The case described here analyses morphological change at the boundary between ecological and evolutionary scales. The size and shape of 8 populations of two sibling species of tenebrionid beetles (Asida planipennis and A. moraguesi) are analysed using landmark-based methods. The two species differ in size, shape and in allometric trajectory. Thin-Plate Spline Analysis (TPSA) combined with Canonical Variate Analysis (CVA) reveal the specific shape changes that allow the best inter-species discrimination. These changes involve the outline of the posterior margin of the pronotum. Moreover, the landmark-based method provides useful tools for interpreting the intra-species variability of some continuously varying morphological characters. In the case of A. planipennis, size and shape are correlated at the inter-population level, but are independent at the intra-population level. Moreover, size and shape do not reflect any spatial (i.e., geographical) structure or phylogenetic inertia at the inter-population level. These facts favour site-specific environmental conditions as the main cause of shape and size variability in this species. One environmental variable is suggested to be the cause of the inter-population morphological differences detected.