I re-assured them that mountain ash = rowan = caorthann = Sorbus aucuparia was completely unrelated to regular ash = fuinseog = Fraxinus excelsior, soon to be of blessed memory like elms. It's easy, at a glance, to be confused: both trees share the feature of pinnate leaves [rowan shown R with defining saw-tooth edges to each leaflet]. When Linnaeus was putting naming-of-parts order on the natural world he used differences and similarities in the sexual apparatus to decide who was more closely related to whom. That method seemed to have utility because Linnaean classification is largely consonant with the DNA evidence on which all life's wonderful diversity ultimately depends. Similarities in leaves are, literally, superficial.
I was annoyed at myself that I couldn't, off the top of my head, tell our mates just how far distant the two tree species were. But, trained researcher me can defo find out. The definitive way would be to think of gene(s) which were present in all the relevant species, pull the protein sequences out of UniProt, align them with Clustalω, and tally up the differences, drawing a phylogenetic tree of trees. But the good enough way is to trawl through Wikipedia and clip out the taxonomic hierarchy for each species:
- Most of the trees we know and love, including rowan are roses! But ash, olive, holly and elder are daisies. That's as different as you can be and still be dicots.
- Rowan is [and a close look at the fruit will make obvious] closely related to apples and pears.
- ditto hawthorn Crataegus
- All the stone fruit cherries to plums are in the same genus Prunus.
- Horse chesnut Aesculus and sweet chestnut Castanea are not closely related.
- Acer - maples and sycamore - is in the same Order as Aesculus.
- Hickory, walnut and pecan are nutty cousins.
- Aspen and poplar = Populus are straight and tall willows.
- I've always believed that holly and oak were closely related. But I was misinformed. Quercus ilex - aka the holm, holly or evergreen oak - has a slightly pointy leaf margin [as R] like holly = Ilex but that is really the only point of similarity.