So, are the dead snakes reproducing?
Or, are other snakes watching and telling their babies not to grow such big mouths?
"evolution" my a$$!
Evolution has as much to do with the least fit organisms not reproducing as it does with the fittest surviving. The snakes which are dying before they can reproduce, or at least bear a large enough number of offspring to pass their genes on to, are those which are least fit to adapt to this intruder-species in their mist. Those which are surviving and are able to reproduce quickly enough are those who have features-be they smaller heads, greater resistance to poison, or whatnot-which allows them to better cope with the invaders.