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 chamaemespilus in corymbs 3–4 cm diameter. The fruit is an oval red pome 10–13 mm diameter. It is the sole species in a group that has been called genus "Chamaemespilus" or "Sorbus" subgenus "Chamaemespilus", distinguished from other subgenera of "Sorbus" by the pink (not white) flowers with forward-pointing petals (not opening flat). More recently, it has become clear that the simple-leafed species traditionally included in "Sorbus" form a monophyletic group, and this species could be included in a clade called "Aria" (genus "Aria" or "Sorbus" subgenus "Aria"). Sorbus chamaemespilus Sorbus chamaemespilus, the false medlar or dwarf whitebeam, is a species of