it seems an odd idea that a tune might float around the tradition for centuries before settling on a definitive mode (though it does occasionally happen). The way people think about mode in Middle Eastern music applies to a lot of Western tunes too: you create new tunes by assembling phrases characteristic of a particular mode (consciously or not). The mode comes first, and the modal system is fairly static. Tunes emerge from it like new plants budding off a cactus.