Accounting for clinal variation and covariation in the assessment of taxonomic limits: why we should remember the ‘rules’

Well over a century has passed since the publication of a number of ideas that later became enshrined as biological ‘rules’: that endotherms tend to be larger in colder environments (‘Bergmann’s rule’; 1847, but see Salewski & Watt 2017), that the proportions of their external appendages change along the same temperature gradient (‘Allen’s rule’; 1877), and that they tend to be more heavily pigmented in more humid (wetter) environments (‘Gloger’s rule’; 1833).