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So I found a fairly complex Rhododendron subgenus to subsection classification list, and despite there being genus > subgenus > section > subsection > species > subspecies > variety > subvariety > form, the only parts that seem to make up a name when things go beyond the species level is picking the last one after that (so subspecies, variety, subvariety, or form). So we have examples like Capsicum annuum var. glabriusculum or Rhododendron campylogynum var. leucanthum.

https://en.wikipedia.org/wiki/Infraspecific_name

You can also have the cultivar epithet added after species.

So my question is, is it always genus species [possibly one deeper]? Or are there any other situations I missed? Also, it sounds like you can insert arbitrary number of levels between the ones you are taught in grade-school (kingdom / phylum / class / etc.), you might insert a few layers there when necessary like phylum > subphylum > superclass > class sort of thing. Like for family we added 4:

  • Superfamily
  • Family
  • Subfamily
  • Tribe
  • Subtribe

So in the end it might be something like this, or even more!?:

  1. empire
  2. kingdom
  3. superphylum
  4. phylum
  5. subphylum
  6. superfamily
  7. family
  8. subfamily
  9. tribe
  10. subtribe
  11. genus
  12. subgenus
  13. section
  14. subsection
  15. species
  16. subspecies
  17. variety
  18. subvariety
  19. form

So from 7 to about 20 levels! They don't make that obvious in your initial pass at learning this. Do I have the naming system correct then? If there is anything below the species it gets a 3rd item in the name. If there is anything between genus and species, that is ignored in the name.

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2 Answers 2

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Binomial nomenclature is the name for the formal naming of species in "Genus species" form. Genus species should always be a unique identifier (also see Are All Taxonomic Groups Uniquely Named?), so this is the "official name" of a given species.

However, as you've pointed out, sometimes after a species is described it is found useful to differentiate between subspecies or smaller varieties, especially in agriculture where artificial selection has taken one native species and transformed it into many cultivars that have become so distinct from each other that they'd hardly be considered the same species had they been discovered that way in the first place (and very unlikely to evolve those discordant traits naturally without some of the reproductive barriers that lead to speciation).

Note that all of these designations are somewhat arbitrary, besides their order. They are all just a way of imposing a categorization for the convenience of humans in describing relationships between different living things.

Some more reading:

https://en.wikipedia.org/wiki/Phylogenetic_nomenclature

https://en.wikipedia.org/wiki/Evolutionary_taxonomy

https://en.wikipedia.org/wiki/Taxonomic_rank

How could humans have interbred with Neanderthals if we're a different species?

Usage of "lineage" over "species"

Why is traditional rank based taxomomy considered by some as logically inconsistent with phylogenetic knowledge?

Difference between Category, Rank and Taxon

Imagine life as a very, very big tree. You can call a part the "trunk", that's pretty well-defined. That might be the original common ancestor of all life, or perhaps all eukaryotes, or perhaps all vertebrates; you can choose where you want to start. The leaves, also, are relatively well-defined. Those are the species. But some leaves have a complex shape and you might even debate when a leaf is one leaf and when it's actually a collection of leaves. In between, though? We use some names for tree parts. Limbs, branches, twigs, etc. But if you want to get into more detail, you may eventually find yourself needing a word to describe something between a branch and a twig, and then sometimes between that thing and a proper twig, etc. There is no real behavior of the universe that is based on these classification points, it's just convenient to draw a line somewhere to say "okay, I want to talk about all parts of the tree past this branch, so I'll give them a name". Nothing constrains the tree of life to grow in any particular pattern where there are a set number of branches between particular extant species; you can easily have leaves that grow directly out of the trunk and serve as their own branch, their own twig, etc.

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  • $\begingroup$ Does NCBI not use the new PhyloCode system then? $\endgroup$
    – Lance
    Jul 8, 2022 at 15:20
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    $\begingroup$ @Lance I have no idea, I assume PhyloCode is some sort of new standard? See xkcd.com/927 and also note that just because there is a standard does not mean it is not ultimately arbitrary in some fashion. For example, the length of a meter is both entirely arbitrary and also exactly the distance light travels in 1/299792458 of a second. Of course, a second is also an arbitrary unit of time, even if it is also precisely 9,192,631,770 periods of the radiation corresponding to the transition between the hyperfine levels of the unperturbed ground state of the 133Cs atom. $\endgroup$
    – Bryan Krause
    Jul 8, 2022 at 15:31
  • $\begingroup$ (from en.wikipedia.org/wiki/Metre and en.wikipedia.org/wiki/Second ) $\endgroup$
    – Bryan Krause
    Jul 8, 2022 at 15:31
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The rules may be different for zoological names, but for botanical names:

Is it always genus species [possibly one deeper]?

This is governed by the International Code of Nomenclature for algae, fungi and plants, Article 24.1, and the answer is yes, the name is always genus species [connecting term infraspecific epithet].

The Code provides the following example:

Ex. 1. Saxifraga aizoon subf. surculosa Engl. & Irmsch. This taxon may also be referred to as Saxifraga aizoon var. aizoon subvar. brevifolia f. multicaulis subf. surculosa Engl. & Irmsch.; in this way a full classification of the subforma within the species is given, not only its name.

Here, the name is "Saxifraga aizoon subforma surculosa" (where subforma can be shortened to subf.), and it is common to list the authors who described that taxon (here, Engler and Irmscher). However, that taxon is placed within a hierarchy of variety, subvariety, and form, which may be listed if the full classification is needed.

One consequence of this is that no other subform within Saxifraga aizoon may have the epithet "surculosa", even if its classified in a different variety, subvariety or form.

It sounds like you can insert arbitrary number of levels

Also yes. Article 2 states, "Every individual organism is treated as belonging to an indefinite number of taxa at consecutively subordinate ranks, among which the rank of species is basic."

If there is anything between genus and species, that is ignored in the name.

Correct, if you're naming a species or infraspecies, only use the genus, specific epithet, and infraspecific epithet.

You can, of course refer to those intermediate ranks if you need to, such as Primula section Dodecatheon.

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