Variability of Angles of Molar Occlusal Surface Pattern in Northern Red-Backed Vole <i>Myodes rutilus </i> (Arvicolinae, Rodentia) of Far East of Russia
Based on materials from long-term collections, the variability of angles of the occlusal surface pattern of the first lower (М<sub>1</sub>) and third upper (М<sup>3</sup>) molars of the northern red-backed voles (<i>Myodes rutilus</i>) living of Far East of Russia has been studied. The spectrums of theoretically possible variants of structural (all possible combinations of characters) (theoretical spectrum - St) for the angles of М<sub>1</sub> and М<sup>3</sup> of red-backed voles of the genus Myodes have been calculated, containing 252 small variants of angles each. The general laws of the qualities of variability of signs М<sub>1</sub> and М<sup>3</sup> are established. The relationship between the theoretical spectrum (St) and the real variability spectrum (real spectrum - Sr) of both molars is determined. The total spectrum of real variability (Sr) М<sub>1</sub> of red voles contains 6 large and 30 small variants, and Sr М<sup>3</sup> contains 6 large and 32 small variants. We have found that the theoretically greatest maximum possible variability (St) of both molars is eight times higher than their realvariability (Sr). A similar range and nature of Sr was established for both teeth, since they realized a similar number of real variants, and the Sr of both teeth are very similarly located in St (the ranges of variability are similarly limited). The found naturally limited placement of Sr in St made it possible to outline the spectrums (possible spectrum - Sp) containing 84 of the most probable small variants of the angles for М<sub>1</sub> and М<sup>3</sup> each. A comparison is made of the real diversity of variants М<sub>1</sub> and М<sup>3</sup> in animals from continental and isolated island populations. An analysis of the material showed that the isolated island populations of <i>M. rutilus</i> do not differ in the real diversity of the М<sub>1</sub> and М<sup>3</sup> variants, either from each other or from the main-land populations. The results of our study confirm the general biological pattern, according to which the real variability of the structure is naturally limited and many times less than theoretically possible. And also our study confirms yet another general biological pattern, showing at the population-species level the stasis of the morphological diversity of the dental system <i>M. rutilus</i>.