Monotypic taxon

The aardvark is an example of a species that is the sole extant member of its genus, family, and order

In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon.<ref>Mayr E, Ashlock PD. (1991). Principles of Systematic Zoology (2nd ed.). McGraw-Hill. Template:ISBN</ref> A monotypic species is one that does not include subspecies or smaller, infraspecific taxa.Template:Contradictory inline 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 in which a genus and a single species are simultaneously described.<ref>Template:Cite book</ref>

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.<ref name="Ebach & Williams 2010">Template:Cite journal</ref>

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.<ref name="Ebach & Williams 2010"/>

Some taxonomists argue against monotypic taxa because they reduce the information content of biological classifications. As taxonomists Backlund and Bremer explain in their critique, Template:" 'Monotypic' taxa do not provide any information about the relationships of the immediately subordinate taxon".<ref name="Backlund 1998">Template:Cite journal</ref> When a monotypic taxon is sister to a single larger group, it might be merged into that group; however, when it is sister to multiple other groups, it may need to remain separate to maintain a natural classification.<ref name="Backlund 1998"/>

From a cladistic perspective, which focuses on shared derived characteristics to determine evolutionary relationships, the theoretical status of monotypic taxa is complex. Some argue that 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.<ref name="Platnick 1976">Template:Cite journal</ref> 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.<ref name="Potter 2005">Template:Cite journal</ref>

Monotypic taxa are part of a broader challenge in biological classification known as aphyly – situations in which 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.<ref name="Ebach & Williams 2010"/>

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.<ref>Template:Cite journal</ref>

Conservation implications

Species in monotypic genera tend to be more threatened with extinction than average species. Studies have found this pattern to be particularly pronounced in amphibians, of which about 6.56% of monotypic genera are critically endangered, compared to birds and mammals, of which around 4.54% and 4.02%, respectively, of monotypic genera face critical endangerment.<ref name="Vargas 2023">Template:Cite journal</ref>

Studies have found that extinction of monotypic genera is particularly associated with island species. Among 25 documented extinctions of monotypic genera studied, 22 occurred on islands, with flightless birds being particularly vulnerable to human impact.<ref name="Vargas 2023"/>

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.<ref>Template:Cite journal</ref>
• In the order Amborellales, there is only one family, Amborellaceae, 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.<ref>Template:Cite web</ref> 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.<ref>Template:Cite journal</ref>
• 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.<ref>Template:Cite journal</ref>
• Delphinapterus leucas, the beluga whale, is the only member of its genus and lacks subspecies.<ref>Template:Cite report</ref>
• Dugong dugon is the only species in the monotypic genus Dugong.<ref>Template:Cite book</ref>
• Homo sapiens (humans) are monotypic, as they have too little genetic diversity to have any accepted living subspecies.<ref>Template:Cite journal</ref>
• The narwhal, a medium-sized cetacean, is the only member of the monotypic genus Monodon.<ref>Template:Cite report</ref>
• The palmchat is the only member of the genus Dulus and the only member of the family Dulidae.<ref name="iucn status 12 November 2021">Template:Cite iucn</ref>
• The salamanderfish (Lepidogalaxias salamandroides) is the only member of the order Lepidogalaxiiformes, which is the sister group to the remaining euteleosts.<ref>A phylogenomic approach to reconstruct interrelationships of main clupeocephalan lineages with a critical discussion of morphological apomorphies</ref>
• Ozichthys albimaculosus, the cream-spotted cardinalfish, which is found in tropical Australia and southern New Guinea, is the type species of the monotypic genus Ozichthys.<ref>Template:Cite journal</ref>
• 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 it is not strictly monotypic.<ref name="IOC">Template:Cite web</ref>

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  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.
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  Beluga "kissing" a human trainer; both are monotypical in their genera.
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  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.<ref name="Seenivasan2013">Template:Cite journal</ref>

See also

• Glossary of scientific naming
• Monophyly

External links

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