Quick question about evolution

[Yoo]

What does a poisonous predator have to gain from very weakly poisoning a prey, like the earliest spiders probably did? Is that really a strong enough pressure for the first incidental venomous spider to have a real advantage?

Injury, incapacitation.  Hurting or disorienting something is halfway to putting it on the plate, at least.  In defense..if your bite just hurts..and mine makes you sick......I'm gonna win that fight.

Yeah I do get the basic reasoning, I just see a few practical issues. Back in evolutionary time when the spiders were just about to become venomous, they probably hunted by ambushing and jumping their prey. (not sure about this, but I think ambushing was there before venom or making a web was.) Now if you've ever seen a spider catch a prey there's not a lot of a fight going on between it and its prey. So that's why I have some doubts about the whole "weakening the prey is a real advantage" claim.

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Wow so much replies! Busy answering them...

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What does a poisonous predator have to gain from very weakly poisoning a prey, like the earliest spiders probably did? Is that really a strong enough pressure for the first incidental venomous spider to have a real advantage?

Hunting prey is only one side of the coin. Defending yourself against a larger predator is another. Survival pressure would thus favour higher toxicity of venom.

This is a reply to you and to Aoi Magi.

I don't really get how weakly poisoning a large predator gets you anywhere. As a small scorpion or spider, you're probably eaten before you can even react, and if you're not, you will be before the poison kicks in, right? Remember that the poison was probably not very strong at first.

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A strong poison is a "don't fuck with me" system. Mammals and reptiles take note.

Yeah but that only works after the poison is a standard in the spider/scorpion population. There must have been a first poisonous (however weakly) spider, and that one spider couldn't have had any advantage from himself being a warning, because it was most likely eaten anyway. And if it wasn't, an individual can't cause such a system in the mammals and reptiles on its own, so the individual venomous spider doesn't benefit.

[Jesster]

I think the confusion here comes from the idea that evolution has a goal. This is false.

Evolution happens when a change makes a species more likely to survive and successfully reproduce. There's no conscious decision to make venom stronger in a species. If its specific venom just happens to help it survive longer than its relatives, then its specific strain will become more dominant.

[Yoo]

google yields an example

http://www.iflscience.com/plants-and-ani...squirrels/

Ok, that really answers my basic question. It's arms races, should have thought of that.
I still have a few questions about how that arms race could have started, and on the workings of it.

[Mermaid]

I think some specific prey animals had evolved increasing immunity to the venom, which led to venom arms races causing some snakes to be incredibly poisonous to other animals that aren't normally prey

I've read about arms races, didn't think of that though, you're porbably right. 
Some thoughts: 

Because the prey is usually very small, the amount of venom they get is a lot relative to their size, so even though they are better adapted to handle it, they react just as strongly to the venom as we do...?

What does a poisonous predator have to gain from very weakly poisoning a prey, like the earliest spiders probably did? Is that really a strong enough pressure for the first incidental venomous spider to have a real advantage?

Btw, will make an intro!

Hi there. Evenomation is not a constant; sometimes there will be a lot, sometimes, not very much, and other times, not enough. It's a trade-off. It can range from about a 1 mg to 1 gram for a rattlesnake, for instance, depending on several factors--age, size, body condition, time since last bite, etc.
Venom is a super interesting thing to study. There are some venoms from spiders and snakes that contain proteins with a whole spectrum of activity. If you fractionate it, you can get venom specific to insects, or mammals, or birds, etc. Venoms can contain proteins that specifically target blood clotting, or some nerve impulse transmission mechanism, or several other things. Some animals have more of a certain mechanism to interfere with than others. What kills a person may only cause a dog momentary paralysis. Black widow and brown recluse spiders, for instance, typically aren't as dangerous in a dog as they are in a person.

[Yoo]

I think the confusion here comes from the idea that evolution has a goal. This is false.

Evolution happens when a change makes a species more likely to survive and successfully reproduce. There's no conscious decision to make venom stronger in a species. If its specific venom just happens to help it survive longer than its relatives, then its specific strain will become more dominant.

I pride myself in thinking I understand evolution quite well, and I know that it's not part of the theory that evolution has a goal in mind, and there's no reason to suppose it has. 
The basic question is already answered by Alex K, there's just a few little things left unanswered for me now.

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I've read about arms races, didn't think of that though, you're porbably right. 
Some thoughts: 

Because the prey is usually very small, the amount of venom they get is a lot relative to their size, so even though they are better adapted to handle it, they react just as strongly to the venom as we do...?

What does a poisonous predator have to gain from very weakly poisoning a prey, like the earliest spiders probably did? Is that really a strong enough pressure for the first incidental venomous spider to have a real advantage?

Btw, will make an intro!

Hi there. Evenomation is not a constant; sometimes there will be a lot, sometimes, not very much, and other times, not enough. It's a trade-off. It can range from about a 1 mg to 1 gram for a rattlesnake, for instance, depending on several factors--age, size, body condition, time since last bite, etc.
Venom is a super interesting thing to study. There are some venoms from spiders and snakes that contain proteins with a whole spectrum of activity. If you fractionate it, you can get venom specific to insects, or mammals, or birds, etc. Venoms can contain proteins that specifically target blood clotting, or some nerve impulse transmission mechanism, or several other things. Some animals have more of a certain mechanism to interfere with than others. What kills a person may only cause a dog momentary paralysis. Black widow and brown recluse spiders, for instance, typically aren't as dangerous in a dog as they are in a person.

Thanks for your answer! It's actually quite funny how the venom intended for killing a dog ends up killing us, while "only" paralyzing the dog. The snakes have lived with dog(/like) animals so the dogs are better adapted to it. But superficially it seems like evolution made the venom weaker!

[vorlon13]

What does the strawberrry or peanut gain from trigering allergic reactions in certain people ??

That a secretion from some bug in a godforsaken swamp somewhere might be toxic to my pansy caucasian ass doesn't strike me as a big surprise. If my ancestors had encountered it and the lucky few with the right enzyme mutation survived, I'd be immune too. No one has ancestors exposed to (and then survived) everything.

[Cyberman]

This is a reply to you and to Aoi Magi.

I don't really get how weakly poisoning a large predator gets you anywhere. As a small scorpion or spider, you're probably eaten before you can even react, and if you're not, you will be before the poison kicks in, right? Remember that the poison was probably not very strong at first.

Right, so the spiders with weaker venom got eaten more often than those with more powerful venom. You don't have to be able to kill your attacker to survive, merely discourage it. Over generations, evolutionary pressure would favour increasing potency of venom; balanced on the other side by increasing resistance to that venom by the spiders' predators and prey.

[Yoo]

This is a reply to you and to Aoi Magi.

I don't really get how weakly poisoning a large predator gets you anywhere. As a small scorpion or spider, you're probably eaten before you can even react, and if you're not, you will be before the poison kicks in, right? Remember that the poison was probably not very strong at first.

Right, so the spiders with weaker venom got eaten more often than those with more powerful venom. You don't have to be able to kill your attacker to survive, merely discourage it. Over generations, evolutionary pressure would favour increasing potency of venom; balanced on the other side by increasing resistance to that venom by the spiders' predators and prey.

You didn't really answer my question. I understand the basic principle, "the spiders with weaker venom got eaten more often than those with more powerful venom" but I still see some complications, stated in my question.

[Arkilogue]

Hi everyone! This is my first thread on this site, and I'm starting small.
I just had a thought today, thinking about venomous animals.

Why is the venom of some animals, like certain kinds of spiders and scorpions, strong enough to kill even a human? 

Surely they don't need venom that strong for their prey, which is not much bigger than mice I'd say, so that can't be the reason. I find it hard to believe that it's for self defence, because the venom doesn't kill or weaken the threatening predator fast enough for it not to be able to kill its prey.

Maybe these sorts of questions shouldn't be on here, because they're too specific, but I'm curious and maybe some of you find this interesting as well.
(English isn't my first language so don't kill me for getting words wrong 'n stuff)

Heard an interesting take on NPR from an evolutionary biologist. She said some developed highly potency toxin, which put them ahead of their predators, and then was re-purposed by their body as a means of subduing prey.

[Yoo]

Hi everyone! This is my first thread on this site, and I'm starting small.
I just had a thought today, thinking about venomous animals.

Why is the venom of some animals, like certain kinds of spiders and scorpions, strong enough to kill even a human? 

Surely they don't need venom that strong for their prey, which is not much bigger than mice I'd say, so that can't be the reason. I find it hard to believe that it's for self defence, because the venom doesn't kill or weaken the threatening predator fast enough for it not to be able to kill its prey.

Maybe these sorts of questions shouldn't be on here, because they're too specific, but I'm curious and maybe some of you find this interesting as well.
(English isn't my first language so don't kill me for getting words wrong 'n stuff)

Heard an interesting take on NPR from an evolutionary biologist. She said some developed highly potency toxin, which put them ahead of their predators, and then was re-purposed by their body as a means of subduing prey.

That's interesting as well! That reminds me of another question I had once, I'm going to put it in the thread-intro.