In my experience, we always found phenotypical variability even in low elevations, but it increases with high level to about 1 000 - 1 500 meters...
This map is an extract of "Berroneau 2014 - Atlas des Amphibiens et Reptiles d'Aquitaine".
It shows the intensity of morph zinnikeri vs aspis in our region. Be careful, I speak about
morph, and not
sub-species, because perhaps genetic will show other thing.
But clearly we find more variability with a high zinnikeri level.