A revised phylogenetic classification for Viola (Violaceae)

The genus <em>Viola</em> (Violaceae) is among the 40–50 largest genera among angiosperms, yet its taxonomy has not been revised for nearly a century. In the most recent revision, by Wilhelm Becker in 1925, the then known 400 species were distributed among 14 sections and numerous unranked groups. Here we provide an updated, comprehensive classification of the genus, based on data from phylogeny, morphology, chromosome counts, and ploidy, and based on modern principles of monophyly. The revision is presented as an annotated global checklist of accepted species of <em>Viola</em>, an updated multigene phylogenetic network and an ITS phylogeny with denser taxon sampling, a brief summary of the taxonomic changes from Becker’s classification and their justification, a morphological binary key to the accepted subgenera, sections and subsections, and an account of each infrageneric subdivision with justifications for delimitation and rank including a description, a list of apomorphies, molecular phylogenies where possible or relevant, a distribution map, and a list of included species. We distribute the 664 species accepted by us into 2 subgenera, 31 sections, and 20 subsections. We erect one new subgenus of <em>Viola</em> (subg. <em>Neoandinium</em>, a replacement name for the illegitimate subg. <em>Andinium</em>), six new sections (sect. <em>Abyssinium</em>, sect. <em>Himalayum</em>, sect. <em>Melvio</em>, sect. <em>Nematocaulon</em>, sect. <em>Spathulidium</em>, sect. <em>Xanthidium</em>), and seven new subsections (subsect. <em>Australasiaticae</em>, subsect. <em>Bulbosae</em>, subsect. <em>Clausenianae</em>, subsect. <em>Cleistogamae</em>, subsect. <em>Dispares</em>, subsect. <em>Formosanae</em>, subsect. <em>Pseudorupestres</em>). Evolution within the genus is discussed in light of biogeography, fossil record, morphology, and particular traits. <em>Viola</em> is among very few temperate and widespread genera that originated in South America. The biggest identified knowledge gaps for <em>Viola</em> concern the South American taxa, for which basic knowledge from phylogeny, chromosome counts, and fossil data is virtually absent. <em>Viola</em> has also never been subject to comprehensive anatomical study. Study on seed anatomy and morphology is required to understand the fossil record of the genus.