The Pteridophyte Phylogeny Group (PPG) is an informal international group of systematic botanists who collaborate to establish a consensus on the classification of pteridophytes (lycophytes and ferns) that reflects knowledge about plant relationships discovered through phylogenetic studies. In 2016, the group published a classification for extant pteridophytes, termed "PPG I". The paper had 94 authors (26 principal and 68 additional).[1]
PPG I
A first classification, PPG I, was produced in 2016, covering only extant (living) pteridophytes. The classification was rank-based, using the ranks of class, subclass, order, suborder, family, subfamily and genus.[1]
Phylogeny
The classification was based on a consensus phylogeny, shown below to the level of order.[1]
The number of genera used in PPG I has proved controversial. PPG I uses 18 lycophyte and 319 fern genera.[1] The earlier system put forward by Smith et al. (2006) had suggested a range of 274 to 312 genera for ferns alone.[2] By contrast, the system of Christenhusz and Chase (2014) used 5 lycophyte and about 212 fern genera.[3] The number of fern genera was further reduced to 207 in a subsequent publication.[4]
The number of genera used in each of these two approaches has been defended by their proponents. Defending PPG I, Schuettpelz et al. (2018) argue that the larger number of genera is a result of "the gradual accumulation of new collections and new data" and hence "a greater appreciation of fern diversity and [..] an improved ability to distinguish taxa". They also argue that the number of species per genus in the PPG I system is already higher than in other groups of organisms (about 33 species per genus for ferns as opposed to about 22 species per genus for angiosperms) and that reducing the number of genera as Christenhusz and Chase propose yields the excessive number of about 50 species per genus for ferns.[5] In response, Christenhusz and Chase (2018) argue that the excessive splitting of genera destabilises the usage of names and will lead to greater instability in future, and that the highly split genera have few if any characters that can be used to recognize them, making identification difficult, even to generic level. They further argue that comparing numbers of species per genus in different groups is "fundamentally meaningless".[4]
^ abcdefPPG I (2016), "A community-derived classification for extant lycophytes and ferns", Journal of Systematics and Evolution, 54 (6): 563–603, doi:10.1111/jse.12229
^Smith, Alan R.; Pryer, Kathleen M.; Schuettpelz, Eric; Korall, Petra; Schneider, Harald & Wolf, Paul G. (2006), "A Classification for Extant Ferns", Taxon, 55 (3): 705–731, doi:10.2307/25065646, JSTOR25065646
^Christenhusz, Maarten J. M. & Chase, Mark W. (2014), "Trends and concepts in fern classification", Annals of Botany, 113 (4): 571–594, doi:10.1093/aob/mct299, PMC3936591, PMID24532607
^ abChristenhusz, Maarten J. M. & Chase, Mark W. (2018), "PPG recognises too many fern genera", Taxon, 67 (3): 481–487, doi:10.12705/673.2
^Schuettpelz, Eric; Rouhan, Germinal; Pryer, Kathleen M.; Rothfels, Carl J.; Prado, Jefferson; Sundue, Michael A.; Windham, Michael D.; Moran, Robbin C. & Smith, Alan R. (2018), "Are there too many fern genera?", Taxon, 67 (3): 473–480, doi:10.12705/673.1
