Sorry for not having concrete evidence but in the beta you unlocked skills upon level up and then the next one down in the column also seemed to increase XP as your character levels. We know from interviews that certain skills they made available early (level 9 and 10 skills such as second grenade). If you look on destinydb.com or anywhere with skill trees, you'll see numbers on each perk which likely correspond to level numbers, topping at level 29 for locking in the subclass. We know from the IGN video that guardians can get up to level 29 (possibly higher). So it is likely that your subclass levels in the same way as your character. We also know that at level 15 you unlock your character's second subclass, but equipping it doesn't reset your character level. This then suggests that your subclass levels up and gains XP in a similar way to your character, but is not tied directly to character level. I think your subclass levels up separately, but all with the same XP earned from bounties, missions, kills, etc.