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Combination of different dispersal modes may itself, without external obstacles, lead to the appearance of subdivided populations and maintain the existence of independent population systems. The common shrew, a mammal convenient for studying different levels of intraspecific differentiation, was the object of the study. Empirical data have been used for simulation taking into account the change of dispersal modes in the population area. The obtained results agree with empirical data on the distribution of races and hybrids in the hybrid zone of chromosome races Moscow and Seliger. Change of the dispersal mode may maintain independent population dynamics and, in the case of chromosome races, prevent the migration of parental individuals into the territory of the other race.

Two chromosome races of common shrew, Moscow and Seliger, differ in the arm combination in 11 diagnostic chromosomes (Robertsonian metacentrics/acrocentrics). Homozygotes of both pure races, simple Robertsonian heterozygotes of Seliger race, and complex heterozygotes (FI hybrids) were detected in the found earlier between hybrid zone of these races, in the spring before the breeding seasonbreeding season. The g/oheterozygote was first discovered in race Seliger, whose chromosome formula typically contains acrocentrics g and o. The m/q heterozygote was recorded for the second time. Meiosis was studied in 16 males representing five detected karyotypic categories. No abnormal in pairing of homologs in either sex trivalent common for the species (XY1Y2) or autosome trivalents (g/o and m/q) was detected at diakinesis--metaphase I. Two hybrids displayed a theoretically expected and unimpaired meiotic configuration in a form of a very long chain comprising 11 monobrachial homologs (g/gm/mq/qp/pr/rk/ki/ih/hn/no/o). The results are discussed in terms of hypotheses on fertility of complex heterozygotes and limited gene flow in hybrid zone.