What is a sub-species anyway?
Species is defined as a number of living organisms that can unterbreed ith each other.
Do you mean by sub-species an off shoot of the population that underwent some evolutionary changes but hasn't diverged so far that interbreeding became impossible?
Which branch becomes the sub-species in that case? The one with the smaller Population? Or the one that diverged more obviously from the nearest common ancestor? Or the cultural less dominant one?