22Phenotypic plasticity allows organisms to change their phenotype in response to shifts in 23 the environment. While a central topic in current discussions of evolutionary potential, a 24 comprehensive understanding of the genetic underpinnings of plasticity is lacking in 25 systems undergoing adaptive diversification. Here we investigate the genetic basis of 26 phenotypic plasticity in a textbook adaptive radiation, Lake Malawi cichlid fishes. 27Specifically, we crossed two divergent species to generate an F 3 hybrid mapping 28 population. At early juvenile stages, hybrid families were split and reared in alternate 29 foraging environments that mimicked benthic/scraping or limnetic/sucking modes of 30 feeding. These alternate treatments produced variation in morphology that was broadly 31 2 similar to the major axis of divergence among Malawi cichlids, providing support for the 32 flexible stem theory of adaptive radiation. Next we found that the genetic architecture of 33 several morphological traits was highly sensitive to the environment. In particular, of 22 34 significant quantitative trait loci (QTL), only one was shared between environments. In 35 addition, we identified QTL acting across environments with alternate alleles being 36 differentially sensitive to the environment. Thus, our data suggest that while plasticity is 37 largely determined by loci specific to a given environment, it may also be influenced by 38 loci operating across environments. Finally, our mapping data provide evidence for the 39 evolution of plasticity via genetic assimilation at an important regulatory locus, ptch1. In 40 all, our data address longstanding discussions about the genetic basis and evolution of 41 plasticity. They also underscore the importance of the environment in affecting 42 developmental outcomes, genetic architectures, morphological diversity, and 43 evolutionary potential.

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Environmental conditions can impact the development of phenotypes and in turn the performance of individuals. Climate change, therefore, provides a pressing need to extend our understanding of how temperature will influence phenotypic variation. To address this, we assessed the impact of increased temperatures on ecologically significant phenotypic traits in Arctic charr (Salvelinus alpinus). We raised Arctic charr at 5°C and 9°C to simulate a predicted climate change scenario and examined temperature‐induced variation in ossification, bone metabolism, skeletal morphology, and escape response. Fish reared at 9°C exhibited less cartilage and bone development at the same developmental stage, but also higher bone metabolism in localized regions. The higher temperature treatment also resulted in significant differences in craniofacial morphology, changes in the degree of variation, and fewer vertebrae. Both temperature regime and vertebral number affected escape response performance, with higher temperature leading to decreased latency. These findings demonstrate that climate change has the potential to impact development through multiple routes with the potential for plasticity and the release of cryptic genetic variation to have strong impacts on function through ecological performance and survival.