Syst. Biol. 48(4):790–807, 1999
Species Names in Phylogenetic Nomenclature
PHILIP D. CANTINO,1* HAROLD N. BRYANT,2 KEVIN DE QUEIROZ,3 MICHAEL J. DONOGHUE,4 TORSTEN ERIKSSON,5 DAVID M. HILLIS,6 AND MICHAEL S. Y. LEE7 1Department of Environmental and Plant Biology, Ohio University, Athens, Ohio 45701, USA; E-mail: [email protected] 2Royal Saskatchewan Museum, 2340 Albert Street, Regina, Saskatchewan S4P 3V7, Canada; E-mail: [email protected] 3Department of Vertebrate Zoology, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, USA; E-mail: [email protected] 4Harvard University Herbaria, 22 Divinity Ave., Cambridge, Massachusetts 02138, USA; E-mail: [email protected] 5Bergius Foundation, Royal Swedish Academy of Sciences, Box 50017, 104 05 Stockholm, Sweden; E-mail: [email protected] 6Section of Integrative Biology, School of Biological Sciences, University of Texas, Austin, Texas 78712, USA; E-mail: [email protected] 7Department of Zoology, University of Queensland, Brisbane, Queensland 4072, Australia; E-mail: [email protected]
Abstract.—Linnaean binomial nomenclature is logically incompatible with the phylogenetic nomenclature of de Queiroz and Gauthier (1992, Annu. Rev. Ecol. Syst. 23:449–480): The former is based on the concept of genus, thus making this rank mandatory, while the latter is based on phylo- genetic definitions and requires the abandonment of mandatory ranks. Thus, if species are to re- ceive names under phylogenetic nomenclature, a different method must be devised to name them. Here, 13 methods for naming species in the context of phylogenetic nomenclature are contrasted with each other and with Linnaean binomials. Afundamental dichotomy among the proposed methods distinguishes those that retain the entire binomial of a preexisting species name from those that retain only the specific epithet. Other relevant issues include the stability, uniqueness, and ease of pronunciation of species names; their capacity to convey phylogenetic information; and the distinguishability of species names that are governed by a code of phylogenetic nomenclature both from clade names and from species names governed by the current codes. No method is ideal. Each has advantages and drawbacks, and preference for one option over another will be influenced by one’s evaluation of the relative importance of the pros and cons for each. Moreover, sometimes the same feature is viewed as an advantage by some and a drawback by others. Nevertheless, all of the proposed methods for naming species in the context of phylogenetic nomenclature provide names that are more stable than Linnaean binomials. {Phylogenetic nomenclature; species names; binomial nomenclature.}
Phylogenetic nomenclature (de Queiroz nized as a species (de Queiroz, 1998, 1999). and Gauthier, 1992, 1994) is a system of bio- Although biologists disagree about which logical nomenclature in which taxon names lineages to recognize as species, we suspect are explicitly applied to evolutionary enti- that most would agree that species are ties by means of phylogenetic definitions. “fundamental units for organizing knowl- In this system, the categories “species” and edge of biodiversity” (Baum, 1998) and, as “clade” are not ranks but different kinds of such, require names. It is not our intent to entities. We consider a clade to be a mono- add to the vast literature on species con- phyletic group of species (de Queiroz and cepts (e.g., Ereshefsky, 1992; Claridge et al., Donoghue, 1990; de Queiroz, 1999). A 1997; Howard and Berlocher, 1998; Wilson, species, in its broadest conception, is a seg- 1999). Rather, we address here the form that ment of a population-level lineage, but species names should take in a system of views vary as to which criteria (e.g., poten- phylogenetic nomenclature and consider tial interbreeding, diagnosability, exclusiv- implications regarding their stability , de- ity) should be used to determine whether a gree of ambiguity , ease of pronunciation, particular lineage is to be formally recog- and potential to convey information (or misinformation) about relationships. *All authors except the first are listed alphabeti- The literature on phylogenetic nomencla- cally. Address correspondence to this author. ture includes extensive discussion of the is- 790 1999 CANTINO ET AL.—SPECIES NAMES IN PHYLOGENETIC NOMENCLATURE 791 sues surrounding the naming of clades the specific epithet sometimes must change (e.g., de Queiroz and Gauthier, 1990, 1992, as well, either because it duplicates a spe- 1994; Minelli, 1991, 1995; Rowe and Gau- cific epithet under the new genus (homo- thier, 1992; Sundberg and Pleijel, 1994; nymy) or because it no longer matches the Bryant, 1994, 1996, 1997; Schander and gender of the new genus in the case of ad- Thollesson, 1995; Lee, 1996a,b, 1998, 1999; jectival epithets. Another drawback of Lin- Wyss and Meng, 1996; Cantino et al., 1997; naean binomial nomenclature is that it en- Crane and Kenrick, 1997; de Queiroz, 1997; courages the creation of monotypic and Kron, 1997; Baum et al., 1998; Härlin, 1998; paraphyletic genera in situations in which Hibbett and Donoghue, 1998; Moore, 1998; relationships among the species in a com- Schander, 1998a; Sereno, 1999), but rela- plex of genera are poorly resolved (dis- tively little has been written about the nam- cussed below). ing of species in this system (de Queiroz Aformal code of phylogenetic nomencla- and Gauthier, 1992; Graybeal, 1995; Schan- ture (the “PhyloCode”) is in preparation. It der and Thollesson, 1995; Cantino, 1998; is being designed so that it can be used con- Schander, 1998b). This may in part be due currently with the traditional system em- to a tacit assumption that there would be no bodied in the existing codes (i.e., Interna- fundamental difference between phyloge- tional Code of Botanical Nomenclature netic and traditional nomenclature in the {ICBN}, International Code of Zoological application of species names (i.e., based on Nomenclature {ICZN}, and the Bacteriolog- nomenclatural types and lacking explicit ical Code {BC}) or similar codes that might phylogenetic definitions), whereas the rules be adopted in the future (e.g., the draft governing the application of supraspecific BioCode: Greuter et al., 1998). In conjunc- taxon names in the two systems are very tion with the phylogenetic code, plans are different. There is no reason why species being developed for an Internet-accessible names could not have phylogenetic defini- database in which all names governed by tions in phylogenetic nomenclature, as the new code would be registered. This clade names do, but the theory underlying would provide an easy means of cross-ref- the application of such definitions to erencing names under different codes, facil- species names has not yet been developed. itate access to the relevant nomenclatural Although the manner in which species literature, and prevent accidental creation names would be applied in phylogenetic of homonyms under the phylogenetic code. nomenclature does not necessarily differ The development of the PhyloCode was from that in the traditional system, the form initiated in preparation for, and discussed that they take in the traditional system— and elaborated at, a workshop that took Linnaean binomials— is incompatible with place at the Harvard University Herbaria in phylogenetic nomenclature. The use of Lin- August 1998 and was attended by 27 peo- naean binomials makes the genus a manda- ple from five countries (see Acknowledg- tory rank, whereas a basic tenet of phyloge- ments). Of the 20 issues on the agenda at netic nomenclature is abandonment of the Harvard workshop, the one that proved mandatory ranks (de Queiroz and Gau- most contentious was the form that species thier, 1992). Linnaean binomials not only names should take. No decision was are logically incompatible with phyloge- reached at the workshop, and the partici- netic nomenclature (Griffiths, 1976; de pants debated the issue intensively during Queiroz and Gauthier, 1992), they also have the next five months in an Internet discus- several practical drawbacks (Cain, 1959; sion group. When it became clear that no Michener, 1964; Cantino, 1998). The most consensus would be reached, it was de- serious of these is instability of species cided to restrict the initial version of the names. Every change in generic limits, PhyloCode to the naming of clades. We re- whether based on phenetic criteria or new main committed to the development of a phylogenetic evidence, necessitates changes parallel set of rules for species names, but in species names. At the least, the genus we think this should be delayed until there portion of the binomial must be altered, but has been an opportunity for the systematics 792 SYSTEMATIC BIOLOGY VOL. 48 community to discuss the issues involved. newly recognized species will require a This article is intended to initiate that dis- description or diagnosis as well as a type cussion. designation. In this article, we describe and contrast Thirteen proposed methods for naming the methods that have been proposed for species in the context of phylogenetic no- naming species in the context of phyloge- menclature are briefly characterized in the netic nomenclature. Many of these methods form of a key in Table 1 and are described in arose in the Internet discussion among the detail in Table 2. Other variants that differ workshop participants and have not previ- in minor ways from the options covered ously been published. Although Linnaean here were considered in the Internet discus- binomial nomenclature is incompatible sion that led to this article but had little with other aspects of phylogenetic nomen- support. The key in Table 1 could have been clature and is not under consideration for organized in various ways. Our choice of adoption in the PhyloCode, it is included properties to include in the early couplets here for comparative purposes. emphasizes what we consider the most fun- damental distinctions among the methods: whether converted names are based on an PROPOSED METHODS FOR NAMING SPECIES entire preexisting binomial or on the spe- In comparing the methods that might be cific epithet alone; and whether species used to name species in a system of phylo- names are fixed or may be changed in re- genetic nomenclature, one must keep in sponse to new information about phy- mind the relationship between the Phy- logeny. Other distinctions among the meth- loCode and the current codes (i.e., the ods include (1) whether species names are ICBN, ICZN, and BC), referred to hence- unique; (2) whether species names are dis- forth as the preexisting codes. Because the tinguishable from clade names; (3) whether PhyloCode is still in preparation, the infor- converted names (or the converted name mation in this paragraph must be viewed as preceded by the name of a clade that in- provisional. The PhyloCode is being de- cludes the species) are identical in form to signed so that it can be used concurrently Linnaean binomials; (4) whether converted with the preexisting codes. Minimizing the species names are identical in form to the disruption of the preexisting nomenclature names of newly recognized species; (5) is a priority. When a previously recognized whether species names begin with a capital species is named under the PhyloCode, its letter or a lower-case letter; (6) whether name will be based on the preexisting name species names contain numbers; and (7) that is considered to be correct (ICBN, BC) whether species names contain nonal- or valid (ICZN). This process is called con- phanumeric symbols (e.g., hyphens or version, and the name under the PhyloCode dots). is the converted name. Conversion is the act The proposed methods can be divided of establishing a name as governed by the into three groups, based on the primary dis- rules of the PhyloCode, and thus formaliz- tinctions described above. In groups I and ing its independence from the concept of II (methods A–J), converted species names genus. If, at the time of conversion, there is are derived from binomials (i.e., retain both disagreement in the current literature as to parts of the preexisting name). In contrast, which name is correct under the preexist- converted species names in group III ing code that governs it, the one that is (methods K–M) are derived from preexist- mostwidely used should be chosen. The ing specific epithets (i.e., do not retain the process of conversion is analogous to pub- genus portion of the preexisting name). The lishing a new combination. Thus, a clear binomial-based methods differ in their de- reference to the author and place of publi- gree of stability. In group I (methods A–H), cation of the preexisting name will be re- species names will not change as a result of quired, but a description or diagnosis will new information about phylogeny . Thus, not. The converted name will retain the although these names are derived from bi- same type as the preexisting name on nomials, they function like uninomials. In which it is based. Publication of names for group II (methods I and J), a name must be 1999 CANTINO ET AL.—SPECIES NAMES IN PHYLOGENETIC NOMENCLATURE 793
TABLE 1.Outline of the proposed methods for naming species in phylogenetic nomenclature, organized as a dichotomous key. Aconverted name is a name established under the PhyloCode and derived from a preexisting Linnaean binomial. Anew name is a name established under the PhyloCode for a species that has no preexisting Linnaean binomial (i.e., a newly recognized species).
1.Converted names are based on the full preexisting binomial; new names may be binomial or uninomial in form, depending on the method. 2.Names do not change with new knowledge of phylogeny. 3.All names consist of two parts, although not necessarily two words (a word being a string of letters that means something by itself), separated by a space, hyphen, or dot. 4.Names are capitalized and their two parts are separated by a blank space, thus they are identical in form to Linnaean binomials. Method A 49 .Names are distinguishable from Linnaean binomials, either by beginning with a lower- case letter or by separation of their two parts by a hyphen or dot. 5.Genus names may not be established as clade names. Method C 59 .Genus names may be established as clade names. 6.New names must consist of two words. Method B 69 .New names may consist of either two words connected by a dot or one word with a dot somewhere within it. Method F 39 .At least some names consist of a single unbroken string of letters. 7.Converted names are binomial in form; new names are uninomial. 8.Names are capitalized. Method D 89 .Names are not capitalized. Method E 79 .All names are uninomial in form; converted names are formed by the fusion of the two parts of the Linnaean binomial on which they are based. 9.Names are capitalized. Method G 99 .Names are not capitalized. Method H 29 .Names sometimes change with new knowledge of phylogeny. 10.Names are capitalized and their two parts are separated by a blank space, thus they are identical in form to Linnaean binomials.Method I 109 .Names are distinguishable from Linnaean binomials, either by beginning with a lower-case letter or by separation of their two parts by a hyphen or dot. Method J 19 .Converted names are based on the epithet of the preexisting binomial; all names are uninomial in form. 11.Names are unique and sometimes contain numbers. 12.Names end in a sequential number (shorter than the registration number) if they are not unique without it. Method K 129 .All names end in a registration number. Method L 119 .Names do not contain numbers and are not necessarily unique (but the combination of name and registration number is unique). Method M changed if its first part is the name of a belong. Methods B and D–H differ in rela- clade to which the species does not belong. tively minor ways concerning form, e.g., Within group I (stable, binomial-based whether the two words of a preexisting bi- names), method Adiffers from the others in nomial are fused or connected by a hyphen providing names that are indistinguishable or dot, and whether the names of new in form from Linnaean binomials. Method species must be identical in form to con- C differs from the others in prohibiting the verted names. Within group II (binomial- use of preexisting genus names for clades, based names with limited stability), the two thereby eliminating the possibility that the methods differ in whether or not a con- first part of a converted species name could verted name is distinguishable in form be the name of a clade to which it does not from a Linnaean binomial. Within group III 794 SYSTEMATIC BIOLOGY VOL. 48
TABLE 2.Descriptions and examples of the proposed methods for naming species in the context of phyloge- netic nomenclature. The person who proposed each method is indicated for the record, but it should not be as- sumed that the proposer prefers that option.
Group I. Stable, binomial-based names (methods A–H): Converted species names are derived from preexisting binomials and are stable; that is, they do not change as a result of new information about phylogeny. Methods A–C (Griffiths, 1976; de Queiroz and Gauthier, 1992): All species names consist of two parts, separated by a space, hyphen, or dot. Converted species names are based on the accepted binomial under the preexisting code. In names of new species, the first part may be the name of a clade (except in method C), descriptive of the organism, or chosen in some other way. Method A: All species names are identical in form to Linnaean binomials and thus indistinguishable from species names governed by the preexisting codes. Example: The preexisting name of the Common Starling, Sturnus vulgaris, remains Sturnus vulgaris under the PhyloCode, but Sturnus may or may not be the name of a clade to which the species belongs. Anew species name must take the same form (e.g., Sturnus phalacrocephalus ). Method B (Cain 1959; Michener 1963, 1964; Cantino 1998): All species names are nearly identical to Linnaean binomials, differing only in a convention that makes them distinguishable from names governed by the preexisting codes (e.g., noncapitalization or separation of the first and second parts by a hyphen or dot). Example: The preexisting name Sturnus vulgaris becomes sturnus vulgaris or Sturnus-vulgaris or Sturnus.vulgaris, depending on which convention is adopted, but Sturnus may or may not be the name of a clade to which the species belongs. Anew species name must take the same form (e.g., sturnus phalacrocephalus or Sturnus-phalacrocephalus or Sturnus.phalacrocephalus ). Method C (proposed by H. Bryant): Method C is identical to method B except that genus names cannot be used as clade names, and new species names must not incorporate a clade name as their first part. (Note: These restrictions could equally well be combined with the features of method A.) Example: The preexisting name Sturnus vulgaris becomes sturnus vulgaris or Sturnus-vulgaris or Sturnus.vulgaris, depending on which convention is adopted, but no clade may be named Sturnus. A new species name must take the same form (e.g., sturnus phalacrocephalus or Sturnus-phalacrocephalus or Sturnus.phalacrocephalus ). Methods D and E (proposed by P. Cantino): Converted species names consist of the two parts of the preexisting binomial, separated by a hyphen or dot, but new species names may be any unique, Latinized word containing only letters (i.e., no hyphen or dot). Method D: Species names are capitalized, thus new species names are indistinguishable from clade names. Example: The preexisting name Sturnus vulgaris becomes Sturnus-vulgaris or Sturnus.vulgaris, depending on which convention is adopted, but Sturnus may or may not be the name of a clade to which the species belongs. Anew species name might take the form Phalacrocephalus or Sturnusphalacrocephalus. Method E: Species names begin with a lower-case letter; thus all species names are distinguishable from clade names. Example: The preexisting name Sturnus vulgaris becomes sturnus-vulgaris or sturnus.vulgaris, depending on which convention is adopted, but Sturnus may or may not be the name of a clade to which the species belongs. Anew species name might take the form phalacrocephalus or sturnusphalacrocephalus. Method F (proposed by M. Donoghue): Converted species names consist of the two parts of the preexisting binomial separated by a dot. New species names may be any unique, Latinized, nonhyphenated string of letters (representing one word or two), with a dot placed somewhere within the name, but not immediately following the first or second letter and not at the beginning or end of the name. Example: The preexisting name Sturnus vulgaris becomes Sturnus.vulgaris, but Sturnus may or may not be the name of a clade to which the species belongs. Anew species name might take the form Phalacro.cephalus (providing that this name, or one differing only in the presence or position of the dot, has not previously been established) or Sturnus.phalacrocephalus. Method G and H (Michener, 1963): Converted species names consist of one word, formed by the fusion of the two parts of the preexisting binomial. New species names may be any unique, Latinized word containing only letters. Method G: Species names are capitalized and thus are indistinguishable from clade names. Example: The preexisting name Sturnus vulgaris becomes Sturnusvulgaris, but Sturnus may or may not be the name of a clade to which the species belongs. Anew species name might take the form Phalacrocephalus or Sturnusphalacrocephalus. Method H: Species names begin with a lower-case letter and thus are distinguishable from clade names. Example: The preexisting name Sturnus vulgaris becomes sturnusvulgaris, but Sturnus may or may not be the name of a clade to which the species belongs. Anew species name might take the form phalacrocephalus or sturnusphalacrocephalus. (Continued on next page) 1999 CANTINO ET AL.—SPECIES NAMES IN PHYLOGENETIC NOMENCLATURE 795
TABLE 2.Continued.
Group II. Binomial-based names with limited stability (methods I and J; proposed by P. Cantino): Converted species names are derived from preexisting binomials. The first part of a species name must be changed if it is the established (under the PhyloCode) name of a clade to which the species does not belong. Method I: Method I is identical to method Aexcept that the first part of the species name must be changed if it is the name of a clade to which the species does not belong. Example: The preexisting name of the Common Starling, Sturnus vulgaris, remains Sturnus vulgaris under the PhyloCode. Anew species name must take the same form (e.g., Sturnus phalacrocephalus ). If Sturnus is the established name of a clade to which this species does not belong, the word Sturnus within the species name must be formally changed to the name of a clade to which this species belongs or to some other Latinized word that is not the name of a clade. Method J: Method J is identical to method B except that the first part of the species name must be changed if it is the name of a clade to which the species does not belong. Example: The preexisting name Sturnus vulgaris becomes sturnus vulgaris or Sturnus-vulgaris or Sturnus.vulgaris, depending on which convention is adopted. Anew species name must take the same form (e.g., sturnus phalacrocephalus or Sturnus-phalacrocephalus or Sturnus.phalacrocephalus ). If Sturnus is the established name of a clade to which this species does not belong, the word Sturnus within the species name must be formally changed to the name of a clade to which this species belongs or to some other Latinized word that is not the name of a clade. Group III. Epithet-based names (methods K–M) (Graybeal 1995; Schander and Thollesson 1995; Schander 1998b): Converted species names are derived from the epithets of preexisting binomials and are stable; that is, they do not change as a result of new information about phylogeny. To provide a reference to the preexisting binomial, the name of the genus to which the species belongs under the preexisting code may (but need not) be cited as a taxonomic address; it is recommended that this be done if the species name alone might be confusing (proposed by M. Lee and T. Eriksson). Method K (proposed by K. de Queiroz): Species names are terminated by a number (which is part of the name) if the rest of the name has previously been established for a different species under the PhyloCode. The number must be the lowest integer >1 that has not previously been used as part of a name that is otherwise spelled the same. The number may be dropped after the first use of the name in a particular publication. When a species name is converted, the nonnumerical portion of the name is the epithet of the accepted binomial under the preexisting code. When a species name is new, the nonnumerical portion of the name may be any Latinized word. If a taxonomic address is cited, the combination of address and species name may or may not be distinguishable from a Linnaean binomial, depending on whether the two names are separated by a delimiter of some sort (e.g., a slash or parentheses). If a delimiter is not used, the combination of taxonomic address and species name will be identical in form to a binomial if the name does not contain any numbers (i.e., if it is the first name established under the PhyloCode based on this epithet) or if the number is omitted subsequent to its first use in a publication. Example: The preexisting name Sturnus vulgaris becomes vulgaris. If vulgaris has already been established for another species, the name vulgaris2 is used (or vulgaris.2 or vulgaris {2}, depending on which convention is adopted). If vulgaris2 has already been established for another species, vulgaris3 is used, and so forth. If the name vulgaris2 by itself might be confusing to readers, it is recommended that Sturnus be cited as a taxonomic address; this would take the form Sturnus vulgaris2 or Sturnus/vulgaris2 or (Sturnus) vulgaris2 or vulgaris2 (Sturnus) or Sturnus:vulgaris2, depending on which convention is adopted. Method L(proposed by T. Eriksson): Species names are terminated by a unique registration number (which is part of the name). The number may be dropped after the first use of the name in a particular publication. When a species name is converted, the nonnumerical portion of the name is the epithet of the accepted binomial under the preexisting code. When a species name is new, the nonnumerical portion of the name may be any Latinized word. If a taxonomic address is cited, the combination of address and species name may or may not be distinguishable from a Linnaean binomial, depending on whether the two names are separated by a delimiter of some sort (e.g., a slash or parentheses). If a delimiter is not used, the combination of taxonomic address and species name will be identical in form to a binomial if the number is omitted subsequent to its first use in a publication. Example: The preexisting name Sturnus vulgaris becomes vulgaris### (or vulgaris.### or vulgaris {###}, depending on which convention is adopted). If the name vulgaris### by itself might be confusing to readers, it is recommended that Sturnus be cited as a taxonomic address. The taxonomic address plus species name would take the form Sturnus vulgaris### or Sturnus/vulgaris### or (Sturnus) vulgaris### or vulgaris### (Sturnus) or Sturnus:vulgaris###, depending on which convention is adopted. (Continued on next page) 796 SYSTEMATIC BIOLOGY VOL. 48
TABLE 2.Continued.
Method M (proposed by D. Hillis and K. de Queiroz): Species names do not contain numbers, but it is recommended that the unique registration number be cited at least once within any publication in which the name is used. When a species name is converted, the name is the epithet of the accepted binomial under the preexisting code. When a species name is new, the name may be any Latinized word. If a taxonomic address is cited, the combination of address and species name may or may not be distinguishable from a Linnaean binomial, depending on whether the two names are separated by a delimiter of some sort (e.g., a slash or parentheses). If a delimiter is not used, the combination of taxonomic address and species name will be identical in form to a binomial. Example: The preexisting name Sturnus vulgaris becomes vulgaris. If the name vulgaris by itself might be confusing to readers, it is recommended that Sturnus be cited as a taxonomic address. The species name with its taxonomic address could take the form Sturnus vulgaris. Other options for combining the species name and the taxonomic address (see previous examples) could also be used with this method.
(epithet-based names), methods K and L cates the “location” of the species within the provide unique names by including num- nested hierarchy of subsuming clades. A bers as part of the name. In contrast, names similar practice is already used when one provided by method M do not contain wishes to impart more phylogenetic infor- numbers and are not necessarily unique but mation than is conveyed by the genus name are complemented by a unique registration in a Linnaean binomial; e.g., Oxyuranus number. Aunique registration number ex- scutellatus (Serpentes, Reptilia). ists under all methods, but only methods L and M take advantage of it to clear up po- DESIRABLE FEATURES tential ambiguities when names are not In evaluating the merits of the proposed unique. Method Lrequires that the registra- methods for naming species, we consider tion number be part of every name to en- several features to be desirable. No method sure that names are unique (in contrast, the has all of them (Table 3); indeed, none could number is not part of the name in method have all of them because some desirable M). In method K, only those names that features are mutually exclusive. have been previously established for a dif- ferent species require addition of a number, and the number used is not the registration Stability and Continuity number (it is generally much shorter than The communication function of nomen- the registration number). clature is best served if names remain stable All of the proposed methods differ from through time. In phylogenetic nomencla- Linnaean binomial nomenclature in that ture, there are two components of stability: species names do not attempt to convey in- continuity of converted names with the formation about supraspecific relationships. names used under the preexisting codes, Even when a species name begins with a and stability of names once they are estab- word that is a genus name under one of the lished under the PhyloCode. preexisting codes, one cannot assume that When applied to taxa that already have the species belongs to a clade bearing that names, phylogenetic nomenclature does name. For example, the genus (under the not attempt to replace the existing names preexisting code) may be paraphyletic, or it with new ones but, rather, governs the ap- may correspond to a clade for which a name plication of the existing names in a different has not yet been established under the Phy- way. Thus, existing species names ideally loCode. This is further discussed below (see should not change when they are con- Information About Phylogenetic Relation- verted. The proposed methods attempt to ships). To indicate relationship, the species meet this objective in different ways. Meth- name would have to be combined with the ods A–J preserve the entire binomial, with name(s) of one or more subsuming clades or without minor changes in form (e.g., hy- (i.e., clades to which the species belongs); phenation or fusion of the two parts). Meth- we refer to such a name or names metaphor- ods K–M preserve only the epithet, but the ically as a “ clade address” because it indi- genus name may optionally be cited as a 1999 CANTINO ET AL.—SPECIES NAMES IN PHYLOGENETIC NOMENCLATURE 797
“taxonomic address,” which may or may species than that supposed by the author of not also be a clade address. If the genus is the name) and lumping and splitting at the monophyletic, its name would be a clade species level. However, in these cases, what address. If the monophyly of the genus has is changing is not the name of the species, not been demonstrated, its name can be but rather the application of the name or the placed in quotation marks to avoid confu- hypothesized boundaries of the species. sion with a clade address. (Some of us feel Under the “limited stability” methods, I that only clade names that have been estab- and J, species names are based on binomials lished under the PhyloCode should be cited and must be changed if the first part of the as clade addresses and that all other names name is the phylogenetically defined name cited as taxonomic addresses should be of a clade to which the species does not be- placed in quotation marks or marked in long (e.g., see the Caryopteris example de- some other way to indicate that they have scribed below under Information About not been established under the PhyloCode; Phylogenetic Relationships). However, they others of us consider it acceptable to cite as do not change if the first part of the name is a clade address any taxon for which there is not a phylogenetically defined clade name. evidence of monophyly, whether or not its Nor do they change because of the other name is established under the PhyloCode. causes, unrelated to phylogeny, that lead to In any case, such conventions will develop name changes under the traditional system. gradually through usage by the systematics Thus, methods I and J provide species community and will not be legislated by names that are more stable than Linnaean the PhyloCode.) binomials but less stable than the other op- In all proposed methods except I and J, tions proposed here. species names are stable once they are estab- lished under the PhyloCode. This is in marked contrast to Linnaean binomial Uniqueness and Ambiguity nomenclature, in which species names One of the principles underlying the Phy- change frequently as a result of several phe- loCode is that names must be unique; that nomena: (1) “ lumping” and “ splitting” at is, each taxon should have only one ac- the genus level, based on new phylogenetic cepted name, and each accepted name evidence that indicates a genus is not mono- should refer to only one taxon (de Queiroz phyletic; (2) lumping and splitting based on and Gauthier, 1994). This presents a prob- phenetic or other nonphylogenetic consid- lem for epithet-based species names be- erations; and (3) discovery of older names in cause many epithets are not unique. The the literature (although changes due to strict critical feature that taxon names must have application of priority are now discouraged is not uniqueness per se, but lack of ambi- by the ICBN, and will be discouraged in the guity; however, uniqueness is the simplest fourth edition of the ICZN {S. Minelli, pers. and surest way to make names unambigu- com.}). The first two phenomena will not ous. Nonunique species names can be ren- cause name changes under any of the meth- dered unambiguous by citing a clade ad- ods proposed here because the genus con- dress that includes only one species with cept is not part of phylogenetic nomen- that name, but this approach presents prac- clature. However , new evidence about tical problems. Users of names who are not phylogeny (i.e., phenomenon 1 uncoupled specialists on the group may not know from the genus concept) may result in name whether a clade that might be cited as a change under methods I and J, as discussed clade address contains more than one species below. The third phenomenon will be elimi- with a particular epithet-based name, un- nated by the implementation of a registra- less the clade address is the name of the tion system for names governed by the Phy- genus to which the species belongs under loCode. Under methods A–H and K–M, the the preexisting code. Alternatively , one only reasons that a species might appear to might cite the preexisting genus name as a be renamed would be the discovery of typi- taxonomic address regardless of whether it fication errors (i.e., if the type of a species is a clade name (placing it in quotation name is found to belong to a different marks if the genus is not monophyletic). 798 SYSTEMATIC BIOLOGY VOL. 48
Unique names are critical for biblio- lished clade names, many species with graphic searches. Entering the name vulgare common epithets would likely have to be in a computerized search for information named in this way under Graybeal’s on the plant species currently known as method. This might occur when a preexist- Clinopodium vulgare will turn up a vast ing name is converted, or long afterwards. number of irrelevant references. This will Aspecies name that is unambiguous when rarely occur if one enters a Linnaean bino- converted may later become ambiguous mial (e.g., Clinopodium vulgare ), and per- when another species name based on the haps never with binomial-based names same epithet is converted, if both species (e.g., Clinopodium-vulgare ), but the much are part of the same least inclusive named greater frequency of homonymy in the case clade. Thus, Graybeal’s method introduces of epithets creates a serious problem that ci- a source of nomenclatural instability that tation of a clade address will not always does not exist in any of the methods pro- solve. For example, the least inclusive taxon posed here. that contains this species and is unques- Schander and Thollesson (1995) pro- tionably a clade is Nepetoideae (in Lami- posed another way of reducing the confu- aceae; Wagstaff et al., 1995), which includes sion caused by the nonuniqueness of epi- well over 100 genera. Combining Nepetoi- thet-based species names. They suggested deae and vulgare in a bibliographic search that it be required that the author and year would not eliminate irrelevant output be- of publication be cited with the name; thus, cause there are other species of Nepetoi- the species currently known as Polycera deae with the same epithet (e.g., Origanum quadrilineata (Müller, 1776) would become vulgare). Aconvention to cite the preexist- Quadrilineata Müller, 1776. However , as ing genus name as a taxonomic address they pointed out, this would not com- would solve the problem if everyone fol- pletely eliminate the problem because some lowed it, but conventions (as opposed to authors (e.g., Linnaeus) published more nomenclatural rules) may be ignored. The than one name with a particular epithet in only way to guarantee presence of the in- the same year. formation necessary to make a name unam- Adifferent solution to the uniqueness biguous is to include that information in problem is adopted in the epithet-based the name itself. methods proposed here: Numbers are used Graybeal (1995) discussed this problem to eliminate the ambiguity that might oth- and proposed that if the genus is not a erwise result from nonunique names. In clade, and if ambiguity persists even when Linnaean binomials, it is the combination of the least inclusive named clade is cited (be- the epithet and the genus name that is usu- cause of homonymy within that clade), ally unique (though the existence of ho- then the species should be renamed. She monyms can occasionally lead to ambiguity suggested that this be done by fusing the if the authors of the names are not cited); two parts of the preexisting binomial. For under the epithet-based methods described example, given the situation described in here (K–M), it is the combination of the epi- the previous paragraph, vulgare would be thet-like species name and a number that is renamed clinopodiumvulgare, to distinguish unique. These methods thus use a number it from other species named vulgare within rather than a ranked category (genus) to the least inclusive named clade (Nepetoi- compensate for the nonuniqueness of the deae). This resembles our method H and specific epithet. has the same potential for pronunciation In methods K and L, a number is in- problems (discussed below under Other cluded as part of the name to make it Desirable Features). It differs from method unique, whereas in method M a unique reg- H in that names would take this form only istration number (which is not part of the if ambiguity would otherwise result, name itself) provides an unambiguous ref- whereas in method H all converted names erence to the species even when the take this form. In groups for which the phy- nonunique name does not. In method L, the logeny is poorly known, and in which most registration number is added to all names. genus names would therefore not be estab- In method K, a shorter number is added— 1999 CANTINO ET AL.—SPECIES NAMES IN PHYLOGENETIC NOMENCLATURE 799 and only to those names that are not unique not necessarily unique; homonyms exist without it. In both options, one may abbre- both within the jurisdiction of a given code viate a name by dropping the number, as and between codes. The resulting ambigu- long as the number has been included with ity is largely eliminated by citing the au- the name at least once within a particular thorities of names and by contextual in- publication or paragraph (depending on formation. Citation of authorities will be editorial practice). This is analogous to the unnecessary under the PhyloCode if a reg- current practice, in many journals, of drop- istration system is implemented, as planned. ping the author citation or abbreviating the Use of a registration database will prevent genus name after the first use of the name. the accidental creation of homonyms as Method M differs from Lin that the regis- well as provide an easy way to access publi- tration number is not part of the name; cation data for all names. thus, some names will not be unique. The resulting potential for confusion is miti- gated by a convention that a unique regis- Information About Phylogenetic Relationships tration number be cited the first time a Linnaean binomials contain information name is used in a particular publication. In about genus membership. When genera are method M, the registration number func- monophyletic, such species names convey tions in a manner analogous to a U.S. social phylogenetic information. This information security number, which can be used to dis- is provided at the expense of stability (dis- tinguish between people whose names are cussed above), because species names must identical. In practice, methods L and M change when generic boundaries are re- would probably work similarly , even drawn. Arelated drawback of Linnaean bi- though the number is part of the name in L nomials is their inability to accommodate but not in M. However, a nomenclatural lack of knowledge about the genus-level re- code can determine only the form that a lationships of a species. New species are name takes when it is first published, not sometimes discovered that cannot be re- the subsequent use of that name. Thus, the ferred with confidence to a genus because PhyloCode will have no power to enforce of incompleteness of preservation or the ab- inclusion of the registration number when sence of a diagnostic structure or ontoge- citing a previously published name under netic stage. This problem occurs most fre- either method L or M (or inclusion of the quently when species are described on the shorter numbers used in method K). This basis of fossilized remains but may also oc- will be determined by editors, authors, and cur with extant species (e.g., when a new the community of biologists. If method Lor plant species is described on the basis of M is adopted, we anticipate that most au- flowering material but fruits are needed to thors will include the registration number diagnose the genus). However, Linnaean for a name at least once within any publica- nomenclature requires the systematist to tion in which the name is cited, out of com- place the species in a genus to be able to de- mon sense and self-interest; the registration scribe it as new. Furthermore, this require- number is the only thing that eliminates the ment creates taxonomic dilemmas when the potential for ambiguity in options Land M, phylogenetic relationships among species and authors will presumably want their in a complex of genera are poorly resolved publications to be found by other biologists (Cantino, 1998; Cantino et al., 1999). If only who are searching for literature on the cited some of the species in the complex can be species. However , some of us believe it is referred to clades, then one must choose be- important to emphasize the importance of tween two alternatives, both of which are the registration number by making it an in- unacceptable to many phylogenetic system- tegral part of the name (method L), whereas atists: creating a genus that is likely to be others think this is unnecessary and object paraphyletic or polyphyletic to accommo- to the inclusion of numbers within names. date the species of uncertain relationships, The problem of avoiding ambiguity is or classifying each of these species as a not limited to phylogenetic nomenclature. monotypic genus, thereby increasing the re- Under the current codes, species names are dundancy of the classification. This prob- 800 SYSTEMATIC BIOLOGY VOL. 48 lem is not inherent to Linnaean binomial bifurca would be more closely related to Al- nomenclature but arises when systematists chemilla-alpina than to Potentilla-norvegica. try to combine the Linnaean method with Moreover, the first part of the species the requirement that all supraspecific taxa name may be the established name of a clade be monophyletic. to which the species does not belong. For ex- None of the naming methods proposed ample, the genus Caryopteris (Lamiaceae) here attempts to convey phylogenetic infor- has both a wide and a narrow circumscrip- mation within the species name (although tion (Cantino et al., 1999). Caryopteris s. str. is methods C, I, and J are designed to avoid a clade, but Caryopteris s. l. is not. The species the appearance of conveying incorrect in- widely known as Caryopteris divaricata is not formation, as discussed below), so igno- a member of Caryopteris s. str. For the sake of rance of phylogeny is not an impediment to argument, suppose that the implementation naming species. It may seem paradoxical of the PhyloCode had preceded our current that phylogenetic nomenclature requires understanding of the phylogeny of this less knowledge of relationships to name group. If the name Caryopteris divaricata had species than the traditional system does, been converted to Caryopteris-divaricata un- but this occurs because the naming of der the PhyloCode, and subsequent phylo- species is separated from their referral to genetic study led to the establishment of the clades. Abenefit of this separation is greatly name Caryopteris for the clade correspond- improved stability of species names. ing to Caryopteris s. str., then Caryopteris-di- Although species names are not intended varicata would not belong to the clade Cary- to convey phylogenetic information in any opteris. This would be misleading for users of the methods proposed here, names that who had not yet adjusted to a system in consist of two parts may be misunderstood which species names more or less resemble to imply relationship, when encountered by Linnaean binomials but are not intended to people who assume that they function like convey information about relationship. Linnaean binomials. In some cases, the rela- It is possible to avoid the most severe tionships inferred will be correct, but in manifestation of this problem, the inference some cases they will not. For example, the of relationships that are known to be incor- first part of a converted species name may rect, in two ways. The PhyloCode could be a preexisting genus name that has not prohibit using genus names for clades been established as a clade name under the (method C). Alternatively, a “limited stabil- PhyloCode, perhaps because that name tra- ity” method could be used (methods I and ditionally refers to a paraphyletic group. J), in which species names must be changed Even if the name of a paraphyletic genus has if the first part of the name seems to imply been established as a clade name (with its membership in a clade to which it does not membership expanded so that it is mono- belong. Each of these solutions introduces phyletic), the paraphyly of the genus can its own problems. Methods I and J reduce cause confusion. T wo species whose con- the stability of species names (although not verted names begin with the same word (the as seriously as the Linnaean binomial name of a paraphyletic genus) are not neces- method does). Prohibiting the use of genus sarily more closely related to each other than names for clades (method C) would require to a third species whose name begins with a abandoning many familiar names, even different word. For example (Eriksson et al., though many of them currently apply (at 1998), Potentilla is a large, paraphyletic plant least implicitly) to monophyletic groups. genus that includes the sister groups of sev- The problems discussed in this section ap- eral smaller, segregate genera. If the current ply only to binomial-based names. Infer- names for the species in this complex were ence of incorrect relationships cannot occur converted to binomial-based names (e.g., with epithet-based names (methods K–M). method B), and if Potentilla were established as the name of the clade comprising the pa- raphyletic group to which it is traditionally Distinguishability applied plus all of the segregate genera (as Some people feel that species names discussed by Eriksson et al.), then Potentilla- should differ in form from clade names to 1999 CANTINO ET AL.—SPECIES NAMES IN PHYLOGENETIC NOMENCLATURE 801 make it immediately evident to which kind certain circumstances. If more than one of entity a name refers. All of the proposed clade is cited or if the species name and methods satisfy this requirement except clade name are separated by a symbol such methods D and G, in which the names of as a slash, colon, or parentheses, the names new species (method D) or all species together are distinguishable from a Lin- (method G) are capitalized uninomials and naean binomial. However, if only one clade thus indistinguishable from clade names. is cited, and if its name precedes the species Capitalization of species names under these name and is separated from it by a blank options is based on the argument that all space, the two names together are indistin- scientific names should be capitalized be- guishable from a Linnaean binomial if the cause they are proper nouns. For uninomial species name does not contain numbers. species names, there is a conflict between Names produced by method M never con- this consideration and distinguishability tain numbers, which makes this method at- from clade names. One solution might be to tractive to those who would like the combi- use italics for species names but not clade nation of a PhyloCode name and a names, permitting both to be capitalized taxonomic address to look exactly like a but still distinguishable. However, unless Linnaean binomial. Numbers may also be some other font (e.g., boldface) were used absent from names produced by methods K for clade names, they could be confused and Lif the numerical portion of the name with vernacular (common) names. Many is omitted after its first use in a particular genus names, some of which would even- publication or (in method K) if the name is tually be converted to clade names, are the first one established under the Phy- identical to vernacular names, which may loCode based on a particular epithet. not even apply to the same organisms. For Those who favor indistinguishability of example, the plant genus Lotus and the PhyloCode species names from Linnaean plant with the vernacular name Lotus are binomials argue that, to minimize disrup- very distantly related. T o avoid confusion tion of the preexisting nomenclature, bino- with vernacular names, the draft Phy- mials should not be changed in any way loCode recommends that all names estab- when converted under the PhyloCode. In lished under it be italicized. addition to this philosophical argument, Another consideration is distinguishabil- some of us have a practical concern that ity of names governed by different codes. people will refuse to use names that differ This issue has been a major source of dis- in appearance, even in minor ways, from agreement among the authors of this paper. the ones with which they are familiar. Oth- Some of us maintain that species names ers of us believe this is unlikely to be a governed by the PhyloCode should be im- problem; i.e., users of names will be flexible mediately distinguishable from the Lin- enough to accept minor changes in format naean binomials governed by the preexist- such as a hyphen or a dot connecting the ing codes, whereas others of us argue that two words of a binomial. Those who favor they need not, or even should not, be distin- distinguishability of species names gov- guishable from Linnaean binomials be- erned by the PhyloCode are concerned cause the context will usually clarify which about potential confusion if a single species code governs the name. Methods A and I has two different names that are indistin- are the only ones in which species names guishable in form, one of them correct un- are identical in form to Linnaean binomials. der the PhyloCode and the other one cor- In the other binomial-based options, con- rect under the preexisting code governing verted names (and in some methods, all it. This will occur whenever a species name names) are similar in form to Linnaean bi- changes under the preexisting code (e.g., nomials but differ in minor ways, such as because of generic realignment) after it has hyphenation or fusion of the two parts of been converted and thereby stabilized un- the name. In the epithet-based methods der the PhyloCode. In most cases, only the (K–M), the combination of species name genus portion of the binomial will change and a taxonomic address may be indistin- under the preexisting code, but homonymy guishable from a Linnaean binomial under under the new genus will sometimes cause 802 SYSTEMATIC BIOLOGY VOL. 48 the epithet to change as well. The longer the species that was first recognized after the two codes coexist, the greater will be the di- implementation of the PhyloCode. In meth- vergence of species names under the two ods D and E, new species names take a dif- systems. This is a serious problem, particu- ferent form from that of converted names. larly if names governed by the two systems Converted names are based on binomials, are not distinguishable. However, the draft whereas new species names must have a PhyloCode suggests a convention for dis- uninomial form, the name consisting of ei- tinguishing between PhyloCode names and ther one word or two fused words. These names governed by the preexisting codes methods were proposed in response to a when confusion might otherwise result. criticism of methods such as Aand B, in which names with a binomial form would have to be found for new species in perpe- Other Desirable Features tuity, even though the genus category, and Because scientific names are spoken as thus the need for a binomial name, does not well as written, they must be pronounce- exist under the PhyloCode. Methods D and able. Most of the naming methods pro- E attempt to combine the advantages that a posed for the PhyloCode are no worse than binomial-based method has for converted the current one in this regard, but the fusion names (i.e., continuity through retention of of the two parts of a binomial to form a sin- the entire preexisting binomial; uniqueness gle word in methods G and H may lead to without addition of numbers) with the ad- pronunciation problems. When faced with vantages that an epithet-based method has long names like Amsoniatabernaemontana for new names (i.e., simplicity of form; and Agastachescrophulariaefolia, or even rela- species names cannot be misunderstood to tively short ones based on binomials in imply relationships). However, this is done which the genus name ends in an unusual at the expense of consistency of form. vowel, people who are unfamiliar with the Method F also attempts to combine the binomials on which these name are based advantages of binomial-based and epithet- are likely to have trouble figuring out how based methods, while avoiding the pronun- to split them into syllables. For example, ciation problems of methods G and H and someone who is unfamiliar with the genus providing greater flexibility and consis- Muscari, might assume that Muscaribotry- tency of form than methods D and E. In oides is based on Musca ribotryoides or Mus method F , converted species names are caribotryoides and pronounce it accordingly. based on preexisting binomials, with a dot It is likely that such names will also be more (or period) inserted between the two words difficult to memorize because pronuncia- as a pronuncation aid. New species names tion plays an important role in memoriza- may be binomial-like with a dot between tion for many people. One could argue that the parts (as for converted names), or they the inclusion of numbers in species names may be epithet-like with a dot inserted (methods K and L) would make verbal somewhere within the name to maintain communication more awkward by length- consistency of form with converted names. ening the names, but the more likely result The stipulation that the dot may not be would be omission of the numbers (particu- placed after the first or second letter is nec- larly the registration numbers in method L) essary to avoid confusion with abbreviated when the name is spoken. The citation of a species names governed by the preexisting clade address or the context of the conver- codes, in which the genus name is com- sation would generally make it clear to monly abbreviated as a single capital letter which species the name refers, making it or occasionally by the first two letters (e.g., unnecessary to say the number. Ph. arundinacea for Phalaris arundinacea ). In most of the proposed naming meth- ods, all species names have the same form, whether they are new or converted. This COMPARISONOF PROPOSED has the advantage of consistency , but one NAMING METHODS might argue to the contrary that it is useful The distinctions among the proposed to be able to tell at a glance whether a methods for naming species are summa- species name is converted or applies to a rized in Table 3. No method combines all of 1999 CANTINO ET AL.—SPECIES NAMES IN PHYLOGENETIC NOMENCLATURE 803 - - t r e f r a y i
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e p e s M L A B C D E F G H I J K L M r n s a s n d i 804 SYSTEMATIC BIOLOGY VOL. 48 the desirable features discussed above; each cans12 the first time it is used in a particular has advantages and disadvantages. The au- publication, and Rhus radicans or R. radicans thors of this paper have not been able to subsequently. If Rhus is not a clade, the reach a consensus about the best approach species could be cited as “Rhus” radicans12 because we disagree about the relative im- (and subsequently abbreviated as “Rhus” portance of the advantages and drawbacks radicans or “R.” radicans ). However, the in- of the various options, and because some clusion of a taxonomic address is optional, features are viewed as advantages by some and if one is used, it does not have to be the of us and as drawbacks by others (e.g., see name of the genus of the preexisting name. Distinguishability). Currently , the epithet- For example, an author could cite this based methods have the greatest support species as Anacardiaceae radicans12 (assum- among the participants in the PhyloCode ing Anacardiaceae is a clade), thereby greatly project, but there is also substantial support reducing the continuity with the preexist- for method B among the binomial-based ing species name. In contrast, under the bi- methods. nomial-based options, continuity with the Afundamental dichotomy among the preexisting name at the time of conversion proposed methods distinguishes those in is ensured. which converted species names are based However, the epithet-based methods of- on binomials (A– J) from those in which fer continuity of a different kind. As species they are based on epithets (K–M). Epithet- names change under the preexisting codes based names are preferred by those who are because of generic realignments, while attracted to their logical simplicity and by PhyloCode names remain stable, the two those who object to the fact that binomial- sets of names will gradually diverge. The based names may be misinterpreted as im- epithet-based methods allow one to track plying relationships. Binomial-based names some of the name changes that occur under are preferred by those who place a pre- the preexisting codes if one wishes, as long mium on continuity and view the binomial as the epithet doesn’t change. For example, as the full preexisting name and by those if the name Toxicodendron radicans were to who object to nonunique names (method become more widely accepted for poison- M) or to changing the name by adding ivy than Rhus radicans under the traditional numbers to it (methods K and L). system, one could choose to combine the Continuity with the preexisting nomen- species name radicans12 with the clade ad- clature is a complex issue. In Linnaean dress Toxicodendron instead of Rhus. This nomenclature, the binomial is traditionally would not be possible with a binomial- viewed as the name of the species, and this based method. If Rhus radicans were con- is reflected in how the preexisting codes verted to Rhus-radicans (method B), this deal with homonymy. Names are treated as name would have to be retained, even if homonyms only if the entire binomial is users of the traditional system were to identical. However, there is a way in which abandon Rhus radicans in favor of Toxicoden- the epithet functions more like the true dron radicans. Even under the limited stabil- name of the species: It is the only part of the ity methods (I and J), one could not change name that remains constant when new Rhus-radicans to Toxicodendron-radicans un- combinations are published. less Rhus was the established name of a Even if the binomial is considered the clade that did not include Rhus-radicans. If true name of a species, it is possible to pre- generic realignments under the preexisting serve continuity with preexisting names codes result in an increasing proportion of when using an epithet-based method. This monophyletic genera, the use of epithet- can be accomplished by citing the genus based names and citation of the appropriate portion of the preexisting name as a taxo- genus name as a clade address may to a nomic address preceding the converted large extent avoid divergence of the names species name (epithet). For example, under governed by the two systems. method K, if Rhus radicans were converted Some of the differences among the ten to radicans12, and if Rhus is a clade, it would options that involve binomial-based names be possible to cite the species as Rhus radi- concern distinguishability of PhyloCode 1999 CANTINO ET AL.—SPECIES NAMES IN PHYLOGENETIC NOMENCLATURE 805 species names from Linnaean binomials (A names despite the fact that both kinds of vs B, I vs J), or from clade names (D vs E, G names are capitalized (which is consistent vs H). Some methods (C, G, H, I, and J) with the fact that they are proper names). In were introduced to circumvent or mitigate method E, species names begin with a a major problem with the binomial-based lower-case letter and thus are also distin- options—the potential for confusion when guishable from clade names. In method D, users incorrectly assume that the first part species names are capitalized, and new of the name implies relationship—or to re- species names are indistinguishable from duce the frequency of this problem by per- clade names. mitting new names to have a uninomial Afundamental dichotomy among the form (D and E) or a uninomial-like origin three epithet-based methods is whether (F). All of these methods except F have the names are required to be unique (K and L) disadvantage of decreasing the stability (I or are permitted to be nonunique (M). The and J), consistency of form (D and E), or practical consequences will depend on the ease of pronunciation (G and H) of species degree to which everyone follows two con- names or limiting the choice of clade names ventions that would eliminate the potential (C). Those of us who prefer binomial-based ambiguity caused by nonunique names: cit- methods tend to feel that the confusion will ing the unique registration number associ- pass with time, as users of names grow ac- ated with a species name when the name is customed to the idea that relationships can- first used in a publication, and citing an ap- not be reliably inferred from the first part of propriate genus name as a taxonomic ad- a species name governed by the Phy- dress. A particular concern is the impact loCode. Some of us believe this is likely to that rampant homonymy would have on occur more quickly if PhyloCode names are bibliographic searches if these conventions readily recognizable as such (B–H, J) rather were not followed. Some of us are confident than being identical in form to a Linnaean that they would be followed, thus the binomial (Aand I), because distinguishabil- nonuniqueness of names in method M ity of names governed by the two nomen- would not lead to confusion; others of us clatural systems will hasten popular recog- strongly doubt this. An underlying dis- nition that two systems are in operation agreement is whether the registration num- and will stimulate people to ask how they ber is more likely to be cited if (a) it is offi- differ. cially part of the name but may be dropped Methods D– F are superficially similar as an abbreviation (method L) than if (b) it but differ in some important ways. In meth- is not an official part of the name but there ods D and E, new species names must con- is a recommendation that it be cited at least sist of only one part. Because binomial form once in any publication in which the name is not permitted for new names, it would is used (method M). not be possible to start the species name Methods K and L differ in whether the with the name of a clade whose members registration number is attached to every the new species resembles (unless the clade name (L) or a shorter number is added to a name is fused with the rest of the species name only when needed to make it unique name, as in methods G and H, which would (K). In method K, no number is added if the often yield names that are difficult to pro- name is based on a unique epithet or is the nounce, as discussed above). Methods D first instance of a particular epithet being and E were proposed so that the binomial converted to a PhyloCode name. Thus Zea form would not be required for new species mays would probably not require a number names, but the corresponding drawback is under method K, because the epithet mays that the binomial form is not permitted is probably unique. Under method L, how- even when it would be useful. Method F is ever, the name would end in a registration more flexible. Anew species name may de- number. The advantages of method K are rive from one word or two; the only re- that many names would not require num- quirement is that there be a dot somewhere bers, and when a number is used it would within the name. The presence of the dot be shorter than the registration number and distinguishes species names from clade thus easier to remember. On the other hand, 806 SYSTEMATIC BIOLOGY VOL. 48 every name will have a registration number Robeck, Timothy Rowe, Christoffer Schander, Per regardless of which naming method is Sundberg, Mikael Thollesson, and André Wyss. 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