Within a species there may be subspecies that are named using trinomial nomenclautre. For example the Grizzly Bear, Ursus arctos horribilis is a subspecies of the Brown Bear Ursus arctos.

The dictionary definitions of subspecies seldom expand beyond this:

A taxonomic category that ranks below species, usually a fairly permanent geographically isolated race.

I'm assuming redefining species into subspecies is not a completely arbitrary process as the definitions would imply, but is based on biology. What are the biological criteria for a group within a species to be "upgraded" to a subspecies?

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    $\begingroup$ This is the definition by ICZN which is equally un-informative "The species-group rank below species; the lowest rank at which names are regulated by the Code. (2) A taxon at the rank of subspecies." $\endgroup$
    Commented Sep 14, 2015 at 13:34
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    $\begingroup$ @WYSIWYG The definitions are all very tortological rather than insightful. Of course it's below a species rank! What else could 'sub' mean in this context? There must be criteria that make some strains, or groups biologically different enough to be subspecies, but that threshold is apparently hard to come by if you're not 'in the know'. $\endgroup$
    – James
    Commented Sep 15, 2015 at 0:19
  • $\begingroup$ The wikipedia article is more helpful, if accurate: "a polytypic species has two or more subspecies, races, or more generally speaking, populations that need a separate description" and "A monotypic species has no distinct population or races, or rather one race comprising the whole species. A taxonomist would not name a subspecies within such a species." $\endgroup$
    – jzx
    Commented Sep 15, 2015 at 1:11
  • $\begingroup$ @jzx It sounds better until you realise that it's still missing that nugget of information; specifically what factors warrant the 'need' to be separately described? $\endgroup$
    – James
    Commented Sep 15, 2015 at 1:14
  • $\begingroup$ To clarify, it might be better to refer to subspecies on equal footing as Ursus Arctos Brunneis (brown) and Ursus Arctos Horribilis, both of which would be members of species Ursus arctos. $\endgroup$
    – jzx
    Commented Sep 15, 2015 at 1:17

1 Answer 1


In practice, subspecies are often fairly loosely-defined, reflecting a degree of uncertainty and ambiguity at this level of taxonomy. There are systematists who take the view that subspecies-level classifications should not be used as they are not rigidly definable.

In general, a subspecies will fulfil the following criteria:

  • it will occupy a distinct geographic range from other subspecies within its species,
  • it will have limited gene flow to other subspecies within its species, while still being capable of interbreeding
  • it will have some recognizable shared character, used to distinguish it from other subspecies within its species

Subspecies classifications, and their usefulness, are discussed in the Turtle Taxonomy Working Group's 'Turtle Taxonomy: Methodology, Recommendations, and Guidelines', for those interested in further reading.

  • $\begingroup$ Great answer, albeit not what I was hoping for! I'm with the systemists on this one. I can't understand the usefulness of unsystematic subspecies nomenclature. $\endgroup$
    – James
    Commented Sep 15, 2015 at 12:24
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    $\begingroup$ There can be good pragmatic reasons to hold on to subspecies labels. For instance, imagine you're after old DNA samples from museum specimens, and the species falls into two subspecies, one in the Pacific Ocean and one in the Indian Ocean. However, the old specimens do not have a lat/long of collection associated with them. In that case, you can get geographic subsets of the museum's catalogue by searching based on subspecies, and there may not be any other useful way to get that geographic subset. $\endgroup$
    – bshane
    Commented Sep 16, 2015 at 7:14
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    $\begingroup$ There are also conservation reasons for wanting a "subspecies" designation. If a species is cosmopolitan, but a local unique population is threatened, being a "subspecies" may provide for governmental protection. For instance, many freshwater fish species on the US endangered species list are subspecies. $\endgroup$
    – Kara
    Commented Apr 12, 2017 at 13:06
  • $\begingroup$ Subspecies are useful becasue species are not uniform homogenous populations, subspecies, may gave different behaviors, gene ratios, morphology, ect. For example there are several subspecies of Canus lupus including dogs, dingoes, and a variety of wolves ranging from the high tundra to the equatorial desert with adaptations to those environments. $\endgroup$
    – John
    Commented Apr 12, 2017 at 13:50

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