Relationships within "B." subg. "Isostylis" still remain unclear. Though Mast's studies found "B. cuneata" to be the most basal of the three species, a 2004 study of genetic divergence within the subgenus yielded both other possibilities: some analyses suggested "B. ilicifolia" as basal, while others suggested "B. oligantha". Further complicating the situation is the existence of a population of "B. cuneata" having both genetic and phenetic affinities with "B. oligantha". The origin of this population is unknown. It might have arisen through hybridisation, or it may be a transitional or even ancestral form. Finally, biogeographical factors suggest that "B. ilicifolia" would be the most basal of the three species: it occurs in the High Rainfall Zone where relictual species are most common, whereas the others are restricted to the Transitional Rainfall Zone, where more recently evolved species are most common.