= Type species
A <type species> is the <species> serving as the <nomenclatural type> of a <genus (biology)> under a naming code that uses this form of typification. It anchors application of the genus name. It is not necessarily an average, an ancestor or the most familiar member of the <genus (biology)>.
In <prokaryotic nomenclature>, the Code explicitly identifies the type of a genus or subgenus with a type species and specifies how that species is fixed. If a genus originally included only one species when validly published, that species is its type even without an explicit designation. These are naming rules, not measurements of evolutionary distance.<International Code of Nomenclature of Prokaryotes. Prokaryotic Code (2025 Revision) by Aharon Oren and others>[Rule15 and Rules20a–20c, printed pp.10 and13: types, designation and a genus originally containing one species]{ref}
An original hypothetical example separates the reference from a summary statistic. Suppose a genus contains species T, U and V, with T the designated <type species>. A measured trait has values1,10 and11, whose <arithmetic mean> is $22/3\approx7.33$. U lies nearer that mean, but this does not make U the type. The designation and the statistical comparison answer different questions.
Now suppose new evidence from <phylogenetics> divides the hypothetical group into T+U and V. The existing reference is associated with the group containing T; how the remaining group is named requires the relevant naming rules and publication history. Merely discovering that V has an unusual trait does not transfer the reference to it.
The anchor has another level beneath it. In the ordinary prokaryotic case a species itself has a <type strain>, whereas the genus has a <type species>. A stored culture accession, a species reference and a genus reference therefore do not all denote the same rank or the same physical object.<International Code of Nomenclature of Prokaryotes. Prokaryotic Code (2025 Revision) by Aharon Oren and others>[Table2 under Rule15 and Rule18a: species and genus types; Rule20a: type species]{ref}
A <type species> stabilizes the connection between a name and its reference while <taxonomy (biology)> remains open to revised evidence. The detailed code governing the organisms concerned must be stated; one nomenclatural example should not silently impose every prokaryotic rule on other branches of biology.
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