A subspecies is a population that occurs within a portion of a species' range that has certain traits that distinguish it from populations that occur in other parts of the range.



Domesticated animals like gerbils are biologically irrelevent. Definitions of species and subspecies depend on behavior in the wild, so by trying to apply them to captive gerbils you're sort of missing the point.



I don't know anything about garter snakes. It's quite possible for a single subspecies to have multiple morphs (external appearances). For example, the california kingsnake can be striped or banded. There can be both in the same clutch. Only if the striped ones lived in a separate area from the bandeds would they be considered two different subspecies.



Two subspecies will produce intergrades where their ranges meet. (If they didn't then they would be considered different species). Of course, in some cases a subspecies will be geographically isolated, in which case obviously no natural intergrades can occur.

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