Quite simply, one can theoretically approach the study of something however one wishes. To reiterate and/or clarify, talking about "tune evolution" _under the rubric of ethnomusicology_ simply presents a problem because, historically, scholars in the field have taken issue with the application of ideas of evolution to musical forms.

One quick fix, as I'd mentioned is to simply disclaim ethnomusicology as the field of reference. Say it's Folklore or English Lit. or something (might work -- I can't speak to how those disciplines would react).

Once clear, however, of any claim that the topic is one covered by ethnomusicology, one might still be interested in the potential insights from ethnomusicology.

I don't have time to go over the history of ethnomusicology's position. It's in the references I gave. I will only mention that just mention of the word "evolution" is enough to raise the hackles of ethnomusicologists. Whether or not one thinks this is justified, it is helpful to know that this will be the case.

Does this mean that someone may not use the term "evolution" in an ethnomusicology-based course, or, more substantially, that one may not apply ideas from biological evolution to the study of musical forms? No, it does not. It means that if one chooses to do so, one is fairly obligated (by the "rules of academia," if you will!) to demonstrate knowledge of the past discourse on the issue and to explain why, despite common objection, one thinks this idea is worth reconsidering.

I happen to think that "old" ideas ARE worth reconsidering, and have faith that others, being people of good faith, will be open to hearing about it. To make a crude analogy, if you chose to refer to Black Americans as "colored people," the response would not be favorable. You could go on to explain why, for some thoughtful reason, you chose to say "colored people" and, hopefully, reasonable people would hear you out. But to remain oblivious to peoples historical objection to "colored people" would not be wise.

Ethnomusicology as we know it was a field configured in the wake of WWII, and its founders (who tended to come out of anthropology) were reacting to the way that ideas from biological evolution had been applied to culture. These were people who had either escaped or been exiled from Nazi Germany, or were the students of such people. It was thought that the misapplication of ideas of evolution to the study of culture was, broadly speaking, a forerunner of Nazi racialist ideology. One may make of that what one will, but the fact remains that the school of anthropology with which ethnomusicologist allied themselves was anti-evolutionism.

Leaving aside the politics and historical trends within the field, ethnomusicologists are disinclined to think about "tune evolution" (since the mid-20th c.) because they don't think about music as an object. As such, there is no "thing" to undergo evolution.