A computer-assisted composition tool for investigating the application ofevolutionary techniques in the composition of music is presented. The natureof such an application is examined in terms of defining the possiblemechanisms that provide a means for automated creativity. These mechanismstake inspiration from processes found in Darwinian-based evolution theory,genetic algorithm theory and similar aesthetically based uses of a geneticsearch heuristic in the visual arts. A formal model of ‘musicalevolution’ is proposed, with particular emphasis placed on the waysin which a genetic algorithm can be used to effectively manipulate avariety of compositional structures within a hierarchical and generativegrammar-based model of musical composition. The result is a prototypeWindows MIDI application called GeNotator that allows composers toexperiment with a range of musical structures by interactively‘evolving’ them through a familiar and comprehensive graphicaluser interface.