The impact of climate and habitat on body shape and size evolution in charontid whip spiders.

Environmental shifts often dictate Arthropod morphological evolution, yet certain lineages maintain remarkably conserved body plans across diverse habitats. Whip spiders (Amblypygi) emerged over 300 Ma and exhibit striking morphological conservatism; nonetheless, what drives this apparent conservatism remains an open question. Here, we analyzed how body size and shape evolved in response to climate and habitat by examining the contributions of bioclimatic variables and habitat type to morphology, alternative macroevolutionary models, estimating the strength of the phylogenetic signal, and testing whether evolutionary rates differ. Habitat was the strongest predictor of body size, with cave-dwelling species approximately 22% larger than surface species, while climatic gradients accounted for almost none of the shape variation. The evolution of both body size and shape was best described by a single-peak Ornstein-Uhlenbeck model, suggesting stabilizing selection toward a shared adaptive peak. Body size exhibited a moderate phylogenetic signal consistent with a Brownian motion process, whereas body shape lacked a clear phylogenetic signal, indicating stronger stabilizing selection on shape. Together, our findings suggest that the morphological conservatism of whip spiders is maintained by a "jack-of-all-trades" strategy, in which stabilizing selection favors a versatile phenotype that facilitates the occupation of diverse ecological niches without marked morphological divergence.