Morphometric methods allow the quantification of directions of phenotypic changes and their statistical comparison in a morphometric space. We applied this approach to investigate several candidate factors to explain changes in mandible shape occurring in house mice (Mus musculus domesticus, Mammalia, Rodentia) in Corsica and a nearby islet. The role of niche widening and of the concomitant change in diet was evaluated by comparing the micro-evolutionary insular change to the macro-evolutionary difference between omnivorous and herbivorous rodents. Phenotypic plasticity, which may contribute to rapid insular evolution, was assessed by breeding laboratory mice on hard versus soft food. The related change in mandible shape was compared with differences between continental and insular populations. The role of allometry was evaluated by assessing shape change related to size within the continental population and comparing this direction of change with differences on islands. Finally, evolution may be facilitated along the direction of the greatest phenotypic variance. This hypothesis was tested by computing in wild populations vectors corresponding to this direction and by comparing these vectors with those corresponding to estimates of shape changes related to plasticity, micro- and macro-evolutionary processes. In Corsica, the congruence in directions of macro- and micro-evolutionary phenotypic vectors (Corsican/continental mice versus omnivorous/herbivorous rodents) supports the hypothesis of adaptation in mandible shape evolution. By contrast, on the islet, phenotypic divergence follows directions of plastic response to food consistency as well as within-population allometry. Thus, results suggest differences in the relative importance of processes which may influence rodent mandibular shape depending on the size of the islands they colonized. Faster evolution and plasticity may be more evident in small and often ephemeral populations living on small islands, whereas micro-evolutionary processes may have enough time and genetic variability to progressively ‘align’ with macro-evolutionary trends in large populations from big islands. Resume Les methodes morphometriques permettent de quantifier des directions de changements phenotypiques et de les comparer statistiquement. Cette approche a ete appliquee pour evaluer plusieurs facteurs explicatifs des differences de forme de la mandibule observees chez des populations de souris domestiques (Mus musculus domesticus) de Corse, de Sardaigne et d’un ilot voisin de la Corse. Le role de l’elargissement de la niche et du changement alimentaire concomitant a ete teste en comparant le changement insulaire (echelle micro-evolutive) a celui observe entre mandibules de rongeurs omnivores et herbivores (echelle macro-evolutive). La plasticite phenotypique pourrait egalement contribuer a l’acceleration des changements evolutifs sur les iles. Le changement mandibulaire associe a eteevalue en elevant des souris de laboratoire avec deux nourritures de consistance differente, en granules ou gelifiee. La composante allometrique de la variation de forme a eteevaluee sur des populations continentales. Enfin, l’evolution phenotypique peut etre facilitee le long des directions de plus grande variance. Cette hypothese a ete testee en evaluant ces directions dans les differentes populations sauvages et en les comparant aux directions des changements plastiques, allometriques, micro- et macro-evolutifs. La congruence des directions micro- et macro-evolutives observee dans le cas de la differentiation continent – Corse, continent – Sardaigne et omnivores – herbivores, corrobore le role de l’adaptation dans le changement de forme de la mandibule chez les souris corses et sardes. La divergence de la population de l’ilot, en revanche, correspond aux directions de changement allometrique et de remodelage osseux en reponse a la consistance de nourriture. Ces resultats montrent l’interet de comparer quantitativement des directions de changement phenotypique pour analyser les causes complexes de differenciation des populations naturelles. Ils suggerent en outre que differents processus agissent sur des iles de taille contrastee, possiblement a des pas de temps differents : plus long sur les grandes iles comme la Corse, et plus rapides, impliquant une reponse plastique, sur de petits ilots.