The Neogene paleoisland from the area of Gargano, Italy, has yielded numerous fossil vertebrates, some of them showing extraordinary morphological peculiarities due to island evolution. Among them, Microtia (Freudenthal 1976) is the dominant rodent genus in the Gargano palaeofauna and is represented by at least three evolutionary lineages. The incisors are used to describe the size evolution in these lineages, and we come to the conclusion that these lineages did not follow the same evolutionary trend: two of them evolve toward larger size, while the third one shows a slight decrease in size. In addition, we describe the evolution of the curvature of the lower incisor, compared with that of body-size. The evolution of Microtia is characterized by a specialization for burrowing, which may be accompanied by either an increase or a decrease in size. Finally, we propose that the evolutionary change among these three sympatric lineages allowed Microtia to minimize competition between species, by avoiding size overlaps.