The analysis of population structure and dynamics of bank vole, which is a characteristic megarange polytypic palaearctic mammal, has supported earlier suggestion that the population density in the ecological centre (optimum) is not only higher, but also more stable compared to that at the periphery, where it varies greatly (with large amplitude). Under pessimum conditions populations are very sparse, do not possess effective population control and animal number is limited mainly by environmental factors that are extremely variable and arrhythmic. On the contrary, under optimum conditions populations are dense, perfectly organized and more sustainable and rhythmical. Sharp fluctuations in peripheral populations promote genetic turnover (through ‘population waves') and along with selection toughening, specific reconstruction of spatial, age and genetic structure, appearance of temporal isolates, reduction of gene exchange, acceleration of chromosomal recombination and other phenomena that create the necessary prerequisites for rapid genetic turnover and overcoming of evolutionary stagnancy provide for evolutionary transformations, which lead to the conquest of new areas, ecological niche changing and speeding up evolutionary transformations.