Evolution
The trees developed in a highly 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 between S. arranensis and the Mountain Ash (S. aucuparia).
The Sorbus group are apomictic, producing viable seed without the need for pollination and fertilisation. Each time this hybrid cross occurs a new clone is effectively produced.
Smart showed by using physical characteristics that the species were separate and not a result of random variation. Some overlap does however occur and this suggests that some hybridising may occur between the two species.
A number of other Sorbus species have been produced in this way, such as the Devon Whitebeam, the Bristol Whitebeam, the Cheddar Whitebeam, Irish Whitebeam, Lancaster Whitebeam, etc. All are rare and require careful protection and expert habitat management if they are to survive in the wild.
In Scandinavia, particularly Norway, similar species have evolved following similar evolutionary pressures, but quite independently of the Arran Whitebeams.
Islands are well known as sites of endemic species. The Lundy cabbage (Coincya wrightii) is another British example, only growing on Lundy Island off the North Devon coast.
Read more about this topic: Arran Whitebeams
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