No.....if you've noticed, most, if not all cross-generational relatives have very similar colour schemes....sure, Porygon is lighter than Porygon2 and Porygon-Z, but Seviper is completely different to Arbok, as it's really dark....with little purple...also, whereas Ekans' thin neck spreading out into that massive mask-ish flap of skin Arbok has is believable.....where the hell has Seviper hidden all that purple?
It just couldn't happen....and not everything needs to be a 3-tier evolution line...there are very few Gen I pokémon with no relatives...same for Gen II....not sure about Gen III/IV....but a few Gen IV pokémon were related to previous pokémon anyway...