Can someone explain why some snakes are not poisonous while others have a most elaborate tool box with behavioral procedures for target identification et. al. Although not a philosophical question, I just have trouble surmising a simultaneous evolution of all the subsystems as well as why would some snakes have this capability and lose it.
As far as I can find out, once venom had evolved at the base of the advanced snakes (Colubroidea), they've all retained venomous saliva, so that there really are no non-venomous Colubroidea, just Colubroidea that wouldn't cause severe symptoms in humans if one bit us in the leg. See
here for some discussion.
You are right to think that all the subsystems wouldn't evolve simultaneously. But they didn't need to. The first step was the evolution of venomous saliva. Then you get grooved teeth, which give the snakes an improved delivery system; then true hypodermic fangs. From the link above:
Some non-venomous snakes have been previously thought to have only mild 'toxic saliva' but these results shown that they actually possess true venoms. This makes perfect evolutionary sense, there cannot be a strong selection pressure for the development of advanced pieces of architecture like fangs unless there was already a potent venom worth delivering. Therefore, venom preceeded the fang.
This page has interesting information on the various form of delivery system. As you can see, the simplest system involves: no specially adapted teeth; no muscle specially adapted to pump the venom into the prey; no mobility of the fangs; and the toxins being produced in the salivary glands rather than a specialized venom gland. So clearly it isn't necessary for all subsystems to be present in order for snake toxins to be useful; consequently they didn't all need to evolve at once.
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