Subgenera of genus Betula (birch), are; Bark on twigs rich in methyl salicylate (oil of wintergreen).
The above (incomplete) list was compiled from the references cited below.
Birches will hybridise very freely, particularly in cultivation but also in the wild where conditions and species present permit.
While differing chromosome number (diploid, tetraploid, etc.)
Many botanists regard differing chromosome number as a specific discriminant, though not all do so (e.g. some include B. cordifolia and B. neoalaskana as varieties within B. papyrifera).