Morphological evolution in a hyperdiverse clade: The ant genus Pheidole

Understanding patterns of morphological variation and their evolutionary causes is among the main goals of evolutionary biology. In this paper we analyse the pattern of morphological variation in the hyperdiverse ant genus Pheidole (Hymenoptera: Formicidae). We describe the morphospace defined by 231 Pheidole species in the New World using 18 linear quantitative characters. The first ordination axis of the morphospace was found to mainly reflect differences in body size, which explained 83.2 and 78.6% of all morphological variation in minor and major worker subcastes, respectively. Therefore, despite considerable morphological divergence among Pheidole species, most of this variation can be attributed to allometric changes along a size axis, leaving little opportunity for ecological specialization that might be reflected in the studied morphological traits. Simulations suggest that this degree of concentration of variance on the first principal component is unlikely to be due solely to shared phylogenetic history. The second ordination axis reflected mostly variation in head characters, most notably antennal scape length. The third ordination axis described aspects of alitrunk and petiole shape. Although generally concordant, the loading patterns in the second and third axes differed between minors and majors, suggesting different selective pressures affecting the evolution of each subcaste. These general trends in shape were confirmed by additional analyses that corrected for body size. Ordination analyses were complemented by comparing patterns of interspecific morphological covariation in minors and majors of Pheidole using newly developed analyses based on the statistical concept of conditional independence of Magwene. A comparison between worker subcastes indicated substantial reorganization in the pattern of character covariation, particularly through a decrease in character-correlation levels (dissociation). Moreover, graphical modelling indicated correlations between characters in widely different parts of the body, suggesting that the opportunity for morphological evolution in Pheidole appears to be surprisingly limited. This result might explain why morphological evolution in Pheidole has been highly conservative in spite of substantial ecological diversification.