This review addresses the use of comparative studies of development to help understand the mechanisms underlying the evolution of morphological and physiological diversity. Mutation analysis in experimental model organisms is at the core of our understanding of how development works, and these findings have proven important in the identification and functional support of genetic variations associated with character change in natural populations. However, it has been argued that the essential and pleiotropic effect of many developmentally important genes limits the utility of such mutational study of development toward understanding evolution. Here, I argue that an analysis of the genetic regulation of postembryonic development will refine such comparative analysis. By focusing on late developmental events, one limits the type of genes as well as the types of genetic and developmental changes that may underlie evolutionary change. This may refine the predictive value of such comparative analysis. In this review, I discuss the use of fish as experimental and natural models to address these questions. Fish have several established, experimental models for genetic analysis. Additionally, the morphological and genetic diversity among fish provides a wealth of new models in which to look at the genetic and developmental basis of character change. This broadening of comparative genetic analysis may resolve the bias against genetic ‘monsters’ and what they can tell us about evolutionary change.