Morphometric differences between extant and extinct Italian populations of the adder, Vipera berus (Linnaeus, 1758)

Vipera berus (Linnaeus, 1758) is the terrestrial snake showing the widest distribution in the world, occuring from Great Britain, France and Northern Italy to the Sakhalin Island and North Korea (Nilson, 1980; Saint Girons, 1980; Nilson et al., 1994; Nilson & Andrén, 1997a). However, adders do not occur uniformly over their distribution area, but are scattered in several isolated populations (Nilson & Andrén, 1997a). Frequently, ecological traits of borderline and isolated populations differ from those living in the core area of the distribution range of the species, and might be subjected to higher risks of stochastic extinction and higher differentiation rates (Mayr, 1970). For example, meadow vipers (Vipera ursinii) show a highly fragmented distribution, many of isolated groups being different subspecies (Nilson & Andrén, 1997b, 2001).