Quick question about evolution

[The Grand Nudger]

Mmmmm, I get what you're saying. But in spiders, the first of the venomous ones weren't yet adapted to having their own venom. So they would likely just hunt exactly like they used to. Which is jumping prey and killing them very quickly. In such a short fight, how could a weak venom ever come in handy? Same for self defence, a bird catches you, there's nothing your weak venom will do for you. 

Only about a million ways.  They still hunt that way, venom helps them -to- do that.  The venomous among them could take down larger meals, and were a less pleasant meal themselves.  It's about affecting the behavior of populations, not winning an individual spider v bird or spider v bug fight.  Let's say the "weak" venom makes a bird ill for a day or so after eating those little yellow spiders.  The bird starts eating the brown ones.  Suddenly, there are more brown ones than yellow ones hanging around, having spidersex and spiderspawn.  

I think I just found where I went wrong. I probably shouldn't think in terms of a gradual change in venom-strength. A (venom) protein has a very specific effect. The synthesis of a protein happens from your very dna, so a mutated spider can easily stumble across a protein which incidentally has a very strong effect on certain other animals. Like blocking the blood flow or something. There's probably not a gradual chain of ever increasing strength in venoms, because changing the protein sequence probably renders the venom impotent. There's probably a lot of non-relatedness (in the sense of not being anything similar in protein sequence) among venoms. 
So the first venomous spider might not have had a 'weak' venom, but already a very incidentally fatal one. So that solves the problem I had with your answer.
Thanks for helping me out!

There's no need for it to be fatal.  You're still approaching this like it's an all or nothing gambit. It could be incidentally irritating or convenient and that provides a trait to favor.