&quot;As far as Nettl&#39;s take on Eastern Woodla...

"As far as Nettl's take on Eastern Woodland music, his book on The Western Traditions of world music doesn't mention polyphony, just their tendency to sing antiphonally. Since these were the first Indians in N. America to be exposed to Western culture and music (probably hymns) it's hard to evaluate what it would mean if the occasional song were to contain some traces of harmony. In my experience, polyphony among N. American Indians is very rare and when we hear it it's usually an accident (inexperienced singer producing a wrong note). The only exception would be some California Indians who sing in a kind of shouted hocket and it's been reported for NW Coast, though most of what I've heard from there is unison singing."

I looked at Nettl's Music in Primitive Culture. You're right that Woodlands polyphony could result from European influence. Hard to tell, although Nettl would have known, I suppose. But even from what you just said about NW Coast I infer that polyphony and monophony (unison) in some places are very tightly interspersed, which makes a bottleneck idea unlikely, unless of course two populations came together and blended their musical styles.

FYI: the most popular model of phonological evolution (again, as summarized in Blevins) explains changes as resulting from a hearer's misinterpretation of an incoming phonetic sequence. In both music and language, evolution seem to occur as a process by which individual mistakes achieve fixation in society.