When a group adopts a dance or song imported from elsewhere, it doesn't copy it faithfully; it pours in a rhythmic life the original never had. A European dance taken up by black musicians came out livelier than its model, and from it grew a family of new hybrid dances.
When cultures meet, their music doesn't simply sit side by side; meters, melodies and instruments from one fuse with the oral habits of another into a new whole. Spanish instruments and verse forms blended with old African traditions to produce music belonging to neither alone.
Popular music made by people at the bottom gradually seeps upward and changes what elite, formally trained composers write. African-derived popular forms increasingly shaped European-style composition, and eventually forced their way into European culture itself.
A distant outpost can preserve an old tradition better than the place it came from, with only small changes from local singing habits. Old Spanish ballads survived in Cuba as near-copies of regional originals, keeping their classic verse line while gaining a strummed refrain.