When, how, and why does culture change over time? Sociological theory and mechanisms to explain cultural change are underdeveloped. This study links contemporary sociological theories of culture to a burgeoning cultural evolution literature emerging across other social sciences. By shifting our analytical focus from social actors to cultural representations and material "cultural things," our theory is able to explain change over large and long frames of analysis using formal evolutionary mechanisms. Complementing this theory, the paper introduces a suite of novel methods to explain change in the historical trajectories of populations of cultural things (e.g., musical groups, hashtags, laws, technologies, and organizations) through diversification rates. We deploy our theory and methods to study the history of Metal Music over more than four decades, using a complete dataset of all bands active between 1968-2000. Over the course of its history, we find strong evidence that the genre has been fundamentally shaped by competition for the cognitive resources actors can invest in learning about and reproducing this cultural form over time.