twig or branch from these trees could be inserted into a wild tree, (the process of grafting) and the result is propagation of the popular form of olive.

The olive tree has been cultivated this way since the bronze age and has been developed to provide mankind with a fruit that is regarded as one of the healthiest either in its full form as the table olive or as an oil. 

Unfortunately only one cultivar can be eaten off the tree itself in Thassos in the Eastern Mediterranean and all the others have to be processed either preserved in brine or crushed to make the free oil.  The commonest variety in Portugal is Galega Vulgar but there are many others, like Mancillenha and Picaul, which are also propagated and harvested throughout Portugal.Throughout the Iberian Peninsula there is  a mixture of normal farms where generations of farmers have introduced grafting and improved their stock over the centuries and plantations showing intensive olive growing practices.