The view that the zub-zero environment of Siberia would cause animals to evolove to deal with that cold is incorrect then?
A hypothetical for you to consider...
Imagine a species of animal living in a temperate climate in the south of a land mass (group A). These animals have a thin layer of subcutaneus fat and a thin coat of fur making them unable to cope with very cold temperatures. This means they have to confine themslves within a geographical location where the temperature does not get too cold for them to thrive. Now imagine the population grows and its food resources become scarce. Yet there are food sources further north where the climate is too inhospitable. If some individuals possessed a genetic sequence that produced a thicker layer of subcutaneous fat, they could start to colonise the colder regions (group B) and take advantage of the more abundnat food supply. They would also have an advantage if the climate were to become generally colder. This could result in the two groups becoming more isolated from each other until eventually group A and Group B can no longer breed togther. The two groups are now forming two different gene pools.
This process can continue. Some individuals in group B develop a gene sequence that allows for thicker fur. With the thicker fur and subcutaneous fat layer they can form a group even further north, right up into Siberia and form group C. With time this group becomes isolated from B, it forms yet another gene pool. These isolated groups will continue to evolve along different pathways and eventually form 3 different species living in 3 different climates.