The Evolution and Development of Integration and Modularity, and Why It Matters
Modules, semi-autonomous subsets of highly-correlated traits, provide a practical framework for summarizing general patterns of trait interactions, which are often cited as a major influence on morphological variation. However, little comparative data on modularity exists, hindering the identification of trends in modularity and the application of hypotheses of modularity’s evolutionary significance to large-scale evolutionary patterns. Our research has primarily focused on broad, comparative study of cranial modularity and trait integration across vertebrates, spanning over 1800 fossil and extant species of mammals, birds, non-avian dinosaurs, crocodiles, turtles, squamates, frogs, salamanders, and caecilians. Other projects have examined integration in postcranial elements, including limbs of mammals, pterosaurs, and birds, and full skeletal analyses in felids. We have also tested hypotheses of ontogenetic variation and integration and their relationship to adult morphology, particularly as it relates to the marsupial-placental dichotomy in mammal evolution. Our current work expands these analyses with comprehensive sampling across all tetrapods. Most recently, we have started a project examining these questions in arthropods, specifically assessing how developmental mode, integration, and diversification relate across insects.
Our research in this field has taken many directions, including computer simulations, based on empirical and theoretical models of modularity, and analyses of morphological disparity within modules to assess the long-standing hypothesis that trait correlations influence morphological variation and, thus, morphological evolution. Our simulations and empirical analyses demonstrate how trait integration may shape morphological diversity, but also suggests that it does not necessarily affect evolutionary rates, a model that we recently described as the "Fly in the Tube" model of evolution (Felice, Randau, and Goswami, 2019). We have also simulated the effect of character correlations on discrete character states, with relevance for cladistic analyses. We have recently developed a new likelihood tool for the analysis of modularity from correlation matrices, and we have also developed methods to analyse morphological rates of evolution using 3D landmarks and other multivariate metrics.
With funding from the European Research Council, we are now conducting an extensive analysis of modularity and diversity across tetrapods, involving surface scanning and analysis of thousands of living and extinct taxa spanning their past and present ecological, morphological, developmental, and phylogenetic breadth. With these data, we will assess shifts in modularity, evolutionary rates, and morphological disparity through the evolution of terrestrial vertebrates and identify if these shifts are associated with changes in ecology, development, or environment. Thus far, we have published our results for birds, squamates, dinosaurs, salamanders, frogs, and caecilians, as well as related methodological guides and analyses, with studies of mammals, crocodiles, and turtles in progress. These separate analyses, and the ultimate combined study, will provide unprecedented power to reconstruct the major intrinsic and extrinsic factors shaping vertebrate evolution and identify trends in modularity and its evolutionary significance.