From coast to summit - Arran Whitebeams
Rowan Rock Whitebeam
The Rowan Sorbus aucuparia is a common tree and grows well in rocky places. It needs to be fertilised before it produceds seeds but usually it has abundant fruit. There are many trees in Arran and in the past there must have been more Rock Whitebeams too. The Rock Whitebeam Sorbus rupicola is less common and is often only found as one or more bushes on rocky ledges as it suffers severely from being grazed. It is apomictic which means it produces seed without being fertilised, although it still has pollen
When pollen from the Rock Whitebeam landed on Rowan a hybrid was produced. Because the Rock Whitebeam is apomictic the hybrid was also apomictic so it can reproduce and is a new species. It is called the Arran Whitebeam Sorbus arranensis. The new species looks more like the Rock Whitebeam but has lobes that are the beginning of leaflets Rowan Rock Whitebeam Arran Whitebeam
 
Rowan
+ Rock Whitebeam
= Arran Whitebeam
That was not the end of the story and the Arran Whitebeam (that produces good pollen) went on to hybridise again with the common Rowan. The result of this union is the Arran Service-tree Sorbus pseudofennica. This time the leaves are divided into distinct leaflets for at least the lower two pairs. There are hundreds of both new trees but in Scotland they are only found on Arran. Rowan Arran Whitebeam Arran Service-tree
 
Rowan
+ Arran Whitebeam = Arran Service-tree
There has not been no more hybridisation with the Rock Whitebeam as it is no longer found in the north of Arran where the Arran Whitebeams grow in steep-sided ravines. But a third species formed when a another cross occurred between Rowan and the Arran Service-tree giving the Catacol Whitebeam Sorbus pseudomeinichii. This has a larger leaflet at the tip. There is only a single tree on Arran. Rowan Arran Service-tree Catacol Whitebeam
Rowan + Arran Service-tree =Catacol Whitebeam