It is widely agreed that the evolutionary lineage of the grey wolf can be traced back 2 million years to the Early Pleistocene species Canis etruscus, and its successor the Middle Pleistocene Canis mosbachensis. The grey wolf Canis lupus is a highly adaptable species that is able to exist in a range of environments and which possesses a wide distribution across the Holarctic. Studies of modern grey wolves have identified distinct sub-populations that live in close proximity to each other. This variation in sub-populations is closely linked to differences in habitat – precipitation, temperature, vegetation, and prey specialization – which affect cranio-dental plasticity.