The Election of the Tribe of Levi

The story of Korah ends with the flowering of Aharon's staff. The story is immediately followed by God's commandment to the Priests and Levites. Apparently the flowering of the staff was not only an indication of Aharon's position, but of God's choice of the entire tribe of Levi. Why does this choice require reinforcement? Apparently Aharon's special task stems from his tribe, and the unique role of the tribe stems from the nation. Kedusha belongs to the group as a whole, but each component plays a different role to enhance and support kedusha.