Subspecies, Semispecies, Superspecies
James Mallet, Harvard University, Cambridge, MA, United Statesr 2021 Elsevier Inc. All rights reserved.

Introduction
A Brief History of Subspeciﬁc Taxonomy
Variation Below the Level of Species
The Trinominal Revolution in Zoology
Theories of Divergence: Superspecies, Semispecies, and Subspecies
The Subspecies Today
Modern Views of Subspecies and Semispecies
Conclusion: Subspecies, Species, and Conservation
Further Reading
References

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Abstract
Distinct populations that replace each other geographically were recognized either as full species or as lower-level varieties or forms
under the original Linnaean taxonomy. In zoology, a practical resolution of this ambiguity took place largely between about 1880 and
1920, with the recognition of a single additional taxonomic rank below the species, the geographic subspecies. In botany, many
infraspeciﬁc ranks remain valid. Since the 1980s, the fashion has changed once more. Some systematists are again recognizing
geographical replacement forms as full species, even when they blend together freely at their boundaries.

Key Points• It is generally agreed today that evolution is continuous across the species boundary.
• Below the level of species, there may be varieties, ecotypes, ecological races, and geographical races or “subspecies”.
• Above the level of species there are a variety of groups, including genera, families, and phyla. The term “superspecies” isused at the lowest of these levels for a collection of “semi-species” that replace one another geographically.

• A historical approach is adopted to outline past and current usages of these terms.

Introduction
Since the realization that evolution allowed continuity between varieties, species and genera, a variety of ranks in the hierarchy ofthe diversity of life have been recognized. These include recognition of Linnean species, as well as stable groupings below the levelof species, and groupings of species.

A Brief History of Subspeciﬁc Taxonomy
Variation Below the Level of Species
Since the invention of binominal nomenclature by Linnaeus, there has been a conﬂict between “splitters” who named more or lesswell-deﬁned local populations as separate species, and “lumpers” who ignored geographic variation, and united local variants into asingle species. The problem was compounded by early systematists' belief that species had an Aristotelian essence, each fundamentallydifferent from similar essences underlying other species. To Linnaeus' followers, it seemed important to decide which level of variation
was fundamental. The terms “genus” and “species” both result from Aristotelian philosophy, and although Linnaeus is usually creditedwith establishing the species as the basal taxonomic unit, he confused matters, after recognizing that some plant species were of hybrid
origin, by suggesting that genera were a more important taxonomic level (i.e., a separately created kind) than species.
Once evolution was accepted, it became clear that variation at all levels in the taxonomic hierarchy was due to more or lesssimilar causes; the only difference between variation above the level of genus or species and below was one of degree. Darwin
realized that species could evolve from intraspeciﬁc varieties, and he used the term “species” in a new and nonessentialist sense:"the complete absence, in a well-investigated region, of varieties linking together two closely allied forms, is probably the mostimportant of all the criterions of their speciﬁc distinctness." "Geographical distribution is often brought into play unconsciously

Encyclopedia of Biodiversity 3rd edition

doi:10.1016/B978-0-12-822562-2.00023-2

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Subspecies, Semispecies, Superspecies

and sometimes consciously; so that forms living in two widely separated areas, in which most of the other inhabitants are
speciﬁcally distinct, are themselves usually looked at as distinct; but in truth this affords no aid in distinguishing geographical racesfrom the so-called good or true species” (Darwin, 1874). Using this criterion, Darwin classiﬁed all human races as members of thesame species. Darwin showed convincingly that there was no essential difference between species and varieties; species were simply
varieties, which had diverged more, and which could coexist without intermediates being common. However, with his term
“varieties” Darwin did not clearly distinguish between polymorphic variants within populations and the identiﬁable geographic
populations normally today considered as geographic races or subspecies. To Darwin the distinction was unimportant, because
polymorphic variants, clinal variation, geographic races or subspecies, and good species formed a continuum. Darwin demonstrated that this continuum was excellent evidence for an evolutionary origin of the taxa we call species.

The Trinominal Revolution in Zoology
Many systematists wished to preserve the purity of the simple genus–species binominal nomenclature, but by the 1850s, there
were enormous stresses. It began to be realized that identiﬁable geographic replacement forms were an important intermediatestage between insigniﬁcant local variants and good species. Some lumped these replacement forms as varieties within species,
whereas others continued describing these replacement forms as separate species: practices varied widely, leading to considerable
confusion. Although some Europeans had long advocated naming marked geographic forms as subspecies, the accumulation ofmajor North American museum collections during the great push of colonization and railway construction westwards wasprobably the most important catalyst of a revolutionary new systematics. In this new approach, nomenclature consisted of atrinominal: genus–species–subspecies, which is still the dominant taxonomic practice today. The maxim was: “intergradation(at the boundary between two geographic replacement forms) is the touchstone of trinominalism.” Examples from commonly
observed birds, which intergrade are, in North America, the eastern rufous-sided towhee (Pipilo erythrophthalmus erythrophthalmus)
replaced in the west by the spotted towhee (Pipilo erythrophthalmus maculatus), and in Europe the carrion crow (Corvus corone corone)found in the south and west, replaced by the hooded crow (Corvus corone cornix) in Italy, and the north and east of Europe. Amongthe ornithologists responsible for this revolution in North America was Elliott Coues, who published a catalog of American birds
in 1872 incorporating an early version of this trinominal nomenclature, in which subspecies were preﬁxed by “var.” and Robert
Ridgway, who ﬁnally dropped the “var”. in his own 1881 summary of North American bird nomenclature.
The American Ornithologists' Union soon adopted this policy, and the idea then spread to Europe, particularly England where
Walter Rothschild began amassing his vast collection of birds and butterﬂies, and had hired excellent and productive staff, theornithologist Ernst Hartert and the entomologist Karl Jordan, to curate and describe the new material. Jordan was particularly
important in spreading the idea of trinominal nomenclature to entomologists, and in promoting the abandonment of othersubspeciﬁc nomenclature. He was regarded by the Rothschilds as the clever member of the staff (Rothschild, 1983), and publishedenormous systematic treatises as well as papers on the theory of systematics, justifying trinominal nomenclature and the recognition of the subspecies as a valid, identiﬁable taxon in its own right. Both Jordan and Hartert were Germans who contributed toand read European as well as English journals, and in Germany a similar revolution was taking place. Thus, these systematic ideas
were able to spread to the rest of Europe in the time when science was often highly parochial. The standard trinominal
nomenclature for subspecies, and the abandonment of other named ranks, such as semispecies and superspecies, soon became
established in the International Code of Zoological Nomenclature, and has remained there ever since.
Meanwhile, botany, Linnaeus' major expertise, remained much less prescriptive. Many infraspeciﬁc ranks are still consideredvalid taxa under the International Code of Botanical Nomenclature. For instance, Saxifraga aizoon var. aizoon subvar. brevifolia
f. multicaulis subf. surculosa is a valid scientiﬁc name in botany, with names for variety, subvariety, form, and subform as well as
genus and species ranks. Cultivated strains, hybrids, subspecies, are also considered valid nomenclatural ranks in botany, whereas
subspecies do not necessarily refer speciﬁcally to geographic populations, as is the case in zoology. The remainder of this article isconcerned largely with zoological terminology. It is important to realize, however, that neither botanical nor zoological codes
confront controversy by specifying how to decide whether a particular group of specimens is a species or infraspeciﬁc taxon. The
goal of these codes is to merely regulate nomenclature once a decision of rank has been reached.

Theories of Divergence: Superspecies, Semispecies, and Subspecies
It is hard to imagine the diversity of ideas by which the systematists of 100 years ago explained geographic variation. At that time,
evolution by natural selection was far from generally accepted, in fact many believed that it had been disproved. One of the mostinﬂuential ornithologists of the time was Otto Kleinschmidt, who believed that all species suddenly came into being long ago, andsince then had remained completely separate. Replacement forms or subspecies had diverged from the main species but, inKleinschmidt's view, subspecies could never evolve into new species as the Darwinians supposed. To distinguish his new species
concept from the older one in which geographical replacements might be named as separate species, Kleinschmidt called his
theory of variation the Formenkreis (ring of forms) theory. The Formenkreis theory ﬁtted neatly with, and indeed promoted the newpractices of naming subspecies and trinominal nomenclature.
In those times many somewhat peculiar explanations competed to explain geographic variation and speciation, including
Kleinschmidt's “nonspeciation” theory, Lotsy's hybridization theory, saltational evolution via mutation, inheritance of acquiredSubspecies, Semispecies, Superspecies3characters, as well as natural selection. In Britain, Jordan and Rothschild argued eloquently and inﬂuentially against any new
terminology (including Formenkreis) that had theoretical implications and proposed incorporating as little evolutionary theory
into taxonomy as possible, in view of the lack of agreement among scientists at the time. Rothschild and Jordan (1895, 1903),
supported by Hartert, agreed both with the nomenclatural practice of naming subspecies, and that subspecies were valid real taxa
which could evolve into full species. They argued that the Linnaean term “species” should be retained for the whole group of races,
and that the geographic races were not true species, they were simply subspecies or incipient species.
Others felt that the term species was too emotive to be used in the new, multiple-subspecies sense. Some scientists continued
following the Formenkreis doctrine, and had begun to name quite distinct taxa, which did not intergrade at their boundaries, as
subspecies. This situation led in the 1920s and 1930s to the neo-Darwinian ornithologist Bernhard Rensch scrapping the term
Formenkreis because of its theoretical limitations, and instead substituting two new terms, Rassenkreis (circle of races) and Artenkreis(circle of species). Rassenkreise were again considered to be equivalent to species, composed of races or subspecies. However, now
there was an additional layer in the taxonomy, of groups of Rassenkreise that replaced one another geographically, the Artenkreise.
Thus an Artenkreis could consist of multiple Rassenkreise. Rensch and many others believed that the subspecies was an incipient
species, of which the geographic replacement species, Artenkreise were a further development, until ﬁnally divergence was sufﬁcient
to allow complete geographic overlap, whereupon new Rassenkreise could again be formed.
These terms did not catch on, and most scientists came to the conclusion that the Rassenkreise were equivalent to the species
taxa used by Linnaeus and Darwin. Probably a major reason that we do not use these multiple taxonomic terms in zoology is dueto the proliﬁc work published in English by another German, Ernst Mayr. Mayr had worked for Walter Rothschild and knew
Hartert. After Walter Rothschild was blackmailed by a lover, his enormous bird collection of 280,000 skins was sold in 1932 to theAmerican Museum of Natural History, where Mayr happened to have been hired as a curator. Mayr's experience of ornithology,
contact with the European literature, and burgeoning friendship with the geneticist Theodosius Dobzhansky (who convinced himof the lack of evidence for inheritance of acquired characters), lent a unique opportunity to inﬂuence the course of systematics andevolutionary biology. Mayr did not waste this opportunity. Mayr used ideas underlying Rensch's terms but renamed them inEnglish. The Rassenkreis became simply the species or polytypic species, with its geographic races being subspecies, whereas the
Artenkreis became the superspecies, and its component parts semispecies, that is, replacement species, not very divergent but which
may hybridize occasionally where they overlap. Mayr successfully blended the local species concept of Poulton and Dobzhansky
based on interbreeding with the geographic Rassenkreis idea of species, incorporating Jordan and Rothschild's ideas about subspecies. He renamed this combination of ideas “the biological species concept”, a term, which has since remained strongly
associated with Mayr's name. His many inﬂuential articles and books promoted a new program of species study, a science of thespecies that is still with us today. Central to Mayr's system was the belief that discrete taxa such as species or subspecies would
normally diverge in “allopatry”, that is, in complete geographic isolation.

The Subspecies Today
Modern Views of Subspecies and Semispecies
The views of Darwin, Wallace, Rensch, and Mayr that geographic replacement forms, subspecies and semispecies, are in factincipient species, has few critics today. Most geographic replacement species (i.e., semispecies, which intergrade only rarely when
they meet) must indeed have evolved from previously interbreeding subspecies. Modern genetic data have done nothing to cast
doubt on this idea. Meanwhile, superspecies became a specialized term reserved for groups of semispecies that intergraded little attheir boundaries. Because neither of these are valid ranks in the zoological or botanical codes, taxonomists normally name such
replacement semispecies at the species rank.
Under the trinominal approach, taxonomists were now required to describe subspecies, which has been never seen as aparticularly noble activity in comparison to the description of species, especially recently. A strong attack on the zoologicalsubspecies was mounted by Wilson and Brown (1953). Both were systematists working on ants, a group particularly riddled withpoorly conceived trinominals and other named varieties at the time. Wilson and Brown argued that subspecies rarely, if ever, could
be justiﬁed on the basis of multiple characters, and that therefore they were not real taxa. The only real taxa were species, which ina sense were self-deﬁning because interbreeding prevented divergent genes from ﬂowing from one species to another. Subspecies
that interbred at their boundaries, however, were not so endowed, so that genes and morphological characters could ﬂow between
them. Wilson and Brown put forward examples of subspecies that undoubtedly would be hard to justify on multiple charactergrounds. This single paper was enormously inﬂuential on systematics in the US, and generations of insect systematists trained at
Harvard and Cornell, where Wilson and Brown worked, together with their own many intellectual descendants, and their students'
students in turn, have eschewed the practice of naming subspecies.
However, recent work shows that many subspecies separated by hybrid zones do in fact differ at multiple morphological,
behavioral, and genetic characters (Barton and Hewitt, 1985). For instance, the toad Bombina bombina meets its relative Bombina
variegata across a broad front in Europe. The two forms hybridize freely in the contact zone – although the hybrids can be shown tosuffer some inviability – and so should really be classiﬁed as members of the same species under polytypic or biological species
concepts. However, it has always seemed natural to place such well-deﬁned forms in separate species in spite of the fact they havenot truly speciated in the sense of failing to form hybrid swarms whenever they meet. The two taxa differ strongly in call,4

Subspecies, Semispecies, Superspecies

morphology, skin thickness, the sizes of water bodies used for breeding, and egg size, as well as in mitochondrial DNA and protein
sequences. The levels of differentiation suggest that the Bombina taxa have evolved separately for many millions of years. Thissituation of multiple character changes has now been shown to be true across many examples of subspecies as well as (semi-)
species separated by hybrid zones. Effective gene ﬂow can be shown to be almost completely blocked by hybrid zones such asthese, even if hybridization is frequent (Barton and Hewitt, 1985). Thus, although some named subspecies undoubtedly merited
Wilson and Brown's scorn, genetic evidence shows that there are plenty of local replacement forms, which hybridize at theirboundaries but which do form real identiﬁable taxa that are not reproductively isolated, and are therefore valid subspecies under
the Wilson and Brown criteria.

Conclusion: Subspecies, Species, and Conservation
This opposition among modern taxonomists to subspecies taxa can be traced as one catalyst of the recent diagnostic version of thephylogenetic species concept. The adherents of this view of species, led by the ornithologist (Cracraft, 1989), proposed a radical
species concept so that even a single ﬁxed character difference may deﬁne a geographic form as a separate species. Multiple
character justiﬁcation is not considered necessary, even at the species level. The practical result of this new concept is that many
local forms, having been downgraded to subspecies in polytypic species, are again being recognized as species, leading totaxonomic inﬂation (Isaac et al., 2004). In birds and butterﬂies, which often have many morphologically or genetically distinct
subspecies, this could easily result in a 2–10-fold increase in the number of species.
It is probable that the revision of geographic forms upward to the level of species is being driven not only by theoreticalconsiderations, but also by existing conservation legislation, which proposes that endangered species are the valuable units to beconserved. If a reserve contains a taxon recognized as a species rather than just as a local subspecies, it may be seen as morevaluable for conservation purposes. The potential consequences for biodiversity and conservation of the continued instability ofthe term “species” are detailed in the article Species, Concepts of. Today's conservationists are reducing emphasis on species
conservation, and are becoming increasingly aware of biodiversity at all the levels of the hierarchy of life, including well-marked
subspecies. Thus, the legislative need for differentiating local races as species may ultimately become less important provided that
future legislation falls more into line with the prevailing biological thought.

Further Reading
Much of the historical overview in this article is covered by the excellent reviews of (Stresemann, 1936, 1975; Mayr, 1982), and(Rothschild, 1983), as well as by other sources already cited. For plants, a useful historical summary of variation below the species
level is given by Lowry (2012). A recent biography of Karl Jordan (Johnson, 2012) and work on the connections between thespecies concepts of Darwin, Wallace, Poulton, Karl Jordan, and Ernst Mayr (Mallet, 2004a,b, 2008, 2009) may also be of interest.

References
Barton, N. H., Hewitt, G. M., 1985. Analysis of hybrid zones. Annual Review of Ecology and Systematics 16, 113–148.
Cracraft, J., 1989. Speciation and its ontology: The empirical consequences of alternative species concepts for understanding patterns and processes of differentiation.
In: Otte, D., Endler, J. A. (Eds.), Speciation and its consequences. Sunderland, MA: Sinauer Associates, pp. 28–59.
Darwin, C., 1874. The descent of man and selection in relation to sex, second ed. London: John Murray.
Isaac, N. J. B., Mallet, J., Mace, G. M., 2004. Taxonomic inﬂation: Its inﬂuence on macroecology and conservation. Trends in Ecology and Evolution 19, 464–469.
Johnson, K. R., 2012. Ordering life karl jordan and the naturalist tradition. Baltimore: Johns Hopkins University Press.
Lowry, D. B., 2012. Ecotypes and the controversy over stages in the formation of new species. Biological Journal of the Linnean Society 106, 241–257.
Mallet, J., 2004a. Perspectives poulton, wallace and jordan: How discoveries in Papilio butterﬂies initiated a new species concept 100 years ago. Systematics and Biodiversity
1, 441–452.
Mallet, J., 2004b. Species problem solved 100 years ago. Nature 430, 503.
Mallet, J., 2008. Mayr's view of darwin: Was darwin wrong about speciation? Biological Journal of the Linnean Society 95, 3–16.
Mallet, J., 2009. Alfred russel wallace and the darwinian species concept: His paper on the swallowtail butterﬂies (Papilionidae) of 1865. Gayana 73, 42–54.
Mayr, E., 1982. The growth of biological thought: Diversity evolution, and inheritance. Cambridge, MA: Belknap.
Rothschild, M., 1983. Dear lord rothschild: Birds butterﬂies and history. London: Hutchinson.
Rothschild, W., Jordan, K., 1895. A revision of the Papilios of the eastern hemisphere, exclusive of Africa. Novitates Zoologicae 2, 167–463.
Rothschild, W., Jordan, K., 1903. A revision of the lepidopterous family Sphingidae novitates zoologicae. 9. Wentworth Press.
Stresemann, E., 1936. The Formenkreis theory. Auk 53, 150–158.
Stresemann, E., 1975. Ornithology from aristotle to the present. In: Epstein, H. J., Epstein, C., Cottrell, G. W. (Eds.), Foreword and epilogue. Harvard University Press.
Wilson, E. O., Brown, W. L., 1953. The subspecies concept and its taxonomic application. Systematic Zoology 2, 97–111.