If I could turn back time… then I might understand how it came to be that Ruellia fulgens evolved as the sister taxon to a massive South American radiation of Ruellia: the Physiruellia clade. A minimum of 50 and as much as 100 species.
Ruellia fulgens was previously treated in the genus Polylychnis, which was separated from Ruellia by plants having shorter calyx lobes, weakly opened corollas, and dentate nectary discs. The former two features are unreasonable distinctions given the tremendous morphological diversity in Ruellia, and the latter has not been systematically surveyed in Ruellia such that nectary shape is more or less unknown throughout the genus. See Tripp (2007, Systematic Botany) for the taxonomic combination into Ruellia. Out you go, Polylychnis.
There is more appeal the sister group relationship of Ruellia fulgens to the rest of the Physiruellia clade than doing away with a terrible name such as Polylychnis. Ruellia fulgens is one of very few Acanthaceae (period!) that occurs in the Guiana Shield…. an area conspicuously lacking in Acanth diversity. I pay a lot of attention to Neotropical phylogenies, and numerous are the instances in which the sister taxon to some massive Andean or Amazonian radiation occurs in the Guianas. The Guiana Shield has long been hypothesized to represent a biological refuge during Pleistocene glaciations and other climatologically dynamic events.
There is at least some phylogenetic evidence to support the above hypothesis, including our beloved red-flowered Ruellia baby.
Wild collected, French Guiana, voucher unknown, photo by Carol Gracie (thank you Carol!)