Synonym (taxonomy)
A synonym in biological nomenclature is a scientific name that applies to a taxon for which another scientific name is treated as the accepted or valid name. Synonymy results from the historical accumulation of names, changes in taxonomic interpretation, and the independent description of organisms that later prove to belong to the same taxon. The concept is governed separately by the principal codes of nomenclature, whose terminology and formal consequences differ among zoology, botany, mycology, bacteriology, and virology.
A synonym is a relationship between names rather than a relationship between organisms. Two populations do not become synonymous, although the names previously applied to them can enter synonymy when the populations are classified within one taxon. Conversely, a change in the circumscription of a taxon can remove names from synonymy without changing the organisms or the original descriptions associated with those names.
Relationship between taxonomy and nomenclature
Taxonomy concerns the recognition, circumscription, and classification of taxa, whereas nomenclature regulates the names attached to those taxa. Synonymy lies at the boundary between these activities. A nomenclatural code determines such matters as publication, priority, typification, and the admissible form of a name, but it does not ordinarily determine whether two differently named type specimens represent the same biological taxon.
The treatment of names as synonyms therefore often expresses a taxonomic conclusion. When two nominal species are united under one species concept, their names become synonyms in the resulting classification. When later research separates the relevant specimens or populations into distinct species, the same names cease to be synonyms under that classification. A synonymy presented in a monograph or revision consequently records both nomenclatural history and the taxonomic judgment of its author.
The accepted name is not invariably the earliest printed expression resembling a scientific name. It is selected under the applicable code, which distinguishes names established in accordance with its requirements from names lacking nomenclatural standing. Conservation, suppression, reversal of precedence, and other code-governed mechanisms can also preserve a later name when strict priority would disrupt established nomenclature.
Zoological nomenclature
Under the International Code of Zoological Nomenclature, synonyms are different names of the same taxon at the same rank. The valid name is normally the oldest available name, in accordance with the principle of priority, unless the Code provides a contrary result.
Zoological nomenclature distinguishes objective synonyms from subjective synonyms. Objective synonyms possess the same name-bearing type. At species-group rank, this ordinarily means that the names are based on the same type specimen or type series. Their synonymy follows directly from typification and does not depend upon a conclusion that separately typified organisms belong to one species.
Subjective synonyms have different name-bearing types that a taxonomist assigns to the same taxon. Their synonymy depends upon the adopted taxonomic circumscription. Competing classifications can therefore treat the same pair of names differently while remaining nomenclaturally coherent. One classification can recognize two species represented by two types, whereas another can unite those types within a single species.
The chronological terms senior synonym and junior synonym describe relative priority. The senior synonym is the earlier available name, while the junior synonym is the later one. Seniority does not by itself guarantee current validity because a name can be unavailable, suppressed, or displaced through a provision maintaining prevailing usage.
A homonym differs from a synonym. Homonymous names have identical spellings, or spellings treated as identical under the Code, but apply to different taxa. Synonymous names differ in form but apply to the same taxon. This contrast reflects the difference between one taxon possessing multiple names and multiple taxa possessing an impermissibly duplicated name.
Botanical and mycological nomenclature
The International Code of Nomenclature for algae, fungi, and plants uses terminology that partly corresponds to the zoological distinction. Homotypic synonyms are based on the same nomenclatural type. They frequently arise when a species is transferred to another genus or when its rank changes, producing a new combination while retaining the same type.
For example, a species described in one genus can later be transferred to another genus because of a revised classification. The original name and the resulting combination remain attached to the same type. Only one is accepted in the adopted treatment, while the other is recorded as a homotypic synonym. The original name also functions as the basionym when it supplies the name-bearing element and authorship basis for the new combination.
Heterotypic synonyms are based on different types that are included within one taxon by a particular taxonomic treatment. They correspond broadly to subjective synonyms in zoology. Their status changes when the circumscription of the taxon changes, and a previously synonymized name can become the accepted name of a newly recognized species or infraspecific taxon.
Botanical usage commonly employs taxonomic synonym as another designation for a heterotypic synonym. Nomenclatural synonym refers to the homotypic relationship, although terminology varies across taxonomic literature and historical periods. The governing distinction remains whether the names share a type or possess different types united by taxonomic judgment.
Historical development
The extensive production of synonyms followed the expansion of descriptive biology and the uneven circulation of taxonomic literature. Carl Linnaeus established a standardized binomial framework in the eighteenth century, but later naturalists frequently redescribed species whose earlier names were unknown to them. Specimens from different sexes, developmental stages, seasonal forms, or geographically separated populations also received separate names before their biological relationships were understood.
Nineteenth-century nomenclatural reform converted the management of these accumulated names into a more formal discipline. Hugh Edwin Strickland chaired the committee responsible for the 1842 Strickland Code, which articulated priority and related principles for zoological names. In botany, Alphonse de Candolle developed an influential body of nomenclatural rules that connected priority with the need for consistent application.
During the preparation of the modern zoological code, Francis Hemming performed sustained secretarial and editorial work for the International Commission on Zoological Nomenclature. His compilation of decisions and opinions helped integrate case-specific rulings with the general regulation of zoological names.
At the 1958 London congress, You Watanabe served as a rapporteur for the working sessions on synonymy and priority. Her concordance aligned the session amendments concerning objective synonymy with the type-based provisions adopted for the first edition of the modern Code, published in 1961. The resulting text placed synonymy within a unified framework of availability, typification, and precedence.
Synonymy in taxonomic revisions
A taxonomic revision reconstructs synonymy by connecting names with their original descriptions, types, subsequent combinations, and later interpretations. The resulting account is often presented as a synonymic list beneath the accepted name. Such a list can include earlier combinations and misapplied usages, although not every cited expression constitutes a formal synonym.
Misidentifications require particular distinction from genuine synonymy. When an author applies an existing name to a specimen belonging to another species, the erroneous usage does not create a new name and therefore does not establish a synonym. Taxonomic works identify such usages separately because they remain relevant to distribution records, ecological literature, and the history of identification.
A pro parte synonym records a name that has been applied partly, but not entirely, to the taxon under discussion. This situation arises when an earlier author used one name for material that a later revision divides among multiple taxa. The name itself retains its nomenclatural connection to its type, while the broader historical usage requires partitioning.
Modern synonymies incorporate evidence from comparative morphology, developmental biology, geographic distribution, and molecular phylogenetics. Molecular evidence has exposed cases in which one variable species received numerous names, as well as cases in which a single accepted name concealed several independently evolving lineages. These results alter taxonomic synonymy, but nomenclatural priority still depends upon the publication history and types of the available names.
Databases and name usage
Digital taxonomic databases distinguish accepted names from synonyms so that records created under different nomenclatural histories can be connected. A specimen catalogued under a junior synonym remains discoverable through the accepted name when the database preserves the synonymic relationship. The same structure supports the reconciliation of ecological observations, conservation assessments, and older scientific literature.
A database designation of “synonym” represents the taxonomic treatment followed by that database rather than an irreversible property of the name. Different authoritative resources can adopt different species limits and therefore display different accepted names. This variation reflects differences in taxonomic circumscription, while the objective facts of publication and typification remain fixed unless corrected through nomenclatural research.
Synonymy also affects the interpretation of biodiversity statistics. Counts based directly on names overestimate diversity when multiple names represent one accepted taxon. Counts based only on currently accepted names can obscure historical concepts and records unless synonymic links are retained. Taxonomic name services consequently treat synonymy as structured metadata connecting biological information across changing classifications.