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 torminalis the bark on old trees. The species has sometimes been placed in its own genus, "Torminalis". Recent molecular phylogenetics confirms an earlier taxonomic opinion that the genus "Sorbus" as defined in the broad sense is not monophyletic, but the simple-leafed species traditionally included in "Sorbus" are a monophyletic group; this species would be included in a clade called "Aria" (genus "Aria" or "Sorbus" subgenus "Aria") as the species "Aria torminalis" Beck. It is a sexually reproducing diploid species that forms diploid and polyploid hybrids with members of subgenus "Aria" (whitebeams). A number of often very localised stable apomictic polyploid species