Integration and modular patterns shaping the morphological evolution in Neotropical hognose pitvipers <i>Porthidium</i>

Abstract The evolutionary mechanisms driving morphological diversification remain poorly understood for many organismal lineages. We investigated the morphological evolution of the hognose pitviper genus Porthidium (Viperidae: Crotalinae). Using a comprehensive dataset of size, scales, and shape across head and body regions from 484 specimens, we tested patterns of modularity and integration through standardized major axes correlations and evolutionary rates (σ²; estimated with Monte Carlo Markov Chains) and assessed trait evolutionary trajectories with three models: Brownian motion (BM), Ornstein-Uhlenbeck (OU), and early burst (EB) and determined if morphological traits were constrained by their phylogenetic relationships while accounting for ontogenetic shifts between juveniles and adults. Our analyses revealed a dichotomy: while direct trait correlations indicated a current integration, our analysis of evolutionary rates supported modularity. The BM model was consistently supported across all traits, indicating a pattern resembling neutral evolution, but we also found an optimum directed evolution fit for adult snakes. Finally, we found no significant phylogenetic signal for any trait or stage. Our findings suggest that variable selective pressures on complex traits likely drive rapid phenotypic differentiation. Our results highlight that the forces shaping morphological diversity for these pitvipers are dynamic and context-dependent, emphasizing the need to consider complex evolutionary trajectories when studying morphological diversification.