Fashion or function? Relaxed selection and stasis are key features of the evolution of stripes, bands and collars in Sceloporus lizards (Squamata: Phrynosomatidae)

Dorsal markings are common, and can function in predator evasion, as sexual signals, or be the remnants of developmental or physiological processes. We used phylogenetic comparative analyses to test whether black dorsal markings (stripes, bands or collars) on Sceloporus lizards reflect responses to different selective pressures over evolutionary time, or whether these markings have been linked in an integrated phenotype. We found that longitudinal striping evolved in small-bodied, terrestrial and egg-laying species, consistent with an antipredator function, especially when sexual selection was weak. Transverse bands evolved more slowly, and primarily in arboreal species, where they may enhance crypsis. Evolutionary shifts in collars were strongly linked to sexual size dimorphism, suggesting a function as sexual signals, and were tightly linked to parity mode. In two viviparous lineages, collars have been retained without loss for more than 12 Myr. Future studies are needed to determine whether this stasis is due to hormone transfer between females and their offspring or to other underlying physiological mechanisms. Our findings emphasize relaxed selection and stasis as important forces shaping the evolution of animal colours and signals.