Monotypy

In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. 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.

Theoretical implications

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.

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.

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". 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.

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. 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.

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.

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.

Conservation implications

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.

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.

Examples

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:

Plants

* The division Ginkgophyta is monotypic, containing the single class Ginkgoopsida. This class is also monotypic, containing the single order Ginkgoales, which has only the single family Ginkgoaceae, containing a single genus ''Ginkgo'' with a single species ''Ginkgo biloba''.
* 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 conifer ''Sciadopitys verticillata'' is the only species in the monotypic genus ''Sciadopitys'', and also the only member of the family Sciadopityaceae. Multiple other conifer genera are monotypic, but are members of larger families; examples include '' Cathaya, Diselma, Fitzroya, Glyptostrobus, Metasequoia, Microcachrys, Nothotsuga, Parasitaxus, Saxegothaea, Sequoia, Sequoiadendron, Sundacarpus, Tetraclinis, Thujopsis'' and '' Wollemia''.
* 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.

Animals

* The platypus is the only member of the monotypic genus ''Ornithorhynchus''.
* The aardvark is the only extant member of the genus '' Orycteropus'', the family '' Orycteropodidae'', and the order '' Tubulidentata''.
* 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.
* ''Delphinapterus leucas'' or the beluga whale is the only member of its genus and lacks subspecies.
* ''Dugong dugon'' is the only species in the monotypic genus ''Dugong''.
* ''Homo sapiens'' (humans) are monotypic, as they have too little genetic diversity to have any accepted living subspecies.
* The narwhal is a medium-sized cetacean that is the only member of the monotypic genus ''Monodon''.
* The palmchat is the only member of the genus ''Dulus'' and the only member of the family Dulidae.
* The salamanderfish (''Lepidogalaxias salamandroides'') is the only member of the order Lepidogalaxiiformes, which is the sister group to the remaining euteleosts.
* ''Ozichthys albimaculosus'', the cream-spotted cardinalfish, found in tropical Australia and southern New Guinea, is the type species of the monotypic genus ''Ozichthys''.
* The bearded reedling is the only species in the monotypic genus ''Panurus'', which is the only genus in the monotypic family Panuridae; it does however have three subspecies, so is not strictly monotypic.

File:Amborella trichopoda.jpg, 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''.
File:Beluga Whale Kissing its trainer.jpg, Beluga "kissing" a human trainer; both are monotypical in their genera.
File:Cephalotus follicularis Hennern 3 (cropped).jpg, The family Cephalotaceae has only one genus, ''Cephalotus'', which contains only one species, ''Cephalotus follicularis'', the Australian pitcher plant.

Other

* ''Picomonas judraskeda'' is the only known species in the division '' Picozoa''.

See also

* Glossary of scientific naming
* Monophyly

References

External links

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{{DEFAULTSORT:Monotypic Taxon

Conservation biology
Speciation