Evolution: Birds of a feather evolve together

Substantial variation in the evolution of birds is uncovered by the analysis of skeletal structures from more than 200 species, published in Nature this week. In particular, striking differences are uncovered in the mode of evolution between water birds and land birds, which highlights the important role that the environment has on shaping features of animals.

Birds are ecologically diverse and structurally complex, with this diversity expected to have resulted at least in part from a mass extinction event at the end of the Cretaceous period 66 million years ago. The ecological diversification that followed this extinction is thought to have given rise to distinct water- and land-based groups. While the understanding of evolution has advanced through new methods and technology, studies are often limited to a focus on a few traits or parts of the body that are considered to be most important.

Guillermo Navalón, Roger Benson and colleagues quantify patterns of evolution across birds’ entire skeletons, from relative body proportions, to combined shapes of connected bones, to shape changes in individual bones. They analyse 13 skeletal elements in 228 species that span the entire family of living birds, looking at components of the head, wings and legs. The authors find that, contrary to expectations, the end-Cretaceous mass extinction played only a small role in explaining the diversity of living birds in the study group. Instead, subsequent pulses of evolution may have played a more important role. Additionally, the authors find that water and land birds show striking differences in modes of evolution, which suggests that their environments were crucial to defining their evolutionary paths. For example, land birds developed specific adaptations for movement that are reflected in the legs, while water birds (such as penguins) were more evolutionarily versatile in their adaptations for movement.

This study provides a highly detailed 3D quantification of skeletal variation to observe macroevolutionary patterns. The authors demonstrate the value of looking across multiple body features, as different parts of the body may show diverging patterns of evolution. The findings also reframe the discussion of mass extinctions’ impact on evolutionary paths, as well as the role of divergent land and water environments in bird evolution.