Part of the problem - if it is one - is that "evolution" usually implies an increasing complexity or sophistication over time.

Do traditional tunes and songs really exhibit this? A few songs do generate new stanzas over time, but these rarely get combined into a single version. (Think, for example, of "The Frog and the Mouse" and/or "Three German Officers," both unusually popular for decades or centuries.

Tune "evolution" is even more problematical. Chronologically "The Three Ravens" and "John Anderson" come long before "When Johnny Comes Marching Home Again." But has a single tune actually "evolved"? Has the "Ravens" tune somehow "improved"? And how do we know that these recognizably distinct tunes, despite their similarities, are genetically related? They may be, but then again they may be the result of independent, or nearly independent, inspiration. Would that count as "evolution"?

Look at the various tunes related to "Captain Kidd" and "Wondrous Love." They're certainly related in form, and "Ye Jacobites by Name" was even said in the 18th century to go to a (lost, perhaps) version of "Captain Kidd." Some - or even all- of these tunes may have resulted from preconscious influences (i.e., not "unconscious" but just under complete awareness). Others may have been intentional improvements. Do these dynamics count as "evolution"?

Folk song "evolution" - even if clearly defined, as it usually isn't - is rarely an obvious or straightforward process. Did "Wrap Me Up in my Tarpaulin Jacket" really derive or evolve in any meaningful way from the Unfortunate Rake"? Or are they simply quite different songs on broadly similar subjects?

Folk songs and tunes are certainly subject to individual and collective variations, alterations, omissions, intentional deletions, additions, distortions, rewriting, parodies, etc, etc. But do these "folk processes" count as "evolution"?

BTW, one of the attractions of folk song to beginning students is the idea that these songs were so widely known among "the folk" that they represents some sort of communal view of things, or at least communal taste. But what percentage of the population really sang, or even cared much about, any given piece?

That question seems to be unanswerable. All we can say for sure, think, is that traditional songs/tunes in general were in wide circulation for centuries, and some (like "Barbara Allen" and "The Unfortunate Rake") were very popular indeed.