# Combinatio nova

In biological taxonomy, a combinatio nova (abbreviated comb. nov. or n. comb.) refers to the formal renaming of an organism's scientific name when it is transferred to a different genus, reclassified within a different species, or its taxonomic rank is altered. Unlike the naming of a new species (species nova), a combinatio nova does not describe a previously unknown organism but reorganizes an existing name to reflect updated understanding of its relationships or classification. For example, when a species is moved to a new genus, its specific epithet is retained and combined with the new genus name, forming the new combination. This process ensures consistency and accuracy in naming while adhering to the rules established by nomenclature codes.
The concept of combinatio nova plays a vital role in maintaining the stability and traceability of scientific names as taxonomic classifications evolve. Creating a valid combinatio nova requires proper citation of the original name, known as the basionym, and compliance with strict publication standards. These rules differ between taxonomic groups, such as animals, plants and fungi, and bacteria, and are integral to preventing ambiguities in name usage. Superseded names, resulting from these changes, preserve the historical record of taxonomy and facilitate the integration of updated names into biological databases. This systematic approach supports the self-correcting nature of taxonomy, where scientific names are continually revised to reflect new discoveries and evidence.

## Overview

Combinatio nova (abbreviated as comb. nov. or n. comb.) is a taxonomic term used when creating a new scientific name based on a previously published legitimate name. The term literally means "new combination" in Latin and is essential in biological nomenclature when taxonomists need to transfer a species to a different genus based on new research indicating different relationships, change the rank of a taxon, or move an infraspecific taxon to a different species.
A combinatio nova consists of two key elements: the new name being proposed and its basionym (the original name on which the new combination is based). The basionym provides the final epithet, name, or stem of the new combination. For example, when transferring a species to a new genus, the specific epithet from the basionym is retained and combined with the new genus name. The new name supersedes the older one, which is then classified as a "superseded combination". Superseded combinations specifically denote cases where a species' name changes due to its transfer to a different genus or reclassification within the same hierarchical rank. This is distinct from synonyms or preoccupied names, which arise from different taxonomic circumstances.
This taxonomic procedure differs from describing a new species (species nova) or creating a replacement name (nomen novum). Unlike a new species description, a new combination does not require a new diagnostic description but must include a clear reference to its basionym. The validity of a new combination depends on proper citation of the basionym and adherence to the relevant nomenclatural codes.
Recent analyses of taxonomic practices in lichens have highlighted gaps in reporting for new combinations. To enhance clarity, best practices include explicitly stating whether the type material has been examined and including detailed ecological and distributional data in the publication. These steps ensure that new combinations remain well-grounded in taxonomic evidence.

## Nomenclatural stability and longevity

The stability of taxonomic names plays a critical role in ensuring effective communication and the reliability of biological databases. However, names often undergo revision due to improved understanding of relationships among taxa. Superseded combinations are one of the most common outcomes of these revisions, as species are reassigned to different genera or reclassified based on new evidence. Studies show that superseded combinations account for two-thirds to three-quarters of all unaccepted species names in some taxonomic groups, such as helminths.
The longevity of superseded combinations varies widely. On average, unaccepted species names persist in scientific use for about 29 years before being replaced, but this can range from just a few years to over a century. Older names proposed before the mid-20th century tend to have greater longevity, often remaining in use for decades before their invalidation. In contrast, names proposed more recently are revised or replaced more quickly, reflecting advances in molecular methods and taxonomic practices. This pattern highlights the self-correcting nature of taxonomy, where names are systematically revised to align with the latest scientific understanding. Superseded combinations, while no longer valid, are an important part of this process, preserving the historical record of taxonomic decisions and facilitating the traceability of name changes.
Many nomenclatural challenges can lead to uncertainty or a proliferation of invalid species names. Amid these difficulties, the concept of combinatio nova serves as a vital tool for preserving clarity. By ensuring that taxonomic revisions—such as transferring a species to a different genus—are formalized under established rules, combinatio nova prevents ambiguities and maintains a consistent historical record of name changes. This structured process allows researchers to trace the lineage of scientific names, understand their taxonomic context, and avoid confusion caused by informal or unsystematic name changes. Over time, the self-correcting nature of taxonomy, supported by combinatio nova, helps align nomenclature with evolving scientific understanding.

## Requirements for valid publication

The requirements for validly publishing a new combination have evolved significantly over time. Before 1 January 1953, a new combination could be established simply by adopting a previously published name or epithet for the same taxon, even without explicit reference to the basionym. Stricter standards were introduced after this date, requiring authors to clearly indicate the basionym and provide a complete, direct reference to its original publication, including the author, place of publication, page or plate reference, and date. These changes ensured clear links between new combinations and their original names, preventing ambiguities.
The importance of explicitly indicating new combinations was further emphasized in 1968, when the International Code of Nomenclature of Bacteria (ICNP) mandated that names "merely mentioned incidentally" without clear indication of their new status would not be valid. This requirement aimed to prevent confusion and ensure that readers could distinguish newly proposed names from older references.
Since 2007, explicit citation of the basionym has been mandatory. The basionym must meet specific rank requirements and cannot itself have a basionym. In cases where a name change is based on an illegitimate name, it is treated as a replacement name (nomen novum) rather than a combinatio nova. These requirements differ from those for new taxa, as new combinations do not require a new description or diagnosis but must adhere to citation and reference rules outlined in Article 33 of the nomenclatural code.
Across all fields governed by the ICN, ICNP, and ICZN, a new combination involves reassigning a species to a different genus or changing its rank while retaining the type of the original name (basionym). This principle is universal, though codes differ in implementation details. For example, the ICZN requires that the original authority's name be placed in parentheses to denote its relationship to the new combination. These shared rules ensure taxonomic stability, allowing revisions to reflect updated classifications while preserving historical and typological continuity.
In fungal taxonomy, including lichen-forming fungi, tools such as MycoBank provide registration systems to ensure valid publication and traceability of names. Similarly, databases like WoRMS integrate taxonomic records across groups, enhancing accessibility and consistency in nomenclature.
A key principle is that new combinations are only valid when made at identical ranks. Changes in rank, such as elevating a subspecies to species status or reducing a species to subspecies level, do not constitute new combinations and follow separate rules.

## Nomenclatural rules and restrictions

Taxonomic nomenclature follows specific rules governing how new combinations can be created and validated. A fundamental principle is that true new combinations can only be made between taxa of identical ranks. When taxonomic rank changes occur, such as elevating a subspecies to species status, these are denoted by the term status novus (stat. nov.), indicating a change in rank rather than a new combination.
For prokaryotic nomenclature, when changing ranks between species and subspecies, the epithet from the original name must be maintained unless this would create an illegitimate name. While the International Code of Nomenclature for algae, fungi, and plants (ICN) treats such rank changes as new combinations, there is ongoing debate about this treatment in prokaryotic nomenclature. Author citations in parentheses track the origin of reused epithets during rank or genus changes, with formatting rules differing between the botanical and zoological codes.
For rank changes, taxonomists must follow distinct protocols. The epithet from the original name must be maintained in the new designation unless this would create an illegitimate name. For example, when elevating a subspecies to species status, the subspecific epithet becomes the new specific epithet. Similarly, when reducing a species to subspecies rank, its specific epithet becomes the new subspecific epithet.
These rank-change procedures are governed by specific articles in various nomenclatural codes. In prokaryotic taxonomy, Rules 50a and 50b of the International Code of Nomenclature of Prokaryotes explicitly regulate the elevation of subspecies to species and the lowering of species to subspecies, respectively.
Although the primary goal of combinatio nova is to formalize name changes, the inclusion of ecological, distributional, and morphological data can significantly improve their utility. A 2021 study revealed that over 70% of new combinations lack such information, emphasizing the need for more comprehensive taxonomic descriptions. Modern taxonomic databases, such as the World Register of Marine Species, incorporate tools to manage combinatio nova by linking new combinations to original names. In this database, "superseded combination" is a standardized term used to describe names replaced by a combinatio nova. Such terminology helps ensure clarity in cases where species names are reassigned to new genera or taxonomic contexts.

## Types of new combinations

New combinations in taxonomy typically occur in three main situations, each serving a different taxonomic purpose:
* Generic reassignment: When research indicates a species belongs in a different genus, taxonomists create a new combination by transferring the species epithet to the new genus. The specific epithet remains unchanged while the genus name changes.
* Rank changes: When modifying a taxon's hierarchical position, such as elevating a subspecies to species status or changing a subgenus to genus level. When changing ranks between species and subspecies, the epithet from the original name must be maintained unless this would create an illegitimate name. For example, when elevating a subspecies to species status, the subspecific epithet becomes the new specific epithet. Similarly, when reducing a species to subspecies rank, its specific epithet becomes the new subspecific epithet.
* Infraspecific transfers: When moving varieties or subspecies between species while maintaining their infraspecific rank. The infraspecific epithet is preserved but associated with a different species name.

## Examples

In taxonomic practice, new combinations occur across different biological domains. In bacteriology, when Calymmatobacterium granulomatis was reassigned to a different genus, it became Klebsiella granulomatis comb. nov., maintaining its specific epithet while changing its genus placement. Other bacterial examples demonstrate rank changes: Bifidobacterium pseudolongum became B. pseudolongum subsp. pseudolongum when reduced to subspecies rank, while Lactobacillus bulgaricus was reclassified as L. delbrueckii subsp. bulgaricus when its taxonomic status changed.
In botanical taxonomy, new combinations often result from genus reassessments. For instance, Illiciaceae A.C.Sm. represents a new combination based on Illicieae DC., demonstrating the transfer of a taxon while preserving its taxonomic root. Similarly, Irvingiaceae Exell & Mendonça was established as a new combination from Irvingioideae Engl., showing how family-level taxonomy can be adjusted through new combinations. Under Article 7.3 of the ICN, a new combination or a name at a new rank must retain the type of its basionym, even if it was erroneously applied to a different taxon. For instance, Pinus mertensiana Bong. was transferred to the genus Tsuga by Carrière, who (based on his description) was actually referring to Tsuga heterophylla (Raf.) Sarg. Nonetheless, the combination Tsuga mertensiana (Bong.) Carrière remains permanently linked to the original basionym, Pinus mertensiana Bong. The citation "(Bong.)" in parentheses under Article 49 indicates that Bongard's name is the basionym, and thus determines the type for the new combination—even though Carrière mistakenly applied it to a different species.