Patterns of Trophic Evolution: Integration and Modularity of the Cichlid Skull

" A key axis of diversification among cichlid species is trophic specialization, and much effort has been dedicated to revealing the factors that have contributed to the vast array of feeding morphologies that have evolved in this group. While the physical environment, especially within the large East African lakes, has undoubtedly facilitated their evolutionary success, there must also be intrinsic attributes of cichlids that have increased their evolutionary potential. The modular design of organisms has been credited for promoting diversity across metazoans. This is because the mixing and matching of preexisting anatomical “units” is a more efficient way to expand ecological diversity than the independent evolution of each component part. We suggest that modularity may also play an important role in cichlid craniofacial diversification. We review work focused on describing modularity in cichlids at multiple levels of biological organization—e.g., genetic, anatomical, and functional—and how each can explain the pattern and pace of morphological evolution in this group. We show further how changes in the action of a single molecular pathway—the Hedgehog signal transduction pathway—can promote integration at multiple levels. We consider this pathway to be a molecular “hotspot” for early and ongoing diversification in this group. Understanding the factors that promote and constrain biological diversity is an important topic in evolutionary biology. As cichlid researchers, we are in a unique position to contribute key insights into this question "