Perhaps your question was posed too long ago for this reply to make any difference.
But, for my two cents, I can't see any way that the rattlesnake DID evolve. By profession I'm a mechanical engineer involved in systems engineering, and venomous snakes are marvelous creatures with a complex killing mechanism. A folding pair of hypodermic needles, connected to a remote sac of pressurized venom via a venom ducts ... in an animal having a targeting mechanism and musculature to match. And that's just the hardware. Instinct is the accompanying software that contains precise instruction sets of range, appropriate targets, etc. Wow.
Trying to get there (step by small step) from non-venomous snakes seems quite problematic. Non-venomous snakes are quite specialized in other killing mechanics, like constriction. And regardless, a hollow fang does no good without poison, which does no good without instinct, which does no good without associated musculature ... I mean, I've tried drawing diagrams of possible intermediates, and it just gets quite difficult. You have to have several design innovations simultaneously.
So, maybe a good fable would be that a fairy godmother appeared and zapped the creature and made it a rattlesnake!
