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 aucuparia be applied to other species in "Sorbus" subgenus "Sorbus", and mountain ash may be used for several other distantly related trees. The species is unrelated to the true ash trees (genus "Fraxinus"), which also carry pinnate leaves or the species "Eucalyptus regnans", also called mountain ash, native to Tasmania and Victoria in southeastern Australia. "S. aucuparia" was previously categorized as "Pyrus aucuparia". The author citation "Sorbus aucuparia" L. belongs to Carl Linnaeus. "Sorbus aucuparia" occurs as a tree or shrub that grows up to between 5 and 15 m in height. The crown is loose and roundish or irregularly shaped