| Abstract: | A new procedure for defining taxa upon a single character state is developed. It is centred on the designation of two specimens, belonging to two distinct species, exhibiting the same given character state, as type material, and referred to as ‘cladotypes’. A taxon name/definition designates a monophyletic group until one of the following assumptions is falsified: (1) the character state typified by cladotypes is homologous in individuals that are designated as cladotypes, and (2) cohesion mechanisms isolated individuals exhibiting the type‐character‐state from those that do not. A taxon defined by a character state that is found to be a combination of several character states is to be redefined upon a character state shared by its cladotypes. If several character states are available, the character state that makes the taxon the least inclusive taxon including cladotypic species (i.e., species to which belong cladotypes) is to be preferred. Taxon names designate obsolete phylogenetic hypotheses if the first assumption is falsified (such names are to be kept for this purpose, i.e., they are not to be recycled in another definition). Rules governing adaptation of previously erected names are proposed. Main cases of taxa synonymy involve definitions based on different pairs of cladotypes but referring to the same type‐character‐state; and definitions based on the same character‐state initially hypothesised as acquired by convergence in cladotypic species pairs, but later demonstrated as originating from a unique ancestor. Taxa could be synonyms if a permanent splitting event did not segregate individuals exhibiting a new character state, qualified as type‐character‐state, from individuals already assigned to a previously erected taxon. This procedure accommodates potentially any species concept, but is not tied to any; it is an extension of the composite species concept. Species are treated in a different way than other taxa: they are defined as sets of individuals belonging to the same evolving (segments of) metapopulation lineages as a holotype specimen, and do not need a defining character state. |
