Species concept and guidelines for infraspecific taxonomic ranking in Dactylorhiza (Orchidaceae)

In order to prepare for a biosystematic treatment of Dactylorhiza , a search is made for applicable taxonomic concepts (species, subspecies, variety)–aiming at a hierarchical system in which a unique kind of biological unity applies to each level in the hierarchy. The degree of reproductive isolation between D. purpurella s.str. and each of D. maculata s.str., D. majalis s.str., and D. purpurella ssp. majaliformis (delimitation and nomenclature according to Løjtnant 1993) is assessed from morphometric data. These three cases represent three putative levels in a phylogenetic hierarchy. The results are compared to previously published data on corresponding groups of taxa (mainly from northern Europe), and a positive correlation is found between the putative level of phylogenetic relationship and the degree of reproductive isolation within the genus. Furthermore, taxa defined on each level are found to be morphologically recognizable. The three empirically defined categories are matched with general species concepts from the literature, subsequently leading to recommendation of the following taxonomic concepts for Dactylorhiza (slightly shortened): (1) As “species” are designated taxa complying with the biological species concept in a modern, botani‐cally focused sense; (2) as “subspecies” are designated taxa complying with the ecological, but not with the biological species concept; (3) as “varieties” are designated taxa complying with the phenetic, but neither with the biological nor the ecological species concept. In a postscript the taxonomic consequences for the taxa specifically examined in the case studies are briefly sketched, and reference is made to a forthcoming revision of the “purpurella complex”.