On the evolution of shape and size divergence in Nebria (Nebriola) ground beetles (Coleoptera, Carabidae)

Abstract

Ground beetles include many species of rather recent evolutionary origin that are extremely similar in their external morphology. We used geometric morphometrics to quantify the relative degree of morphological divergence in three closely related polytypic alpine species of the genus Nebria, subgenus Nebriola (Nebria cordicollis, N. laticollis and N. fontinalis). To examine evolutionary patterns, we adopted a hierarchical design, using both shape and size to discriminate between species, subspecies within species, and populations within subspecies. A semilandmark-based approach was used to describe the pronotum, elytron and median lobe of the aedeagus. The three closely related species diverged significantly in shape and size. There were clear interspecific differences in the shape of external traits (elytron and pronotum) and of the median lobe of the aedeagus. Furthermore, species also diverged in body size, and in the relative size of traits (evaluated by computation of their static allometries). Shape differences among subspecies and populations were more limited. However, the three subspecies of N. cordicollis considered, N. c. cordicollis, N. c. kochi and N. c. winkleri, showed three distinct body sizes and differences in relative size of the elytron. A few size differences were also evident between distinct populations. These results seem to suggest that size may diverge more rapidly than shape in Nebriola ground beetles. Both at the species and subspecies level, body size order was not reflected in the relative size order, indicating that body size and relative size of traits do not necessarily evolve in concert.