Abstract <p>The occurrence of discrete nonmetric threshold traits (NTTs) of the axial skull and mandible was studied among strains of aggressive and tame American minks (<i>Neogale vison</i> Schreber 1777), obtained after selecting for characters of defensive behavior at an experimental fur farm. Non-selected caged and wild Canadian minks were taken as control groups. After culling the NTTs with invariant frequencies, unclear topologies, single, rare (&lt;5%), and high-frequency (&gt;95%), three versions in the array of traits were used: “expanded” (50 traits), allowing for their connection with sex and size; “restricted” (30), excluding such a connection; and “combined” by sex (50), where the frequencies of males are only taken for sex-related traits. An assessment of the mean measures of divergence (<i>MMD</i>) in terms of the frequency of occurrence of NTT phenes in all versions revealed significant differences between the strains, as well as both control groups. In the first version, the differences between the sexes were most pronounced, compared to those between the strains in the second and third ones. In all versions, aggressive and tame minks differed to the maximum degree, whereas the caged non-selected individuals occupied an intermediate position. When comparing samples, wild Canadian minks are the closest to caged non-selected minks, the divergence between aggressive and tame exceeding the difference between caged and wild. Canonical analysis of the principal components characterizing the manifestation of individual phenetical compositions for a constrained set of 30 NTTs (with lower environmental and greater hereditary conditionality) revealed the same intergroup variations as on the basis of <i>MMD</i>. The effect of selection based on characters of defensive behavior for 16–17 generations was found to be accompanied by a greater differentiation of aggressive and tame American minks than wild and caged ones as a result of their almost century-long isolation of the latter in fur farms. The values of the indices of epigenetic variability (<i>EV</i>) and the volume of within-group morphospace (<i>Vm</i>) characterizing the degree of destabilization of development, are significantly higher in tame minks than in aggressive ones. The results are in good agreement with Belyaev’s theory of destabilizing selection and indirectly indicate a high rate of epigenetic changes in experimental strains of the American mink, this accounting for the high adaptive potential of this invasive species during its range expansion in Eurasia.</p>