I'm not sure if I've ever had cause before to present my concept of the Evil Old Genus. The Evil Old Genus is one that has been used in the past to refer to a massively broader concept than it does currently, and so has been used to refer to many more species in the past than now. This makes dealing with the taxonomy of the genus a major headache, as one has to consider a whole host of now hidden or forgotten combinations. I can't say what would be the most evil of the Evil Old Genera out there, but a definite leader has to be the fern genus Polypodium. When the name was used by Linnaeus way back in the mid-1700s, Polypodium referred to nearly the whole gamut of ferns. Over time, as botanists have come to appreciate that all ferns are not the same, Polypodium has been progressively cut down. Still, it seems that if you go back into the taxonomy of nearly any fern, you'll come up against a 'Polypodium' sooner or later.
At present, Polypodium refers to a group of ferns with creeping, often scaly stems. It is the appearance of these stems that gives them their genus name, meaning 'many feet', as well as the common vernacular name of polypody. The circumscription of the genus can still vary somewhat between authors: some would include about 250 species in the genus, but Smith et al. (2006) restricted Polypodium to only about 30 species found primarily in temperate regions of the Northern Hemisphere, and in Central America. Many of these belong to what is known as the Polypodium vulgare complex. Recognised in the past as a single species Polypodium vulgare, this complex is now recognised as including a number of species found across Eurasia and North America. Ten of these are diploids, but another seven are polyploids. The polyploid species are believed to have originated from hybridisations between the diploid taxa; for instance, the Eurasian Polypodium vulgare sensu stricto is a tetraploid derived from a hybridisation between the diploid species P. glycyrrhiza and P. sibiricum (Sigel et al. 2014). Sigel et al. (2014), investigating the relationships between its diploid species, estimated an early Miocene origin for the P. vulgare complex. A fossil species from the early Oligocene, P. radonii, may belong to the complex or may be closely related (Kvaček 2001).
Distinguishing species of the P. vulgare complex is no easy task, often requiring evaluation of subtle differences in leaf or stem form, or close examination of sporangium morphology. Another feature that has been used in distinguishing Polypodium species, however, is taste: the stems of some species in the complex have distinctive flavours. The Eurasian P. vulgare has been used to impart its bittersweet flavour to confectionary, while the vernacular name of the licorice fern P. glycyrrhiza of North America and eastern Asia is fairly self-explanatory (but like licorice, does it also give you a good run for your money?) The key to Polypodium species in the Flora of North America contains the somewhat unexpected advice that "the reader is cautioned to taste clean rhizomes from uncontaminated soils". And honestly, who could argue with that?
Kvaček, Z. 2001. A new fossil species of Polypodium (Polypodiaceae) from the Oligocene of northern Bohemia (Czech Republic). Feddes Repertorium 112 (3-4): 159-177.
Sigel, E. M., M. D. Windham, C. H. Haufler & K. M. Pryer. 2014. Phylogeny, divergence time estimates, and phylogeography of the diploid species of the Polypodium vulgare complex (Polypodiaceae). Systematic Botany 39 (4): 1042-1055.
Smith, A. R., H.-P. Kreier, C. H. Haufler, T. A. Ranker & H. Schneider. 2006. Serpocaulon (Polypodiaceae), a new genus segregated from Polypodium. Taxon 55 (4): 919-930.