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Priority (biology)

In biological , the principle of is a fundamental rule that designates the valid scientific name of a as the oldest available name applied to it, provided that name was validly and effectively published and has not been invalidated or given way to a by international authority. This principle underpins the stability of taxonomic naming across disciplines, ensuring that names reflect historical precedence while allowing for revisions based on new evidence, and it applies to synonyms (different names for the same taxon) and homonyms (identical names for different taxa) by favoring the earliest legitimate usage. The origins of priority trace back to the 18th century with Carl Linnaeus's introduction of binomial nomenclature, where his Species Plantarum (1753) serves as the starting point for priority in plant names and his Systema Naturae (10th edition, 1758) for animals. Formalized in the 19th and 20th centuries through international agreements, it was enshrined in codes such as the International Code of Zoological Nomenclature (ICZN, first edition 1964, current 4th edition 1999) for animals and the International Code of Nomenclature for algae, fungi, and plants (ICN, evolving from the ICBN with the Madrid Code effective 2025) for plants, fungi, and algae. These codes limit strict application to promote nomenclatural stability, allowing exceptions like name conservation for widely used junior synonyms or suppression of senior names that cause confusion. In practice, priority resolves conflicts when taxa are merged, , or reclassified, requiring taxonomists to trace publication dates and validate descriptions—often in Latin or English under modern rules—to determine legitimacy. For instance, in cases of homonymy, the senior (older) name prevails for its original , rendering later identical names illegitimate and necessitating new epithets. While primarily applied at and levels, it extends to higher ranks with designated starting points, and tools have recently facilitated its enforcement by enabling rapid searches and registration of new names. Exceptions also exclude certain categories, such as cultivars, hybrids, and infrasubspecific names, to avoid unnecessary disruption in applied fields like and .

Historical Development

Origins in Botany

Before the establishment of systematic , plant naming practices were highly inconsistent, relying on long descriptive polynomials in Latin that varied widely among herbalists and scholars, with no formalized principle of priority to resolve conflicts. These names often combined multiple adjectives to describe a 's characteristics, leading to synonymous designations for the same species across different works and regions. For instance, Caspar Bauhin's Pinax Theatri Botanici (1623), a comprehensive index of approximately 6,000 , introduced proto-binomial naming by pairing generic and specific terms based on morphological similarities, but it did not enforce precedence based on publication date, allowing later authors to override earlier names without constraint. Carl Linnaeus revolutionized botanical naming with the publication of Species Plantarum on May 1, 1753, which systematically applied to nearly 6,000 plant species organized into about 1,000 under his sexual classification system. This work implicitly introduced a first-come, first-served approach by treating the names it provided as authoritative references, though Linnaeus himself did not articulate explicit rules for priority in cases of later synonyms. The format—consisting of a name followed by a specific , such as —provided a stable foundation that reduced ambiguity, but conflicts arose as subsequent botanists published alternative names without a binding mechanism to determine validity. Post-Linnaean efforts sought to formalize to promote nomenclatural , culminating in Alphonse de Candolle's de la Botanique (1867), adopted at the first in . These laws explicitly established as a core principle, stipulating that the valid name for a is the earliest one published after , with authorship cited to confirm precedence (Laws 3 and 4). De Candolle's framework addressed the proliferation of synonyms by prioritizing the oldest legitimate name, thereby predating similar formal codes in other disciplines by decades and laying the groundwork for international consensus. The principle of was further refined at the third in in 1905, where delegates adopted the International Rules of Botanical Nomenclature (published 1906), retroactively designating Linnaeus's (1753) as the universal starting point for botanical names, except in specific cases like fungi. This retroactive application ensured that applied consistently from that date onward, resolving ambiguities from earlier works and emphasizing publication date as the key criterion for validity. These rules marked the maturation of in , tracing its conceptual roots to Linnaeus while providing the first comprehensive, internationally binding code.

Emergence in Zoology

In the mid-18th century, zoological naming practices were marked by inconsistency despite the introduction of by in the 10th edition of Systema Naturae (1758), which applied the system to animals but did not enforce a strict principle of priority, resulting in multiple names for the same taxa and widespread synonymy. This lack of a chronological rule for name validity led to confusion, as subsequent authors often ignored earlier names in favor of their own descriptions. By the 19th century, efforts to reform zoological nomenclature gained momentum to address these issues, with the British Association for the Advancement of Science appointing a committee in 1842—chaired by Hugh Edwin Strickland—that proposed rules emphasizing priority to establish uniform naming based on the earliest valid publication. These rules, known as the Strickland Code (published 1843), marked an early formal push for priority as a stabilizing mechanism, though adoption remained inconsistent across and . Further proposals in the late 19th century, including discussions at international gatherings, built on this foundation, culminating in the 3rd International Congress of Zoology in (1895), where the (ICZN) was established and priority was formalized retroactively from Linnaeus's 1758 edition as the starting point for zoological names. This retroactive application aimed to resolve synonymy by prioritizing the oldest available name, with the ICZN tasked with overseeing implementation. Subsequent refinements strengthened the principle's role in promoting nomenclatural stability. At the 9th International Congress of Zoology in (1913), the ICZN was granted plenary powers to suspend strict priority in exceptional cases threatening stability, allowing flexibility while upholding the core rule. Post-World War II, the first full edition of the (1961) codified priority more comprehensively, followed by the fourth edition (1999), which further emphasized its application to ensure universality and resolve ongoing synonymic conflicts through international oversight. Zoology's priority starting point of 1 January 1758—five years after botany's adoption of 1753 in Linnaeus's —reflected these efforts to harmonize with botanical practices while addressing animal-specific naming challenges.

Core Principle

Definition and Objectives

In biological nomenclature, the principle of priority establishes that the earliest validly published name for a takes precedence over any later synonyms, thereby ensuring that each bears a single correct scientific name. This rule applies to and higher taxa, where valid necessitates a or along with the designation of a type specimen to anchor the name objectively. The principle serves as the foundational mechanism for resolving nomenclatural disputes by favoring chronological order over subjective judgments of merit or utility. The primary objectives of are to promote nomenclatural , minimize confusion arising from synonymy, and provide clear, objective criteria for selecting names that facilitate universal communication among taxonomists worldwide. By prioritizing the oldest available name unless invalidated or overridden, it prevents the proliferation of competing names that could disrupt established usage and . This approach addresses the historical chaos of pre-code eras, when unchecked naming led to rampant synonymy and inconsistent , thereby supporting consistent and reliable identification across global research. The principle is formally enshrined in the current edition of the International Code of Nomenclature for algae, fungi, and plants (ICN; Madrid Code, 2025) under Article 11 and in the (ICZN) under , where it operates as the default rule for name validation, subject only to specific exceptions for or . These codes emphasize that applies only among names deemed legitimate and available, reinforcing its role in maintaining an orderly and predictable system for biological classification.

Criteria for Establishing Priority

In biological nomenclature, establishing priority requires that a name be validly published according to the applicable code. Under the International Code of Nomenclature for algae, fungi, and (ICN), valid publication entails effective publication, meaning the work is made effectively and permanently available to the public (including via electronic means since 2012)—accompanied by a or (in Latin until 1 January 2012, thereafter in Latin or English), an indication of the type (such as a or lectotype), and the name properly formatted in Latinized form with correct rank indication. Similarly, the (ICZN) mandates availability through in a scientific work, including a or in any (post-1930), fixation of a type (e.g., for ), and adherence to spelling, though is recommended but not required for validity. These requirements ensure the name is objectively verifiable and tied to specific material evidence, such as specimens or preserved animals. The date of priority is the date of the earliest valid publication, with fixed starting points to standardize application across taxa. For , , and fungi under the ICN, priority generally commences from 1 May 1753, the publication date of Carl Linnaeus's (1st edition), though exceptions apply for certain families and fossils. In , the ICZN sets 1 January 1758 as the starting point, corresponding to the 10th edition of Linnaeus's , ensuring consistency while allowing pre-Linnaean names only if conserved. When multiple names are published on the same date, ties are resolved by internal order: in the ICN, by page number followed by line order within the page; in the ICZN, by the first reviser who explicitly selects among them in a subsequent publication. Additional elements for name validity include proper and avoidance of prohibited forms, reinforcing the objective basis of . Both codes require names to be Latinized, with specific epithets derived from personal names typically in (e.g., smithii for Smith); the ICN recommends obtaining consent before honoring living persons with an epithet to respect , though this is not mandatory for validity. Type material, such as a (a single specimen) or lectotype (subsequently designated from syntypes), must be explicitly indicated or implied, serving as the nomenclatural anchor regardless of taxonomic opinion. Prohibited names include obvious errors or those conflicting with code appendices (e.g., no epithets like nov. sp. in the ICN), ensuring clarity and preventing ambiguity. Specific rules further delineate , with taking precedence over it to maintain uniqueness. A —two identical names for different taxa—is illegitimate if , regardless of name's ; for instance, a later homonym is rejected even if the earlier one lacks a description, as seen in both codes. Tautonymy, where the genus and specific epithet are identical (e.g., Gorilla gorilla), is permitted without invalidating in under the ICZN, provided other criteria are met, while the ICN prohibits tautonyms, rendering them illegitimate and unavailable. These rules prioritize nomenclatural stability over descriptive quality. Fundamentally, operates objectively, hinging solely on and compliance, without regard to subjective assessments like the superiority of a or taxonomic judgment. This contrasts with subjective synonymy, where two names apply to the same based on interpretation, but only the senior (earliest) prevails unless conserved otherwise; for example, a later name with a more detailed does not supersede an earlier valid one. This mechanical approach minimizes disputes and promotes universal consistency in naming.

Operational Framework

Application in Botanical Nomenclature

In botanical nomenclature, the principle of priority is governed by the International Code of Nomenclature for , fungi, and plants (ICN; Code, 2025), particularly through Articles 11 and 12, which address the priority of legitimate names and their status. Article 11 establishes that the correct name for a at the of family to genus is the earliest legitimate name validly published at that , while for infrageneric taxa, it is the earliest legitimate epithet combined with the correct name of the genus or . Legitimacy under Article 12 requires that a name not be illegitimate due to factors such as tautonymy or later homonymy, ensuring only validly published names (meeting requirements like a description or diagnosis in Latin or English since 2012) compete for priority. The starting point for priority in most groups, including spermatophytes, pteridophytes, , and bryophytes below the family , is 1 May 1753, the publication date of Carl Linnaeus's (first edition), though names from his earlier works, such as Genera Plantarum (1737–1738), are excluded unless specifically conserved. The application of priority involves systematically searching for the earliest valid or that establishes a name, with subsequent names for the same in the same (congeneric) treated as synonyms if they lack . This process prioritizes names based on their date of valid publication, as determined by bibliographic records, and applies the general criteria of legitimacy and validity outlined elsewhere in the ICN. Unique to the ICN, orthographic variants—such as differences in spelling due to or minor errors—are generally treated as identical for purposes under Article 60, allowing corrections without affecting nomenclatural status, provided the intent is clear. Additionally, for genera named directly from specific s (e.g., forming a generic name like from a ), is assigned based on the publication date of the generic name itself, not the originating species . Since the 2011 , fungal nomenclature has been aligned under a "one , one name" system, where pleomorphic fungi (with multiple life stages) use a single name based on or , eliminating for anamorphs and teleomorphs effective from 30 July 2011. In practice, botanists use databases such as the International Plant Names Index (IPNI) to verify publication dates, validity, and legitimacy by cross-referencing original sources and bibliographic details. Disputes over priority, such as ambiguous publication dates or conflicting interpretations of validity, are resolved by specialist permanent committees (e.g., for vascular plants, bryophytes, fungi, or algae) under the oversight of the General Committee for Nomenclature, which evaluates evidence and issues rulings to maintain stability. A key mechanism to override strict priority for nomenclatural stability is the conservation of names listed in Appendix B, which requires a formal proposal to the relevant committee, approval by the General Committee, and ratification by the International Botanical Congress plenary session; conserved names then take precedence over earlier synonyms or homonyms.

Application in Zoological Nomenclature

In zoological nomenclature, the principle of priority is implemented through the (ICZN), which establishes the valid name of a as the oldest available name applied to it, dating from the starting point of 1 January 1758, corresponding to the publication of the tenth edition of Carl Linnaeus's . This rule, outlined in , ensures stability by prioritizing the earliest legitimate description while allowing for exceptions to promote nomenclatural consistency. Names published before 1931 are presumed available if they include a description or definition of the taxon and adhere to basic availability criteria under Article 11, reflecting the Code's recognition of historical publication practices with more lenient standards compared to later works. The process begins with identifying the earliest available name, which must meet criteria such as publication in a scientific work, consistent for species-group taxa, and non-violation of homonymy rules. For names published simultaneously, Article 24 assigns precedence to the name at the higher if applicable; otherwise, the first reviser—the initial subsequent author to meaningfully compare and select among the names—fixes the precedence to resolve ambiguities. A distinctive feature of the ICZN is the mandatory fixation of name-bearing types: for new species-group taxa proposed after 1999, a or syntypes must be explicitly designated, providing an objective reference for the taxon's application. Additionally, since 1930, names need not conform strictly to grammar or derivation, allowing terms from any language as long as they are spelled using the , which broadens accessibility while maintaining universality. Practical application involves consulting ZooBank, the ICZN's official online registry established in 2005, where new names in works published on or after 1 January 2012 must have the work registered to be considered available, facilitating verification of priority and publication details. In cases of ambiguity or conflict, taxonomists may seek rulings from the International Commission on Zoological Nomenclature (ICZN), which exercises plenary powers under Article 81 to suppress junior synonyms, senior homonyms, or entire works through formal Opinions, ensuring stability when strict priority would disrupt established usage. For instance, reversal of priority is permitted under Article 23.9 if a junior name has achieved prevailing usage—defined as its application as valid in at least 25 works by 10 authors over 10 years within the last 50 years, with the senior synonym unused as valid since 1899—designating the junior as nomen protectum and the senior as nomen oblitum. Such reversals require evidentiary citation and, if conditions are not fully met, Commission approval to avoid arbitrary changes. This framework emphasizes usage alongside date-based priority, distinguishing zoological from other nomenclatural codes by prioritizing practical stability in animal taxonomy.

Exceptions and Modifications

Conserved and Suppressed Names

In , conserved names (nomina conservanda) allow certain names to take precedence over earlier ones that would otherwise have under the International Code of Nomenclature for algae, fungi, and plants (ICN), thereby promoting stability when strict application of would cause disadvantageous nomenclatural changes. The conservation process in the ICN involves proposals published in the journal Taxon, reviewed by the General Committee on and relevant specialist committees, and ultimately approved by a two-thirds majority vote at the Section of an , with conserved names listed in Appendices II–IV for families, genera, and , respectively. For example, the genus name L. (for roses) was conserved against an earlier homotypic in 2005 to preserve its long-established usage despite lacking strict . In zoological nomenclature, the (ICZN) employs plenary powers to conserve names based on prevailing usage, overriding priority rules to maintain stability in cases where change would disrupt established literature or practice; these decisions are formalized through Opinions published in the Bulletin of Zoological Nomenclature. Since the ICZN's founding in 1895, over 1,000 such Opinions have been issued, many invoking plenary powers to place names on the Official Lists or suppress alternatives. Suppression mechanisms complement conservation by rendering certain names unavailable. In the ICZN, junior synonyms or senior names not in prevailing usage can be suppressed via plenary powers, as exemplified in various Opinions that declare specific works or names invalid for nomenclatural purposes. The ICN, by contrast, employs rejection for names deemed illegitimate (e.g., due to typification issues or later homonyms under Articles 52–54), effectively suppressing them without a separate plenary process, though conservation can also reject competing names. Criteria for overriding priority through conservation or suppression emphasize nomenclatural stability and practical utility, including long-established usage in , economic or significance of the , and the potential for disruption if priority were enforced. These overrides require formal or approval, ensuring rigorous evaluation, and collectively balance the objective application of priority with the need for a stable that supports biological research and communication.

Special Cases in Nomenclature Stability

In biological nomenclature, special cases arise where of is applied with modifications to prioritize nomenclatural stability, particularly for taxa. Under the International Code of Nomenclature for algae, fungi, and plants (ICN), names of taxa (except diatoms) compete for only with other names based on types representing the same organ or part of the , preventing recent extant names from displacing established nomenclature unnecessarily. Similarly, in the (ICZN), ichnotaxa—trace s such as footprints—do not compete in with body taxa, as their names are treated separately to avoid conflation between behavioral evidence and organismal remains. For pre-1931 publications, the ICZN relaxes type fixation requirements for -related names, allowing availability without explicit designation if a description exists, to accommodate historical works while maintaining stability. Hybrid names present another domain where priority interacts uniquely with stability. In botanical nomenclature, the ICN permits nothotaxa for hybrids, with priority determined by the earliest valid publication including parentage, but the subordinate International Code of Nomenclature for Cultivated Plants (ICNCP) modifies this for agricultural hybrids by establishing priority based on the date of registration with an International Cultivar Registration Authority (ICRA), rather than mere publication, to ensure consistent use in horticulture and prevent conflicts from unregistered names. For example, a hybrid cultivar like Rosa 'Peace' gains priority through its 1945 registration, overriding earlier unpublished descriptions. In contrast, the ICZN prohibits the establishment of species-group names for taxa of confirmed hybrid origin, treating such names as unavailable to avoid complicating priority among parental lineages, though formulaic designations (e.g., silvestris × catus) may be used informally. For and infraspecific taxa, is generally enforced, but concerns in often lead to overrides via . In the ICN, the Committee for Vascular Plants can conserve subspecies names against earlier synonyms if displacement would disrupt agricultural or economic usage, as seen with conserved infraspecific names in various crops. The ICZN similarly allows the Commission to rule on conserving subspecies names, balancing with practical needs in applied fields like veterinary science. International commissions play a pivotal role in resolving conflicts through ad hoc rulings to safeguard stability. The ICZN Commission, for instance, has issued opinions conserving names of endangered species to prevent nomenclatural changes that could hinder conservation efforts. Likewise, the ICN's General Committee can suppress names causing instability in lists like the , ensuring that priority does not inadvertently alter protected status designations. A notable recent development enhancing stability involves publications. In Declaration 46 (effective 2023, with ongoing implementation noted in 2024 bulletins), the ICZN amended Article 8 to affirm that subsequent retractions or disclaimers do not invalidate previously published works, thereby stabilizing dates for digitally published names and mitigating risks from post-publication alterations in online journals. This addresses challenges from electronic dissemination, where could otherwise be undermined by editorial changes.

Practical Examples

Botanical Illustrations

In , the application of is vividly illustrated through historical cases where earlier names are upheld or modified to balance stability and accuracy, as governed by the International Code of Nomenclature for , fungi, and (ICN). These examples highlight how conflicts arising from pre-Linnaean or early post-Linnaean publications are resolved, often through to prevent disruption in scientific communication and conservation efforts. The genus Banksia L.f. (Proteaceae), first published in 1782, exemplifies conservation overriding strict priority. An earlier homonym, Banksia J.R.Forst. & G.Forst. (1776), had been used for what is now the genus Pimelea Banks ex Gaertn. (Thymelaeaceae), based on New Zealand collections. To retain the widely used name honoring Sir Joseph Banks—who collected the first specimens during Cook's 1770 voyage—the 1782 name was conserved as a nomen conservandum under ICN Article 14. This decision resolved Linnaean conflicts, as Linnaeus filius initially placed Banksia in the class Tetrandria Monogynia, misaligning it with Proteaceae; 19th-century revisions by Robert Brown and others reclassified it correctly, solidifying the conserved name for over 170 Australian and New Guinean species. The species Rosa canina L. (1753) serves as a classic demonstration of in establishing types, directly applying ICN Article 11, which mandates that the correct name for a is the earliest legitimate one validly published. Designated as the for the genus L. (Rosaceae) to anchor its circumscription, R. canina—the dog rose—takes precedence over later synonyms such as Rosa sylvestris Scop. (1772) and Rosa lutetiana Lem. (1825), which described overlapping variants but were suppressed as illegitimate due to lacking . This ensures nomenclatural for the ~150–200 rose species, preventing confusion in , , and where R. canina represents the European wild type with pinnate leaves and pink flowers. Overrides via further illustrate flexibility in , as seen with the Phoenix L. (Arecaceae), encompassing the date palm P. dactylifera L. Published in 1753, it was conserved over the earlier but obscure and unused genus Dattus L. (1737), proposed for similar palm-like but lacking subsequent adoption. A 1905 proposal at the upheld this conservation under emerging rules (later formalized in ICN Article 14), prioritizing long-established usage in and —where Phoenix names date palms central to ancient economies—over rigid to avoid widespread renaming. Specific data underscore the scale of priority's impact: the (IPNI) records indicate that approximately 66% of published names are synonyms, with many resulting from resolutions that render later legitimate names superfluous. Under the "one , one name" principle adopted in 2011, mergers of teleomorph-anamorph taxa in the fungal genus P. Micheli ex Haller (Aspergillaceae) led to unified , suppressing dual names for like A. fumigatus Fresen. to enhance global fungal databases and clinical identification; a 2023 guideline further emphasized stability for medically important fungi, including those in Aspergillus.

Zoological Illustrations

In zoological , the principle of ensures that the name rex, established by in 1905, takes precedence over any potential earlier descriptions, as no senior synonyms have been confirmed since the 1758 baseline set by in . Although fragmentary material named Manospondylus gigas by in 1892 has been tentatively associated with , it remains a due to insufficient diagnostic features, preventing it from challenging the priority of T. rex. This case exemplifies how is applied strictly to adequately diagnosed taxa, stabilizing the for one of the most iconic species. A notable exception to strict priority involves the suppression of the senior homonym Phalangium Linnaeus, 1758, originally applied to harvestmen (), to conserve modern usage in arachnology. The (ICZN) exercised its plenary powers to suppress Phalangium for harvestmen, as the name had become a junior for a genus, allowing Opilio Fabricius, 1798, to prevail for the harvestman group and maintain nomenclatural stability. This ruling, documented in early ICZN opinions, prioritized widespread contemporary usage over chronological precedence to avoid widespread disruption in taxonomic literature. Another override of priority occurred with the genus Fallén, 1823, encompassing the fruit fly D. melanogaster Meigen, 1830, where the junior name was conserved under Article 23.9 of the ICZN Code for reasons of stability. In Opinion 2245 (2010), the ICZN reversed potential precedence of the senior genus Sophophora Latreille, 1829, affirming as the valid name due to its extensive use in over a century of genetic and evolutionary research, preventing taxonomic upheaval in a model organism. This application of the stability clause demonstrates how the ICZN balances priority with practical utility when a junior name has become entrenched in scientific communication. The ZooBank registry, the official ICZN database for zoological names, highlights the prevalence of synonymy in , with many entries involving multiple junior synonyms that underscore the challenges of establishing . Recent ICZN rulings, such as those addressing publications under the 2012 amendment to Articles 8, 9, 10, 21, and 78 of the , have clarified that electronic works registered in ZooBank before can establish , resolving disputes over "revived" names from digitized pre-2012 sources. For instance, in cases involving , these rules have prevented digital scans of obscure journals from upending established priorities, ensuring stability amid the surge in data. tools like ZooBank continue to aid in enforcing by enabling rapid verification of dates and name availability.