SUBSPECIES, SEMISPECIES, SUPERSPECIES James Mallet University College London I. A Brief History of Subspecific Taxonomy as separate species, and ‘‘lumpers’’ who ignored II. The Subspecies Today geographic variation, and united local variants into a III. Further Reading single species. The problem was compounded by early systematists’ belief that species had an Aristotelian ‘‘essence,’’ each fundamentally different from similar essences underlying other species. To Linnaeus’ fol- DISTINCT POPULATIONS THAT REPLACE EACH lowers, it seemed important to decide which level of OTHER GEOGRAPHICALLY were recognized either variation was fundamental. The terms ‘‘genus’’ and as full species or as lower-level ‘‘varieties’’ or ‘‘forms’’ ‘‘species’’ both result from Aristotelian philosophy, and under the original Linnaean taxonomy. In zoology, a although Linnaeus is usually credited with establishing practical resolution of this ambiguity took place the species as the basal taxonomic unit, he confused largely between about 1880 and 1920, with the matters, after recognizing that some plant species were recognition of a single additional taxonomic rank, of hybrid origin, by suggesting that genera were a more the geographic subspecies (while in botany, many important taxonomic level (i.e., a separately created infraspecific ranks remain valid). Since the 1980s, the kind) than species. fashion has changed once more. Some systematists are Once evolution was accepted, it became clear that again recognizing geographical replacement forms as variation at all levels in the taxonomic hierarchy was full species, even when they blend together at their due to more or less similar causes; the only difference boundaries. between variation above the level of genus or species and below was one of degree. Darwin realized that species could evolve from intraspecific varieties. Darwin used the term ‘‘species’’ in a new and nonessentialist I. A BRIEF HISTORY OF SUBSPECIFIC sense: ‘‘... the complete absence, in a well-investigated TAXONOMY region, of varieties linking together two closely-allied forms, is probably the most important of all the crite- A. Variation Below the Level of Species rions of their specific distinctness ... Geographical dis- tribution is often brought into play unconsciously and Since the invention of binominal nomenclature by sometimes consciously; so that forms living in two Linnaeus, there has been a conflict between ‘‘splitters’’ widely separated areas, in which most of the other in- who named more or less well-defined local populations habitants are specifically distinct, are themselves usually Encyclopedia of Biodiversity Copyright & 2007 Elsevier Inc. All rights of reproduction in any form reserved. 1 2 ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ SUBSPECIES, SEMISPECIES, SUPERSPECIES________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ looked at as distinct; but in truth this affords no aid in North America were Elliott Coues, who published a distinguishing geographical races from so-called good or catalog of American birds in 1872 incorporating an true species’’ (Darwin, 1874). Darwin showed convinc- early version of this trinominal nomenclature, in ingly that there was no essential difference between which subspecies were prefixed by ‘‘var.,’’ and Robert species and ‘‘varieties’’; species were simply varieties Ridgway, who finally dropped the ‘‘var.’’ in his own which had diverged more, and which could coexist 1881 summary of North American bird nomenclature. without intermediates being common. However, with The American Ornithologists’ Union soon adopted his term ‘‘varieties’’ Darwin did not clearly distinguish this policy, and the idea then spread to Europe, par- between polymorphic variants within populations ticularly England where Walter Rothschild began and the identifiable geographic populations normally amassing his vast collection of birds and butterflies, today considered as geographic ‘‘races’’ or ‘‘subspecies.’’ andhadhiredexcellentandproductivestaff,the To Darwin the distinction was unimportant, because ornithologist Ernst Hartert and the entomologist polymorphic variants, clinal variation, geographic races Karl Jordan, to curate and describe the new material. or subspecies, and ‘‘good’’ species formed a continuum. Jordan was particularly important in spreading the Darwin demonstrated that this continuum was excellent idea of trinominal nomenclature to entomologists, evidence for an evolutionary origin of the taxa we call and in promoting the abandonment of other subspe- species. cific nomenclature. He was regarded by the Rothschi- lds as the ‘‘clever’’ member of the staff, and published enormous systematic treatises as well as papers on the B. The Trinominal Revolution in Zoology theory of systematics, justifying trinominal nomen- clature and the recognition of the ‘‘subspecies’’ as a Many systematists wished to preserve the purity of the valid, identifiable taxon in its own right. Both Jordan simple genus–species binominal nomenclature, but by and Hartert were Germans who contributed to and read the 1850s, there were enormous stresses. It began to German as well as English journals, and in Germany a be realized that many clearly identifiable geographic similar revolution was taking place. Thus, these sys- replacement forms were an important intermediate tematic ideas were able to spread to the rest of Europe stage between insignificant local variants and ‘‘good’’ in a time when science was often highly parochial. The species. Some lumped these replacement forms as standard trinominal nomenclature for subspecies, and varieties within species, while others continued des- the abandonment of other named ranks, such as semi- cribing these replacement forms as separate species: species and superspecies, soon became established in practices varied widely, leading to considerable con- the International Code of Zoological Nomenclature, fusion. Although some Europeans had long advocated and has remained there ever since. naming geographic forms as subspecies, the accumu- Meanwhile, botany, Linnaeus’ major expertise, lation of major North American museum collections remained much less prescriptive. Many infraspecific during the great push of colonization and railway ranks are still considered valid taxa under the Inter- construction westwards was probably the most im- national Code of Botanical Nomenclature. For in- portant catalyst of a revolutionary new systematics. stance, Saxifraga aizoon var. aizoon subvar. brevifolia In this new approach, nomenclature consisted of a f. multicaulis subf. surculosa is a valid scientific name trinominal: genus–species–subspecies, which is still in botany, with names for variety, subvariety, form, the dominant taxonomic practice today. The maxim and subform, as well as genus and species ranks. was: ‘‘intergradation [at the boundary between two Cultivated strains, hybrids, subspecies, are also valid geographic replacement forms] is the touchstone of in botany, and subspecies do not necessarily refer trinominalism.’’ Examples from commonly observed specifically to geographic populations, as in zoology. birds which intergrade are, in North America, the The remainder of this article is concerned largely eastern rufous-sided towhee (Pipilo erythrophthalmus with zoological terminology. It is important to realize, erythrophthalmus) replaced in the west by the spotted however, that both the botanical and the zoological towhee (Pipilo erythrophthalmus maculatus), and in codes avoid controversy by not specifying how to Europe the carrion crow (Corvus corone corone) found decide whether a particular group of specimens is a in the south and west, replaced by the hooded crow species or infraspecific taxon. The goal of these codes (Corvus corone cornix) in the north and east. Among is to merely regulate nomenclature once a decision of the ornithologists responsible for this revolution in rank has been reached. _________________________________________________________________________________________________________________________________________ SUBSPECIES, SEMISPECIES, SUPERSPECIES______________________________________________________________________________________________________________________________________ 3 C. Theories of Divergence: Superspecies, Rassenkreise were again considered to be equivalent to species, composed of races or subspecies. However, Semispecies, and Subspecies now there was an additional layer in the taxonomy, of It is hard to imagine the diversity of ideas by which the groups of Rassenkreise that replaced one another systematists of 100 years ago explained geographic geographically, the Artenkreise. Thus an Artenkreis could variation. At that time, evolution by natural selection consist of multiple Rassenkreise. Rensch and many was far from generally accepted, in fact many believed others believed that the subspecies was an incipient that it had been disproved. One of the most influential species, of which the geographic replacement species, ornithologists of the time was Otto Kleinschmidt, who Artenkreise were a further development, until finally believed that all species suddenly came into being long divergence was sufficient