Arran whitebeams complex fashion, which involved the common whitebeam ("Sorbus aria") giving rise to the tetraploid rock whitebeam ("Sorbus rupicola") which is still found on Holy Isle. This species is able to survive at higher altitudes and therefore occupies a less competitive niche with fewer tree species able to tolerate the harsher conditions. The rock whitebeam interbred with the rowan / mountain ash ("Sorbus aucuparia") to produce the hybrid, a fertile separate species the Scottish whitebeam ("Sorbus arranesis") which grows well in this zone of reduced competitive growth at higher altitudes. The bastard mountain ash ("Sorbus pseudofennica") arose from a further cross
Sorbus mougeotii which differs only in slightly broader leaves. It is also closely related to "Sorbus intermedia" (Swedish whitebeam), which differs in having the leaves grey-white below and more deeply lobed, with the lobes spreading rather than forward-pointing, the fruit oval and less bright red, and in forming a stouter tree with a single trunk and more horizontal branching. All are tetraploid apomictic species which breed true without pollination, and ultimately of hybrid origin between "Sorbus aria" and "Sorbus aucuparia". It is widely grown as an ornamental tree in northern Europe, though usually unrecognised, misidentified as "Sorbus intermedia". Although described in 1858,