Several prominent evolutionary theories propose mechanisms whereby the evolution of a defensive trait or suite of traits causes significant shifts in species diversification rate and niche evolution. We investigated the role of cuticular spines, a highly variable morphological defensive trait in the hyperdiverse ant genus Polyrhachis, on species diversification and geographic range size. Informed by key innovation theory and the escape-and-radiate hypothesis, we predicted that species with longer spines would exhibit elevated rates of diversification and larger range sizes compared to species with shorter spines. To address these predictions, we estimated phylogenetic relationships with a phylogenomic approach utilizing ultra-conserved elements (UCEs), which are uploaded in this archive, and also compiled morphological and biogeographic trait databases. This study provides a rare investigation of the role of a defensive trait on geographic range size, and ultimately supports the hypothesis that defensive spines are a factor in increased range size in Polyrhachis ants. Furthermore, the lack of support for an association between spines and diversification, which contrasts with previous work demonstrating a positive association between spines and diversification rate, is intriguing and warrants further study.
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