Adaptive evolutionary variation:
Bergman's Rule in North American ermine (Mustela erminea)

In western North America, Least Weasels (ermine) [Mustelidae] exhibit a marked latitudinal gradient, with larger animals found in the more northerly parts of the range. The map indicates the origins of each of the skulls: each typically represents a different named subspecies. Mean and variance of size are highly correlated with latitude. The trend is an example of Bergman's Rule: size increases exponentially as length increases linearly, and bulkier animals are better able to retain heat in cold environments. Bergman's Rule is one of a series of Ecogeographic Rules that occur across many species, and which often are associated with thermo-regulatory adaptation. Other examples include Allen's Rule, which finds more compact extremities in colder northern latitudes, and Gloger's Rule, which finds darker pelage or skin coloration in more humid, tropical environments.

    "Shape-shifting" adaptive change in response to warming global climates has recently been demonstrated in a number of species, especially with respect to Allen's Rule.