This article presents a comparison of the aesthetic classification system of popular music, as employed by newspaper critics in the Los Angeles Times during 1985-86 and 2004-05. Through a relational discourse analysis of popular music album reviews, which applies the ecological technique of measuring niche spaces occupied by genres in an 'aesthetic discourse space', the genre structure of popular music is compared across a time period in which the field of popular music has arguably undergone a growing 'isomorphism' of aesthetic principles and a blurring of boundaries between genres. Comparison of genre structures at two points in time suggests that the perception and interpretation of diverse genres has become more similar. Moreover, the boundary between two exemplary genres - rock music and pop music - has apparently become less salient for music critics, possibly signaling a weakening of commercial boundaries within the popular music field over the last two decades.