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?
Answer
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.
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