Despite a long-standing interest in the genetic basis of morphological diversity, the molecular mechanisms that give rise to developmental variation are incompletely understood. Here, we use comparative transcriptomics coupled with the construction of gene coexpression networks to predict a gene regulatory network (GRN) for leaf development in tomato and two related wild species with strikingly different leaf morphologies. The core network in the leaf developmental GRN contains regulators of leaf morphology that function in global cell proliferation with peripheral gene network modules (GNMs). The BLADE-ON-PETIOLE (BOP) transcription factor in one GNM controls the core network by altering effective concentration of the KNOTTED-like HOMEOBOX gene product. Comparative network analysis and experimental perturbations of BOP levels suggest that variation in BOP expression could explain the diversity in leaf complexity among these species through dynamic rewiring of interactions in the GRN. The peripheral location of the BOP-containing GNM in the leaf developmental GRN and the phenotypic mimics of evolutionary diversity caused by alteration in BOP levels identify a key role for this GNM in canalizing the leaf morphospace by modifying the maturation schedule of leaves to create morphological diversity.