Defensive spines are associated with large geographic range but not diversification in spiny ants (Hymenoptera: Formicidae: Polyrhachis)

Abstract

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 investigate 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 clades with longer spines would exhibit elevated rates of diversification and larger range sizes compared to clades with shorter spines. To address these predictions, we estimated phylogenetic relationships with a phylogenomic approach utilizing ultraconserved elements and compiled morphological and biogeographic trait databases. In contrast to the first prediction, we found no association between diversification rate and any trait (spine length, body size and range size), with the sole exception of a positive association between range size and diversification in one of three trait-based diversification analyses. However, we recovered a positive phylogenetic correlation between spine length and geographic range size, suggesting that spines promote expanded geographic range. Notably, these results were consistent across analyses using different phylogenetic inference approaches and spine trait measurement schemes. 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.