Title: Developmental Modularity and Evolvability of the Head Skeleton

Abstract: Development “structures the phenotypic variation upon which selection acts” (B. Hallgrimsson),and hence could significantly influence evolvability. What is the nature of the structuring? From studies of bone shaping in zebrafish larvae, I’ll argue that that skull morphogenesis exhibits modularity; semi-autonomous development of separate regions of bone controlled by different intercellular signals. Modularity might influence evolvability; for example different modules could separately evolve distinctive new shapes. We tested this hypothesis with stickleback, and, in support, found examples in the skull where evolutionary changes in bone shape strikingly predict the locations of module boundaries. Finally, I’ll describe our studies of a zebrafish mutation where developmental structuring of a module dramatically breaks down, yielding a largely unpredictable variety of bone shapes. Following Waddington and others we suggest that this phenotypic instability is due to loss of buffering (or “canalizing”) mechanisms, that seem to involve the epigenome. Learning more about these mechanisms may yield insight into how novel morphologies evolve.