Hierarchy and clade definitions in Phylogenetic taxonomy

Abstract

The hierarchical organisation of biological entities has important nomenclatural implications. Because of the independence of reproductive events across different organisational levels, species are not (necessarily) clades of organisms, and organisms are not (necessarily) clades of cells: a surviving ancestral species is a paraphyletic assemblage of organisms, and a parental multicellular organism is a paraphyletic group of cells. Thus, clades of species might not be clades of organisms, and clades of organisms might not be clades of cells. Phylogenetic definitions of clade names must employ specifiers (analogous to ‘type taxa’) appropriate to the relevant hierarchical level: for a clade of individual organisms, the specifiers should be organisms, and for a clade of species, the types should be species. If specifiers of the wrong organisational level are used, the entities defined can be highly problematic. At least in sexually-reproducing taxa, definitions of higher taxa cannot circumvent the species problem by simply referring to specimens instead of species.