The use of ranks and nominal-series in zoological nomenclature has recently been challenged by some authors who support unranked systems of nomenclature. It is here shown that this criticism is based on a double misunderstanding: (1) the confusion between nomenclatural ranks and taxonomic categories; (2) the request for a monosemic nomenclatural system, not for scientific reasons, but to please non-taxonomists, especially customers of the web. It is here argued that nomenclatural ranks and taxonomic categories should be clearly distinguished and designated by different terms, and that the Code should be modified in order to make this distinction clear. Whereas taxonomic categories have biological definitions, nomenclatural ranks do not, as they express only a position in a taxonomic hierarchy. If used consistently (which is not always the case), the system of nomenclatural ranks is very useful for the storage and retrieval of taxonomic and phylogenetic information. Taxa referred to a given rank in different groups cannot therefore be considered equivalent by any criterion, so that using ranks for comparisons between taxa (e.g., for biodiversity richness assessment) is irrelevant and misleading. Although the current Code needs to be improved in several respects, the superiority of this nomenclatural system, which is theory-free regarding taxonomy as it relies on ostensional allocation of nomina to taxa rather than on intensional definitions of nomina, is again stressed. It is suggested that all taxonomists should follow the Code for the allocation and validity of nomina, whatever taxonomic theory they favour, and in particular whatever kinds of definitions or diagnoses they wish to use for taxa. This would avoid the considerable loss of manpower, time and energy that would be required by the implementation of a new nomenclatural system (e.g., in order to require “phylogenetic definitions” for nomina, or to make nomenclature fully monosemic), and the confusion that would result for most users of nomina. The new paradigm imposed to biology by the combination of the taxonomic impediment and of the biodiversity crisis requires from taxonomists, who are already considerably much less numerous than required by this new situation, to concentrate on what should be their priority at the beginning of the century of extinctions, namely the inventory of the living species of our planet before they get extinct.

The term species by itself is vague because it refers to the species concept, the species category and the species taxon, all of which are distinct although related to one another. The species concept is not primarily a part of systematics, but has always been an integral part of basic biological theory, It is based on evolutionary theory and applies only to sexually reproducing organisms. The species concept and the phyletic lineage concept are quite distinct although they are related to one another. The important aspect of the species concept is lack of gene flow between different species, and hence the defining criterion of the species is genetic isolation. The species concept is often considered as non-dimensional, both in time and space. Species possess three different major properties, namely genetic isolation, reproductive isolation and ecological isolation; these properties evolve at different times and under the effect of different causes during the speciation process. Speciation requires an external isolating barrier during the initial allopatric phase in which genetic isolation evolves and must reach 100% efficiency. The subsequent sympatric phase of speciation occurs after the disappearance of the external isolating barrier when members of the two newly evolved species can interact with one another and exert mutual selective demands on one another. Much of the reproductive and ecological isolation evolves during this secondary sympatric phase. The species category is a rank in the taxonomic hierarchy and serves as the basis on which the diversity of organisms is described; it is not the same as the species concept. The species category applied to all organisms, sexually and asexually reproducing. The species taxon is the practical application of the species category in systematics with the recognition of species taxa requiring many arbitrary decisions. No single set of rules exist by which the species category can be applied to all organisms. Recognition of species taxa in asexually reproducing organisms is based on amount of variation and gaps in the variation of phenotypic features associated with ecological attributes of these organisms as compared with similar attributes in sympatric species taxa of sexually reproducing organisms. Species taxa are multidimensional in that they exist over space–time and often have fuzzy borders. Because recognition of species taxa, including those in sexually reproducing organisms, depends on many arbitrary decisions especially when dealing with broad geographical and temporal ranges, species taxa cannot be used as the foundation for developing and testing theoretical concepts in evolutionary theory which can only be done with the non-dimensional species concept. Zusammenfassung Der Begriff ‘Art’ ist in sich selbst unscharf, weil er sich auf die Art als Konzept, als Kategorie und als Taxon bezieht, die alle voneinander unterschiedlich sind, wenngleich sie auch untereinander in Verbindung stehen. Das Artkonzept ist nicht primar ein Teil der Systematik, aber es ist immer ein integraler Teil der Grundlagen der theoretischen Biologie gewesen. Es basiert auf der Evolutionstheorie und ist nur bei sich sexuell reproduzierenden Organismen anwendbar. Das Artkonzept und das Konzept der phyletischen Linien sind sehr unterschiedlich, obgleich auch sie miteinander verknupft sind. Ein wichtiger Aspekt des Artkonzepts ist der fehlende Genflus zwischen verschiedenen Spezies und das Entscheidungskriterium ist daher die genetische Isolation. Das Artkonzept wird oft als dimensionslos, sowohl in der Zeit als auch im Raum, angesehen. Spezies haben drei verschiedene bedeutende Eigenschaften, namlich die genetische Isolation, die reproduktive Dimension und die okologische Isolation; diese Eigenschaften evolvieren zu verschiedenen Zeiten und unter dem Einflus verschiedener Ursachen wahrend des Artbildungsprozesses. Die Artbildung braucht eine externe isolierende Barriere wahrend der anfanglichen

In biological taxonomy, a taxonomic rank denotes the level that a group of organisms—either taxon or clade—occupies in a hierarchical system of classification In biological taxonomy, a taxonomic rank denotes the level that a group of organisms—either taxon or clade—occupies in a hierarchical system of classification, which is based on evolutionary relationships. Some authors prefer to use the term nomenclatural rank, contending that, according to some definitions, the ranking of organisms is more accurately described under nomenclature rather than that In…

(software) Semantic network Semantic similarity network Structuralism Systematics Taxon, a population of organisms that a taxonomist adjudges to be a unit Taxonomy is a practice and science concerned with classification or categorization. Typically, there are two parts to it: the development of an underlying scheme of classes (a taxonomy) and the allocation of things to the classes (classification). Originally, taxonomy referred only to the classification of organisms on the basis of shared characteristics. Today it also has a more general sense. It Alpha taxonomy, the description and basic classification of new species, subspecies, and other taxa Linnaean taxonomy, the original classification scheme of Carl Linnaeus rank-based scientific classification as opposed to clade-based classification Evolutionary taxonomy, traditional post-Darwinian hierarchical biological classification Numerical taxonomy, various taxonomic methods employing numeric algorithms Phenetics, system for ordering species based on overall similarity Phylogenetics, biological taxonomy based on putative ancestral descent of organisms Plant taxonomy Virus classification, taxonomic system for viruses Folk taxonomy, description and organization, by individuals or groups, of their own environments Nosology, classification of diseases Soil classification, systematic categorization of soils