Conclusion
In this study, we sought to more fully understand which factors contribute to diversity in the Cyprinodon pupfish system. Our data support the idea that phenotypic variation and covariation, or the relationship between traits, is an additional and important axis of variation that contributes to diversification in the pupfish system. When investigating the factors driving diversification between pupfish groups, we find that the lack of a flexible P matrix may impede the ability to diversify. We also used F2 hybrid P-matrices to make inferences about underlying mechanisms which produce variation in craniofacial traits in the pupfish system. We found that many traits adhere to the expectations of a simple additive genetic model. Yet, key traits such as head depth, maxilla length, palatine height, lower jaw length, and dentigerous arm width appear to be influences by nonadditive genetic effects and produce large phenotypic changes in other traits. Future work should examine whether this relationship between flexibility and diversification is generalizable across many species or is specific to Cyprinodon pupfish.