The Patriarchal Priesthood

It is the duty of the Twelve,in all large branches of the church, to ordain evangelical ministers, as they shall be designated unto them by revelation—the order of this priesthood was confirmed to be handed down from father to son,and rightly belongsto the literal descendants of the chosen seed, to whom the promises were made (Doctrine & Covenants 107:39-40).

I had never thought of evangelists, or patriarchs, belonging to a separate order of the priesthood. I heard about it in class the other day and thought it sounded a little strange, but it's right there in the Doctrine & Covenants. It seems to be, just as everything else is, a subset of the Melchizedek Priesthood, since the power and authority of the higher, or Melchizedek Priesthood, is to hold the keys of all the spiritual blessings of the church (Doctrine & Covenants 107:18).

It's interesting that the text doesn't mention who may act as an evangelical minister in case nobody can be found of the proper lineage. After all, several verses explain what to do if no literal descendant of Aaron can be found to act as a bishop. Is there some other place in the scriptures that tells us where we get our patriarchs in this case?