Global Patterns in Body Size and Colour Lightness Diversity of Swallowtail Butterflies Highlight Biogeographical Legacy Over Common Environmental Explanations

Aim Differences in the functional traits of co‐occurring species drive species' interactions and sustain key ecosystem services. However, the historical and contemporary environmental drivers of, as well as the surrogacy of conventional range‐based diversity metrics for, this functional diversity remain poorly understood. Here, we assess the drivers and spatial congruence of species richness, range rarity, and a major component of the global functional diversity of swallowtail butterflies—the body size and colour lightness dispersion—grounded in global‐scale analyses of their ecophysiological functions. Location Global. Time Period Contemporary. Major Taxa Studied Papilionidae (Lepidoptera). Methods Using comprehensive distribution data and two ecologically important and ubiquitous traits, namely body size and colour lightness, for 511 (80%) species, we examined the environmental drivers of assemblage‐level trait means and dispersion. Using trend surface models that account for environmental drivers, we further assessed the potential contribution of latent biogeographical drivers. Results Species richness and range rarity, but not functional dispersion, decreased with latitude ( R 2 = 34 > 21 > 5%), mainly due to similar effects of temperature and productivity. Hotspots of over‐dispersion uniquely highlighted Indomalaya and adjacent regions, while being largely absent from the Neotropic and Afrotropic. Being weakly driven by contemporary environmental factors, this pattern was even more prominent in the residuals explained by a latent, strongly longitudinally structured driver. Main Conclusions The idiosyncratic relationships with environmental drivers and moderate congruence with conventional diversity measures suggest broad complementarity of functional dispersion. Notably, although environmental drivers strongly shape mean body size and colour lightness, consistent with ecophysiological mechanisms, they do not translate into filters of trait dispersion. Thus, while both traits are highly scalable and have a broad range of ecologically important functions, our results suggest that a complex interplay of ecological and evolutionary processes underpins large‐scale patterns in functional assemblage structure.