But if some of the branches were broken off, and you, although a wild olive shoot, were grafted in among the others and now share in the nourishing root of the olive tree...
Romans 11:17 ESV

The normal process is to graft cultivated branches into a wild root stock because the wild root is hardier, more tolerant to adverse conditions, but God frequently turns the natural order on its head for spiritual purposes. The younger brother rules the elder. The second Adam takes authority over the domain of the first.

The cultivated olive root is the enduring stock into which the wild are grafted in order to survive a great change for which all wild trees are completely unprepared. An extinction event is coming, and only that one tree which has been specially prepared will survive it.