Clan, folk, kith, or kin might work.
('Kith' might be the best choice because it is not commonly used in modern speech.)
The sub-races are (usually) supposed to have formed due to evolution-like processes - two groups of X moved apart and adapted to different conditions (plains vs forests) over time then expanded and met each other again; or a group went into tough terrain to hide, and stayed hid.
A few exceptions - ex: the Descent of the Drow - are divine intervention.
There might be a third category of differentiator events but I can't think of any examples off-hand.