Macroevolutionary Analyses Provide New Evidences of Phasmid Wings Evolution as a Reversible Process

A bstract The concept that complex ancestral traits can never be re-acquired after their loss has grown popular since its initial formulation and it’s often referred to as Dollo’s law. Nonetheless, several macroevolutionary evidences - along with molecular ones - suggest instances where complex phenotypes could have been lost throughout a clade evolutionary history and subsequently reverted to their former state in derived lineages. One of the first and most notable rejection of Dollo’s law is represented by wing evolution in phasmids: this polyneopteran order of insects - which comprises stick and leaf insects - has played a central role in initiating a long-standing debate on the topic. In this study, a novel and comprehensive time-tree - including over 300 Phasmatodea species - is used as a framework for investigating wing’s evolutionary patterns in the clade. Despite accounting for several possible biases and sources of uncertainty, macroevolutionary analyses consistently support a dynamic and reversible evolution of wings, with multiple transitions to ancestral states taking place after their loss. Our findings suggest that wings and flight are decoupled from Phasmatodea diversification dynamics and that brachyptery is an unstable state, unless when co-opted for non-aerodynamic adaptations. We also explored how different assumptions of wings’ reversals probability could impact their inference: we found that until reversals are assumed to be over 30 times more unlikely than losses, they are consistently retrieved despite uncertainty in tree and model parameters. Our findings demonstrate that wings evolution can be a reversible and dynamic process in phasmids and contribute to shape our understanding of how complex phenotypes evolve.