Inbiology, amonotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon.[1] A monotypicspecies is one that does not includesubspecies or smaller, infraspecific taxa. In the case ofgenera, the term "unispecific" or "monospecific" is sometimes preferred. Inbotanical nomenclature, amonotypic genus is a genus in the special case where a genus and a single species are simultaneously described.[2]
Monotypic taxa present several important theoretical challenges inbiological classification. One key issue is known as "Gregg's Paradox": if a single species is the only member of multiple hierarchical levels (for example, being the only species in its genus, which is the only genus in its family), then each level needs a distinct definition to maintain logical structure. Otherwise, the different taxonomic ranks become effectively identical, which creates problems for organizingbiological diversity in ahierarchical system.[3]
When taxonomists identify a monotypic taxon, this often reflects uncertainty about its relationships rather than true evolutionaryisolation. This uncertainty is evident in many cases across different species. For instance, the diatomLicmophora juergensii is placed in a monotypic genus because scientists have not yet found clear evidence of its relationships to other species.[3]
Some taxonomists argue against monotypic taxa because they reduce the information content of biological classifications. As taxonomists Backlund and Bremer explain in their critique, "'Monotypic' taxa do not provide any information about the relationships of the immediately subordinate taxon".[4] When monotypic taxa aresister to a single larger group, they might be merged into that group; however, when they are sister to multiple other groups, they may need to remain separate to maintain a natural classification.[4]
From acladistic perspective, which focuses on shared derived characteristics to determine evolutionary relationships, the theoretical status of monotypic taxa is complex. Some argue they can only be justified when relationships cannot be resolved throughsynapomorphies (shared derived characteristics); otherwise, they would necessarily exclude related species and thus be paraphyletic.[5] However, others contend that while most taxonomic groups can be classified as eithermonophyletic (containing all descendants of acommon ancestor) or paraphyletic (excluding some descendants), these concepts do not apply to monotypic taxa because they contain only a single member.[6]
Monotypic taxa are part of a broader challenge in biological classification known asaphyly – situations where evolutionary relationships are poorly supported by evidence. This includes both monotypic groups and cases where traditional groupings are found to be artificial. Understanding how monotypic taxa fit into this bigger picture helps identify areas needing further research.[3]
The German lichenologistRobert Lücking suggests that the common application of the term monotypic is frequently misleading, "since each taxon by definition contains exactly onetype and is hence "monotypic", regardless of the total number of units", and suggests using "monospecific" for a genus with a single species, and "monotaxonomic" for a taxon containing only one unit.[7]
Species in monotypic genera tend to be more threatened withextinction than average species. Studies have found this pattern particularly pronounced inamphibians, where about 6.56% of monotypic genera arecritically endangered, compared tobirds andmammals where around 4.54% and 4.02% of monotypic genera face critical endangerment respectively.[8]
Studies have found that extinction of monotypic genera is particularly associated with island species. Among 25 documented extinct monotypic genera studied, 22 occurred on islands, with flightless animals being particularly vulnerable to human impacts.[8]
Just as the termmonotypic is used to describe a taxon including only one subdivision, the contained taxon can also be referred to as monotypic within the higher-level taxon, e.g. a genus monotypic within a family. Some examples of monotypic groups are:
In theorderAmborellales, there is only onefamily, Amborellaceae, and there is only one genus,Amborella, and in this genus there is only one species,Amborella trichopoda.
The flowering plantBreonadia salicina is the only species in the monotypic genusBreonadia.
The family Cephalotaceae includes only one genus,Cephalotus, and only one species,Cephalotus follicularis – the Albany pitcher plant.
ThedivisionGinkgophyta is monotypic, containing the single class Ginkgoopsida. This class is also monotypic, containing the single order Ginkgoales.[9]
Picomonas judraskeda is the only known species in the divisionPicozoa.[10]
Themadrone butterfly is the only species in the monotypic genusEucheira. However, there are two subspecies of thisbutterfly,E. socialis socialis andE. socialis westwoodi, which means the speciesE. socialis is not monotypic.[11]
Thenarwhal is a medium-sizedcetacean that is the only member of the monotypic genusMonodon.[16]
Theplatypus is the only member of the monotypic genusOrnithorhynchus.
Thesalamanderfish is the only member of the order Lepidogalaxiiformes, which is the sister group to the remainingeuteleosts.[17]
Ozichthys albimaculosus, the cream-spotted cardinalfish, found in tropical Australia and southern New Guinea, is the type species of the monotypic genusOzichthys.[18]
Thebearded reedling is the only species in the monotypic genusPanurus, which is the only genus in the monotypic family Panuridae.[19]
Canines form the only living subfamily of the dog family,Canidae
In the order Amborellales, there is only one family, Amborellaceae and there is only one genus,Amborella, and in this genus there is only one species, namelyAmborella trichopoda.
Beluga "kissing" a humantrainer; both are monotypical in their genera.
The family Cephalotaceae has only one genus,Cephalotus, which contains only one species,Cephalotus follicularis, the Australian pitcher plant.
^abBacklund, Anders; Bremer, Kåre (1998). "To Be or Not to Be. Principles of Classification and Monotypic Plant Families".Taxon.47 (2):391–400.doi:10.2307/1223768.JSTOR1223768.
^Platnick, Norman I. (1976). "Are Monotypic Genera Possible?".Systematic Zoology.25 (2):198–199.doi:10.2307/2412749.JSTOR2412749.
^Potter, Daniel; Freudenstein, John V. (2005). "Character-based phylogenetic Linnaean classification: taxa should be both ranked and monophyletic".Taxon.54 (4):1033–1035.doi:10.2307/25065487.JSTOR25065487.
^Lücking, Robert (2019). "Stop the abuse of time! Strict temporal banding is not the future of rank-based classifications in Fungi (including lichens) and other organisms".Critical Reviews in Plant Sciences.38 (3): 199–253 [216].Bibcode:2019CRvPS..38..199L.doi:10.1080/07352689.2019.1650517.
^Kevan, P. G.; Bye, R. A. (1991). "The natural history, sociobiology, and ethnobiology of Eucheira socialis Westwood (Lepidoptera: Pieridae), a unique and little-known butterfly from Mexico".Entomologist.110:146–165.
^Fraser, Thomas H. (14 August 2014). "A new genus of cardinalfish from tropical Australia and southern New Guinea (Percomorpha: Apogonidae)".Zootaxa.3852 (2):283–293.doi:10.11646/zootaxa.3852.2.7.PMID25284398.
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The dictionary definition ofmonotypic at Wiktionary
