The unranked group "Isostylis", with its one species, was reclassified as a section in the 1856 arrangement of Carl Meissner, and 1870 arrangement of George Bentham. In his 1981 revision of the genus, Alex George reclassified the group as a subgenus—"Banksia" subg. "Isostylis"—defined by the dome-shaped flower heads, with "B. ilicifolia" joined by newly described species "B. cuneata" and later "B. oligantha". "Banksia ilicifolia" is the only common member of that subgenus; the two other species are rare and threatened, and are protected under the Environment Protection and Biodiversity Conservation Act 1999. Relationships between "B. ilicifolia" and the other members of "B." subg. "Isostylis" still remain unclear. Although DNA 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". 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.