Two quibbles. First, the Rigveda itself didn't separate castes into hierarchical varnas - that was more of a later vedic thing. Second, jatis aren't exactly sub-varnas. Some jatis span across varnas (e.g. because their members engage in both farming (shudra) and soldiering (kshatriya)), and particularly in South India the jati system exists even though the varna system has only been incompletely applied (a lot of regions in South India don't really have any defined varnas outside of Brahmins).