Except when it's in the Gita. There it's a religious principle.
Moslem buddy of mine from Pakistan said "Don't let them fool you. Everyone still knows his own caste and history. You couldn't get married if you didn't."
Varna/Varga as mentioned in Bhagwat Gita has very different connotation then whats is practiced in Indian socity. At some later point that idea merged with strong tribal affiliation into todays “cast system”. The study provides evidence to the fact that Indians have higher FSTs (genetic variation) compared to the more “homogeneous” Europeans. Large scale genetic intermixing occurred between ANIs and ASI until at some point tribal lines became very rigid.
Beside a Christian from India will tell you precisely the same thing about their custom as your Muslim buddy.