On a slightly different tack, here's an example of 'evolution' (or whatever we prefer to call it) in action.

The recently-released double CD 'Early Ballads in Ireland', which it's my pleasure and privilege to have in my review pile right now (spoiler alert: it's wonderful, and essential to any fan of the Child Ballads) includes two versions of the vanishingly rare Child 87. One of the singers learned the ballad directly from the other, yet there are several differences:

A melodic variation in the second line.
Substitution of vocabulary ('cask of wine' for 'bottle', etc).
Alteration of the text to achieve a rhyme.
Two verses (poetically powerful but tangential to the story) omitted.
An additional verse created using an existing verse as template.

And that's just at one remove down the chain of Chinese Whispers!

Thread here:
Early Ballads in Ireland