Quick question about evolution

[The Grand Nudger]

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.

Any advantage is a "real advantage" if the subject is evolution.  That being able to weaken prey is an advantage isn't a claim...it's an observation of reality. Sure, it may be overkill in some cases...but you don't want to risk losing a limb in an unexpected struggle with a particularly nasty snack..not like they have an ER to go to, no such thing as a spider leg brace, eh? Even if you're many times bigger and tougher than a spider... a komodo for example, you don;t want to -have- to fight tooth and nail to eat (or to keep from being eaten). One bite...then wait. One bite, then run. Bonus points if your venom helps you to digest your dinner (as it does in the case of many spiders).

At the population level, the less likely you are to die in any engagement..be it offensive or defensive...the more offspring you are likely to produce.

[Cyberman]

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.

Maybe read my reply again. You'll see I did offer an answer to your question. Is it 'the' answer? I have no idea; I'm just a layman offering a layman's opinion based on what I've accumulated.

[Yoo]

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.

Any advantage is a "real advantage" if the subject is evolution.  That being able to weaken prey is an advantage isn't a claim...it's an observation of reality.  Sure, it may be overkill in some cases...but you don't want to risk losing a limb in an unexpected struggle with a particularly nasty snack..not like they
have an ER to go to, no such thing as a spider leg brace, eh?  Even if you're many times bigger and tougher than a spider... a komodo for example, you don;t want to -have- to fight tooth and nail to eat (or to keep from being eaten).  One bite...then wait.  One bite, then run.  Bonus points if your venom helps you to digest your dinner (as it does in the case of many spiders).  

At the population level, the less likely you are to die in any engagement..be it offensive or defensive...the more offspring you are likely to produce.

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. 
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!

[Mermaid]

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.

It's the other way around. Spiders with stronger venom get to have an easier time eating. It's not so much about defense, it's about food.

[Yoo]

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.

Maybe read my reply again. You'll see I did offer an answer to your question. Is it 'the' answer? I have no idea; I'm just a layman offering a layman's opinion based on what I've accumulated.

Thanks for the effort man, still not sure if you did Anyways, I'm off to bed now!

[Cyberman]

Oh well, if we're in the game of making bald statements then I say I did and proved evolution beyond all doubt forever and ever times infinity no tagbacks.

[Yoo]

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.

It's the other way around. Spiders with stronger venom get to have an easier time eating. It's not so much about defense, it's about food.

Maybe some sort of self defence during the hunting. Ensuring the prey doesn't go down with a fight? That's what Rhythm's been saying.
Anyway, I'm off. Sleep well

[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.

[Anomalocaris]

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)

Edit: The question has been answered, but the discussion is now about how an evolutionary arms race could have started.

Another question!
You know those flashy colored frogs? Poisonous animals often have very distinct colors, this has been interpreted as a warning to predators. There are even 'fake' ones, that aren't even poisonous, but do have the flashy colors to put off predators.
The first poisonous animals probably were poisonous out of self protection, like those frogs we see today. Now let's focus on the moment where those animals got flashy colors. I can't really think of how a poisonous animal would benefit from showing its poisonness to predators. Your first reaction might be: That way predators know it's poisonous, so they won't eat it.
I have a problem with that simple explanation, because: the first poisonous animal that showed its poisonness to predators was, obviously, the first one. The predators had no built-in mechanism for avoiding that  particular 'warning sign', so they wouldn't refrain from eating it. You might say that the color might have just put the predators off, but simply being brightly colored doesn't usually put predators off. Then we would probably see a whole lot more brightly colored animals, like bright-green rabbits or something. And of course bright colors in animals are usually accompanied by some kind of venom. So why did the bright colors evolve?
I also don't get why being poisonous for self defence can you anywhere at all as an insect, because who cares if the predator dies or not AFTER eating you?

1. There are many plausible ways for a small animal to evolve venom strong enough to kill a human: One that comes to mind is the poisonous animal needs the venom to be strong enough to act very quickly on its normal target. The strength sufficient to kill a small animal in a few seconds might also happen to be enough to kill a much larger humans in hours or days.

2. if a predatory isn't adopted to recognize a flashy display as a sign of lethal poison, then obvious the flashy poisonous prey is even more screwed then a non-flashy, ordinary prey. But the predatory is also screwed. So predators are preferentially selected to avoid flashy prey. This selection of the predatory then allows flashy prey to preferentially survive. This is how flashy prey evolves. Not because the first hint of flashiness immediately benefit its wearer. Instead, a certain distinguishing feature from all other prey that allows predators to become condition to recognize one from another and force predatory to evolve poison avoidance instinct. This is what over long periods allow poisonous prey to slowly evolve signatures that makes it gradually easier for predator to tell it from another.

[Minimalist]

http://www.natureworldnews.com/articles/...nimals.htm

Evolution: Study Sheds Light On Arms Race Among Venomous Animals