There are some flaws in the common understanding of survival of the fittest, but I will ignore that for now.

Poison is usually energy intensive to make, so there is an investment from the mushroom in being harder to eat. If there is not a strong pressure from predators, or the mushroom adopts a different strategy like making tons of fruiting bodies (mushrooms), it might not be worth the cost to make the poison.

Finally, poison doesn't automatically win the evolutionary race. We eat tons of stuff that the plant/mushroom intended to be a deterrent or poison (why are peppers spicy?).

there are plenty of animals that would eat fungi, its nutritious. slugs, bugs. death cap is the most toxic, specifically anamita phalloides, destroying angels is one too. the toxin amatoxin-A, and phalloidin both are toxic to the liver, usually kills you within days if untreated. the symptoms are extreme hunger. mammals are most affected, but bugs/slugs evolved a way to deal with it.

anamita phalloides is a problem because its been introduced in the US from imported oak, its arbuscular mycorhizzae as well. and people have mistaken it for a edible version. it depends on the sitiuation, but producing chemicals is actually energy intensive and makes overall fitness for the organism less than the ones that dont rpoduce it. same goes for antibiotic resistant bacteria, they expend more resources in producing it. but its get complicated as bacteria might have other mechanisms to deal with it. if you look a the biosynthesis of amatoxin A or phalloidin, or any toxin, the molecule is quite complex, and requires several enzymatic steps, if you taken biochem you can sorta follow how its created, its quite fascinating.

phalloidin is used in biotech, for its staining ability, it stops actin in cells, so its useful when you want to trace specific proteins, or genes products.

a plant that produce similar effects with a different toxin. Cholchicine comes from the autum crocus, and its used to treat GOUT and mediterrenean fever. apparently gastropods are immune to death caps.

humans cant eat wild berries, but birds can, different evolutionary ways of dealing it.

toxic plants are also quite fascinating, monkshood is extremely poisonious to people, also known as wolfsbane,historically it was used to kill wolves. even touching the plant will poison you.

euphorbia plants, produce a toxic sap that cause severe dermatitis and blistering, burning it is unwise. thats why most animals dont eat things like euphorbia succulents(they look like cactuses). machineel, dynamite tree.

i believe mostly slugs, and insects can deal with death caps, anamitas, while mammals cannot. thats why birds can eat wild berries, because theres a survival strategy for the berries on the plant, they use the birds to spread the seeds to other areas.

I would argue that every species alive today evidently found a strategy that works, otherwise they wouldn't be here.

Perhaps some mushrooms actually rely on being eaten somehow. Or perhaps being poisonous comes with downsides, so it isn't purely an advantage.

If poisonous mushrooms are poisonous because it is advantageous for the survival of the specific species, are non poisonous mushroom relatively speaking weaker, less well equipped for the furthering of their species?

It depends on many factors/variables. For instance, if they spread by being eaten and then shit out in a different area then being poisonous could work against them. If they don't have something that preys on them by eating them, then producing poison can make them do work that would be better spent in some other fashion.

I suppose that the ability to survive and further a species is multifaceted and that being poisonous or the lack thereof is just one of many parameters?

Correct

I know something about poisonous berries.

They're not poisonous to birds but are to mammals with teeth.

If a mammal would eat them they would crush and destroy the seeds. Birds do spread the seeds though.

We have an invasive Arum Italicum here which has poisonous berries and I thought that was stupid but apparently it's targeted deterrence and it's not stupid.

Looking at all the elaborate replies, I cannot help being amazed. Here I thought the Fedi is overrepresented by computer geeks (such as yours truly) but obviously they've got NOTHING on the biologically inclined here! 🥰😍

Another thing to note is that evolution works on everything all at once. The fact that a chemical is poisonous to mammals can be coincidentally when the real purpose is for something else. Fungus have to do lots of decomposing and digesting of different compounds.

Also to note that the mushroom part is just the fruiting body for the fungus that spreads the seeds. So poison there is under a different evolutionary pressure than a poison frog who will die when eaten . If the fruiting body spreads spores then it's done it's job

Well, there’s poisonous, toxic and indigestible. Various things can be one of those three to various other things; since mushrooms came first, you’ll find that most of them tend to be toxic to other organisms that haven’t evolved to consume them. Also, it’s good to differentiate between the mycelium and the fruit; often the mushroom fruits ARE poisonous to mammals because they propagate by airborne spores, so the poison evolved as a protection against being eaten before they could spore.

Other mushrooms depend on other features for their survival, from looking like a poisonous mushroom, to not fruiting, to making digestion by an animal part of their reproductive process.

There’s probably more variety in the mushroom kingdom than there is in the animal kingdom.

IDK anything about biology so this is just supposition...

Aren't fungi one of the oldest forms of life?

If so is it possible the poison isn't really relevant to the fungis survival and reproduction, but just... incidentally toxic to mammals?

You should ask toxic people, they seem to thrive in this economy (ba dum tss).

I would guess it depends on the environment. Energy needs to he expended to develop and secrete toxins, it's not a free perk.

Plus, you have all the fungi (+ other critters) who cheat and simply look like a poisonous guy to avoid getting eaten. I know there's lots of snakes and such that do that, there are probably plenty of copycat shrooms.

Did not do biology so don't know any of the specifics!

what gets me is poisonous berries. usually the point of fruit is to get eaten and spread the seeds. you basically want the seed to be indigestible so that it is eaten around or spit out or survives the process to be deposited in poop.

As for ourselves, and most other mammals: we rely heavily on social relationships, so much so that our survival depends on them. Suddenly becoming poisonous would be a big disadvantage to us. Imagine if all pets and livestock got poisoned from us touching them.

Well we're kind of poisoning the world already. But you get what I mean.