evolution of toxicity

Factual Questions
1

How would an animal evolve to become poisonous? The only way it could be an advantage is if predators are aware that it is toxic to them, and the only way they could find that out is if they eat it. Even if the predator survives the poison (thereby learning from its experience), the prey assumably wouldn’t. Therefore, it wouldn’t make it any more likely that the animal would survive longer, and perpetuate its gene. If an entire species evolved the poison at the same time, then a predator could learn from eating one and avoid that species in the future; but we all know that evolution stems from mutations in individuals, not changes to species as a whole.

I know that poison is often foul-tasting (which could cause the animal to be spit out before it is killed), but it would be easier and more effective to evolve a bad odour than to evolve poisonous flesh.

I also know that toxicity in an animal is usually accompanied by a distinctive colour pattern, but once again this would be impossible to coordinate given that the mutation would have to appear in an individual, and the individual and its all-important genes would be digested at about the same moment that the predator vowed never to eat another brightly coloured thing.

I’m not disputing Darwin’s theories and I don’t want anybody whining about their brain-dead religions. :wink:

2

I’m no evolution expert but I would guess that should the original animal with the toxic mutation manage to avoid being eaten before producing offspring (as must surely be possible, otherwise no examples of the species would exist whether toxic or not) then enough examples will exist in a population so that a preditors first munch would not wipe them out.

3

An evolutionary story
Suppose you had a population that consisted solely of toads (Bofu maincharacterus), frogs (*Rana budwieserous *) and ravenous hephalumps (Winnius poous). The haphalumps need to eat, and do so indiscriminately. Mmmmm…. Delicious!. One day, one of the toad’s genes for wart causing (let’s not go too far back in the story, just consider that the wart-juice-gland already exists) mutates, and now the wart juice is toxic to the hephalump. Does this do this toad any good? No, evolution doesn’t have any motivational force behind it, it is just a random mutation of genetic material. Along comes a hephalump and eats Mr. Toad, keels over and dies. Looks bad for both parties at this point. However, the hephalump ate the toad the day after he had a chance to pass on his genes to a brood of toadlets. Now there are several toads (not necessarily all of his brood, mind you, but some) with the propensity to produce toxic warts.

Suppose now, that in the population of hephalumps there are some that have a predilection towards toad-eating, some that are indifferent, and some that are frog-biased. For the moment, consider just the toad connoisseurs. They lumber (as hephalumps are wont to do) about the region in search of delicious toads. They go on gulping down toads until they come across a toxic toad - and so end their eating career. The one hephalump, by removing twenty ‘safe’ toads from the population, but only one poisonous toad, served to decrease the overall population of toads by 21 and change the ratio of toxic to non-toxic toads. Not only are the hephalumps helping to alter the ratio of toxicity to delectability, but the noxious toads never stop passing their genes on to new generations. Eventually (many, many, many generations later) palatable toads that are not eaten are in the minority, and face increasing odds of getting it on with a lethal toad, creating a new generation with the lethal gene.

Also keep in mind that the toads, frogs and hephalumps are all evolving in the same environment. Of the three groups of hephalumps, the ones that prefer toads will more and more finding themselves shuffling off this mortal coil, leaving the frog preferers and the diehard indifferents. And die the indifferents will, especially as the number of toxic toads increases. So you are left with a population of poisonous toads, frog preferring hephalumps, and market shills, who are soon to be hanging out with Bud McKensie.

Two last things just to consider. One is that the toxicity does not have to be deadly at first (or at all, for that matter). If the hephalump makes the connection between eating a toad and retching violently (ahh, our old friend conditioning!) it will learn to avoid toads and stick to frogs. Also, remember that species evolve together - this is why the cane toad was able to wreck havoc on Australian ecosystems (and make for a nice subtlety in a Simpsons episode). The indigenous population of birds and snakes had no idea that the toad was poisonous, ate them and died in great numbers.

I hope posters will keep in mind I am very much aware of my oversimplifications and gross generalizations, will keep the lambasting to a minimum. (Though lamb basting would be much appreciated. Haven’t had a good crown roast in a while.) I welcome, though, any corrections as needed. Hope this makes sense!
Rhythmdvl
PS Welcome to the SDMB Gordo. Great first question!

4

Nice story Rhythmdvl (except if you are a hephalump). I am not sure that one part of it will work though. I reckon that the chance of a toxic toad being eaten are the same as their proportion in the toad population. How then will they increase their numbers relative to the non-toxic ones? Unless a hephalump learns to discern which is which then any protection given by toxicity will apply to all toads.

5

Hi Ticker,

Sure, the chances of any individual toad being eaten are equal. But he hephalump keeps on eating until he runs into a poison toad, then stops - no hephalump removes more than one poisonous toad from the population, but any hephalump has the opportunity to remove several toad toasties. The overwhelming minority of poison toads at the beginning makes it more likely that the hephalump will eat up many more good toads before he runs into a bad one.

In a last ditch effort to make sense, allow me to switch from animals to marbles. I have a sack of 1000 marbles. 900 are white, 100 are black. I invite fellow dopers in to take marbles home with them under the following conditions. They must randomly select the marbles one at a time, and they must stop when they get to a black one. The odds that the first doper will pick a white marble are much greater than their chances of getting a black one (though it is still possible). As long as there are significantly more white marbles in the bag than black ones, the odds of their having to stop taking marbles stays relatively low. Who will have more marbles, the first people in line or the last people in line? In biological terms, the process does not have to continue all the way until there are only black marbles (or very few white ones). While this change in relative proportions is going on, the toads are still breeding. Whereas at first, the odds of a edible toad meeting a poison toad under a toadstool were fairly slim (i.e. they were much more likely to run into another dinner toad) but as the ratio of toads in the overall population shifts, so too does the chances of meeting Mr. Toxicity.

I don’t know how to paste a table to the MB (or if it is possible) but if you have a spreadsheet on your computer you can see the basic process for yourself. Set up columns with populations, a sum, percentages and odds. Drag-copy the formulas down, and watch the percentages change. It may even look like you are doing real work you your computer! Which, come to think of it, I should get busy doing, afore before my boss comes in a feeds me to his pet hephalump!

Rhythmdvl

6

I see your argument, thanks.

7

Wow, did I just make sense? And to think I lost all my marbles! I am sure that this isn’t the only way to tell the evolutionary story of toxicity. I hope someone else will come along and add to it.

8

I like the idea that a predator will only be able to remove one poisonous animal but it can remove any number of non-poisonous ones. However, I’m not so sure that there could be a genetic predisposition for the haphalumps to eat frogs or toads. And besides, if the toad-connoisseurs died it would provide an advantage to all toads, not just the poisonus ones. You seem to be more educated on the subject than me though, so I’ll take your word on that.

I think that’s probably the best answer I’m going to get. Thanks a lot for the insight! I was thinking too much along the lines of individual organisms competing with each other on a level playing field. I guess it’s not really survival of the fittest, but survival of whoever is left over after everyone else dukes it out. And that is fitness of a sort.

I’d like to modify your marble analogy, Rhythmdvl, to make it more realistic. I thought about if for a while, adding ways for the marbles to mate and such (that would be a sight to see), but I can’t make it realistic without making it more complicated than real life, so I gave up. :o

On a different topic, I think the only way to post a table would be to upload it somewhere else and post a link. Unless you want to try to line it up with these “_” and these “|” and that seems like a waste of time at best and more likely a motivation for suicide.