A. G. Vasil’ev, V. N. Bol’shakov, Yu. M. Malafeev, and E. A. Valyaeva
Institute of Plant and Animal Ecology, Ural Division, Russian Academy of Sciences, ul. Vos’mogo Marta 202, Yekaterinburg,
620144 Russia
Received December 8, 1998
Abstract—On the basis of the multivariate analysis of the cranial characteristics of the common muskrat
Ondatra zibethicus L., it was shown that, at the first early stages of acclimatization between northern and south-
ern groupings of animals originating from one unique, genetically homogeneous group of animals from Can-
ada, there arose steady morphological differences at a population level whose rate has remained almost the
same over a half-century period of isolation. This evolutionary-ecological phenomenon emerged over a very
short period of time and was accompanied by a transformation of the cranial shape and size and the pattern of
nonmetrical threshold characters that govern the frequency of manifestation of certain genes. The effect of
“rapid response” for steady morphological structures is unexpected since the more labile morphophysiological
characteristics studied at the initial stage of acclimatization by Smirnov and Shvarts (1959) have not changed.
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