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 domestica Sorbus domestica Sorbus domestica, with the common name service tree or sorb tree (because of its fruit), is a species of "Sorbus" native to western, central and southern Europe, northwest Africa (Atlas Mountains), and southwest Asia (east to the Caucasus). It may be called true service tree, to distinguish it from wild service tree "Sorbus torminalis". It is a deciduous tree growing to 15–20 m (rarely to 30 m) tall with a trunk up to 1 m diameter, though it can also be a shrub 2–3 m tall on exposed sites. The bark is brown, smooth on young trees, becoming