Going back to the idea I floated earlier of comparing variants from different locations, here's a good example from England: You Seamen Bold

What you have there are four tunes for the same song, sometimes known (courtesy of the Penguin Book) as 'The Ship in Distress'. #2 and #3 are from the same village in Sussex, #1 from a village 20 miles away, and #4 from 200 miles away. They are all clearly variants of the same tune, although they are set in different musical modes: Mixolydian,Dorian and Ionian (incidentally a bit of digging in the Full English should get you midi sound files of all four). As you might expect, the two from Pulborough are almost identical, and share the characteristic of the flattened third and sixth notes of the scale (Dorian), which to our ears imparts a darker tone to the melody. But the other two have a major third and a sunnier temperament - despite the sombre theme of shipwreck, cannibalistic inclinations etc.

However, Percy Grainger believed (and I understand Julia Bishop is of the same mind) that the other song collectors of his day were mistaken in their identification of and fascination with medieval church modes in folk song melodies. According to him, precisely the intervals that determine the four commonest modes - the 3rd, 6th and 7th of the scale - were precisely those likely to be varied or sung ambiguously by traditional singers (think of the Blues as well). There's plenty of evidence of that if you listen to enough recordings. So it may be that those modal variations of 'You Seamen Bold' had a lot less significance for the singers who sang them than they do for scholars or modern revivalists.

It's worth mentioning that this particular song wasn't collected very often, but all the versions I can find had a very similar tune. A song like 'John Barleycorn', which was older and much more widespread, had a whole number of quite different tunes, in every shape, mode and time signature you could wish for.

I think Jack Campin had it right in his post of 27 Dec 15 @ 08:45 PM: there was not some linear evolutionary process going on, even thought tunes clearly changed. Cecil Sharp's metaphor of the pebble shaped by the waves of the sea is pretty, but not necessarily apt - in fact he gave the impression that songs could be refined to some high artistic level at the same time they were becoming degenerate.

Regarding the other point about commercial music before the days of broadcasting, etc: there were constant inputs over centuries from pleasure garden performance, glee club repertoire, the church, music hall, minstrel shows and quite possibly circus music as alluded to above. How much all of that contributed to or impinged on what we think of as the folk song repertoire is something I don't think we know yet. However, you wouldn't mistake 'You Seamen Bold' for a music hall song.