Difference in evolutionary patterns of strongly or weakly selected characters among ant populations

The pronotum shape of scelimenine grasshoppers (Orthoptera: Tetrigidae) likely represents an exaptation for heterogeneous niche colonization

Insects are suitable model organisms for functional morphology research, especially in the context of exaptation, when the same morphological trait represents an advantage in disparate niches. Phylogenetically distant groups of pygmy grasshoppers (Orthoptera: Tetrigidae) have various pronotal projections defining their general appearance and body shape. However, body shape has never been related to niche occupation in these insects, thus the aim of this study is to investigate the relationship between pronotum shape and macrohabitat adaptation in Scelimeninae, a group of Asian and Papuan tetrigids encompassing amphibious and corticolous (bark-dwelling) representatives. With the use of geometric morphometrics and phylogenetic comparative methods, two morphological and functional groups were distinguished, with the body shape exhibiting a significant phylogenetic signal. The first group consists of elongated amphibious taxa (Scelimenini tribe) with highly uniform pronotum morphology, likely due to a strong selection for streamlined body shape. Stouter corticolous taxa (Discotettigini tribe) exhibit more conspicuous body shape variability, possibly increasing camouflage efficiency in tree bark habitats. Ecological divergence associated with macrohabitat adaptation may thus have been the primary driver of speciation in this insect group, but the evolutionary constraints leading to this divergence are still to be identified.

Evolution of morphological crypsis in the Tetramorium caespitum ant species complex (Hymenoptera: Formicidae)

Cryptic species are morphologically very similar to each other. To what extent stasis or convergence causes crypsis and whether ecology influences the evolution of crypsis has remained unclear. The Tetramorium caespitum complex is one of the most intricate examples of cryptic species in ants. Here, we test three hypotheses concerning the evolution of its crypsis: H1: The complex is monophyletic. H2: Morphology resulted from evolutionary stasis. H3: Ecology and morphology evolved concertedly. We confirmed (H1) monophyly of the complex; (H2) a positive relation between morphological and phylogenetic distances, which indicates a very slow loss of similarity over time and thus stasis; and (H3) a positive relation between only one morphological character and a proxy of the ecological niche, which indicates concerted evolution of these two characters, as well as a negative relation between p-values of correct species identification and altitude, which suggests that species occurring in higher altitudes are more cryptic. Our data suggest that species-specific morphological adaptations to the ecological niche are exceptions in the complex, and we consider the worker morphology in this complex as an adaptive solution for various environments.