Tunes don't evolve, but sometimes people change them. Take a tune.

Musician #1 is a beginner, and he makes it easier to play - skips ornaments, makes big jumps smaller, ignores dotted timings, drops accidentals, omits pick-up notes, plays slow.

Musician #2 is in a polka band, and he makes it hop up and down more.

Musician #3 wants a tune for a class of six-year-olds, so he moves it from minor to major.

Musician #4 wants authenticity, so he revives Version 1. (He is lucky to find an old version in a published book.)

Musician #5 wants a barbershop sound, so he throws in new sharps and flats.

Musician #6 wants to impress music critics, so he plays it fast with lots of ornaments.
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Let this go on for 100 or 200 years, and chances are nobody will know what the original tune was like.