Bronson has v. interesting things to say on evolution of tunes.
Readings
Bronson, Bertrand. "Folk-Song and the Modes," "Habits of the Ballad as Song," and "Words and Music in Child Ballads." In The Ballad as Song. Berkeley: University of California Press, 1969, pp. 79-132.

1. General theory about tunes shifting as they slip from one mode to another.
2. Comparison of "Captain Kidd," "Sam Hall," and "Admiral Benbow," all ballads written to the "same tune" (according to Bronson) from around 1700. But the tune does change, at least based on the way these are sung today.