Humm...even if they did originate from a singular form...they're still distinct enough these days to warrant separate classification aren't they?
Okay, if not that, than Eastern and Western RPGs maybe? From what I've seen, although there are exceptions, in 'general' Eastern style RPG's tend towards flashy visuals and heavy characterisation (such as it is in -some- of them)with regards to their main protaganists/allies. Western examples tend towards...I guess you could say the mundane?
Where Eastern style RPG's are often bombastic, bright and free with their use of color, Western RPGs tend to go more with 'real' or muted colors, or to be more specific, take Oblivion, that's actually quite a colorful game, but it's tones still go together more naturally as to emulate a kind of real word aesthetic. Now, take an Eastern style like Eternal Sonata, or any of the Final Fantasy's from 9 and up. Bright, bold, stand out colors.
But, that's not to say that -all- Eastern style RPG's are bright bubblegum colors and -all- Western style RPG's are -all- muck grey n' green. There are exceptions as always. And, I'm still rather fond of Eternal Sonata's bright and cheery palette, and enjoy the everpresent brown grit and grime that almost envelops games like Fallout and S.T.A.L.K.E.R.
As for characterisation. With Eastern, characters in the main I guess, have a generally layed out script that they will with few exceptions follow. Some games do offer choice when playing through the story, but that often consists of simple yes or no actions, although I've seen one or two that go into a little more detail.
Western RPG's tend to leave the main char as something of a blank slate, oh, he may have a history, friends and family etc. like the Eastern style char, but he is essentially faceless, or rather, it's up to the player to fill in the rest. In this, it is also in the player's interactions that the player's char is defined. This, seems most often to take the form of the almost ubiquitous 'chat box' style of conversation trees. Something akin to a choose your own adventure book I guess. In this regard however, the Eastern sytle RPGs can sometimes be better at holding a structure for it's plot, rather then risk losing the plot completely and having the player go off doing something they find more interesting in the game. Which, considering games like Baldur's Gate or Neverwinter Nights I was wont to do myself, and actually wonder if they would have been so good with -only- the main storyline holding them up.
Still...