Monotypic taxon

In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon.^[1] A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispecific" or "monospecific" is sometimes preferred. In botanical nomenclature, a monotypic 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 in biological 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 organizing biological diversity in a hierarchical system.^[3]

When taxonomists identify a monotypic taxon, this often reflects uncertainty about its relationships rather than true evolutionary isolation. This uncertainty is evident in many cases across different species. For instance, the diatom Licmophora 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 are sister 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 a cladistic 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 through synapomorphies (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 either monophyletic (containing all descendants of a common 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 as aphyly – 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 lichenologist Robert Lücking suggests that the common application of the term monotypic is frequently misleading, "since each taxon by definition contains exactly one type 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 with extinction than average species. Studies have found this pattern particularly pronounced in amphibians, where about 6.56% of monotypic genera are critically endangered, compared to birds and mammals 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 term monotypic 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 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, Amborella trichopoda.
• The flowering plant Breonadia salicina is the only species in the monotypic genus Breonadia.
• The family Cephalotaceae includes only one genus, Cephalotus, and only one species, Cephalotus follicularis – the Albany pitcher plant.
• The division Ginkgophyta 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 division Picozoa.^[10]

• The madrone butterfly is the only species in the monotypic genus Eucheira. However, there are two subspecies of this butterfly, E. socialis socialis and E. socialis westwoodi, which means the species E. socialis is not monotypic.^[11]
• Erithacus rubecula, the European robin, is the only extant member of its genus.^[12]
• Delphinapterus leucas or the beluga whale is the only member of its genus and lacks subspecies.^[13]
• Dugong dugon is the only species in the monotypic genus Dugong.^[14]
• Homo sapiens (humans) are monotypic, as they have too little genetic diversity to harbor any living subspecies.^[15]
• Limnognathia maerski is a microscopic animal and the only species in the monotypic phylum Micrognathozoa.
• The narwhal is a medium-sized cetacean that is the only member of the monotypic genus Monodon.^[16]
• The platypus is the only member of the monotypic genus Ornithorhynchus.
• The salamanderfish is the only member of the order Lepidogalaxiiformes, which is the sister group to the remaining euteleosts.^[17]
• Ozichthys albimaculosus, the cream-spotted cardinalfish, found in tropical Australia and southern New Guinea, is the type species of the monotypic genus Ozichthys.^[18]
• The bearded reedling is the only species in the monotypic genus Panurus, 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, namely Amborella trichopoda.
• 
  Beluga "kissing" a human trainer; 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.

• Glossary of scientific naming
• Monophyly

• The dictionary definition of monotypic at Wiktionary