Separation and variation—that is, variation not overwhelmed by crossing—is all that is necessary to secure divergence of type in the descendants of one stock, though external conditions remain the same and though the separation is other than geological.
For divergence to occur among the descendants of a single lineage, it suffices for there to be variation that does not get submerged by crossing. This can happen regardless of whether external circumstances stay unchanged or the separation between groups isn't geologic.