Hipsley, C. | Search bioRxiv

bioRxiv2026

Morphological integration of the avian beak facilitates evolution along lines of least resistance

Innovation of the avian beak has facilitated a grand radiation of >11,000 species, with vast morphological disparity suggesting limited developmental constraints on beak diversification. We assess four macroevolutionary currencies - integration, disparity, phenotypic evolutionary rates, and ecological specialization - using 3D beak landmarks for 8,627 species mapped to a complete avian supertree with a resolved genomic backbone. We introduce a Gini coefficient-based metric of ecological specialization, measuring evolutionary time spent across trophic niches. Phylogenetic regressions show that lineages with faster phenotypic rates exhibit stronger beak integration (landmark covariation) and more generalised diets, while beak disparity declines with greater trophic specialization. These results suggest that integration facilitates, rather than constrains, phenotypic evolution, by channeling variation along lines of least resistance. Future work should explore modular structure of the bird beak, which arises from multiple genetic and developmental factors.

evolutionary biology↗

bioRxiv2025

Neural crest cell biology shapes lizard skull evolution across evolutionary time scales

The vertebrate skull originates from two embryonic lineages, the mesoderm and the neural crest, offering a unique framework to study how developmental mechanisms connect phenotypic variation and evolutionary diversification. Using 3D geometric morphometrics, we analysed skull shape variation in lacertid lizards. Mesoderm- and neural crest-derived bones formed distinct, conserved modules at both micro- and macroevolutionary scales. In the common wall lizard (Podarcis muralis), rapid evolution of skull shape under sexual selection was primarily driven by neural crest-derived bones. While the primary axis of shape divergence in P. muralis aligned with a major axis of variation across lacertids, neural crest-derived bones exhibited slower evolutionary rates and lower morphological disparity than mesodermal-derived bones. We propose that this discrepancy between the role of the neural crest for skull evolution on micro- and macroevolution reflects constraints imposed by neural crest cell biology: although developmental plasticity enables rapid, correlated responses under sexual selection, pleiotropy may limit long-term evolvability of neural crest-derived skull regions. Teaser textThe bones of the vertebrate skull come from two developmental sources: the mesoderm and the neural crest. This dual origin allows to study how development influences evolution. Using 3D models of lizard skulls--including a species with exaggerated male traits linked to the neural crest--we examined patterns of skull variation. We found that neural crest-derived bones contribute to rapid changes driven by sexual selection. However, across different species, these same skull regions evolve more slowly and show less variation. This suggests that while neural crest cells may constrain long-term evolution because of their wide influence, they can also enable fast adaptations. The developmental biology of a trait therefore shapes its evolution.

evolutionary biology↗