Specific/Subespecific status

[Borji Heras]

Most of the autors describe Nikolskii as a subespecific form of vipera berus , altough the situation is still unclear i think they have many differences with beru berus , for example venom-colour, body size and shape, most of melanistic populations , yellowish-green tail , newborns have a characteristic colour , and the habitat is drier.

Maybe someone can help me with a doubt :
if a SPECIE to be called "specie" must be reproductively isolated ,why species like Vipera ammodytes and Vipera aspis in the Tyrol can Hybridate? i can understand this with berus berus and berus nikolskii , because they are more similar and closely related , they have not been isolated for so much time and they share similar characteristics, but what about aspis-ammodytes , aspis -latastei , or ammodytes-berus intermediate forms?

(I´m sorry,my english is not the best hope you understand it )