Music evolves the way life does: through change in the heritable characteristics of populations over successive generations. Most of the heritable characteristics of music are abstractions like rhythm patterns and chord progressions. However, you can also see heritability at work more obviously in the form of sampling. It's especially illuminating when a song samples a song which in turn samples yet another song. The longest such chain that I know of: “Workin’ On It” by Dwele (2008)