ALTERATION: voluntary

EVOLUTION (in either sense): involuntary

I wouldn't disagree with that. Voluntary vs involuntary is not critical in discussing whether songs evolved or not (Darwinian evolution is involuntary); it's just an additional complicating factor in determining how the songs might have changed.

'Involuntary' might cover mishearing of the source, misremembering it later, or inaccurately reproducing it in performance. I'm not one of those people who believe that traditional singers never sang a wrong note.