8/17/07

The Puzzle of Existence


Why, may I ask, is there poison ivy? I write this with teeth clenched, using all the self-control I have not to turn on myself in an itching, scratching frenzy. My little friend is paying it's yearly visit this week--the oozing, spreading rash caused by that mysterious three-leafed plant. Yes, yes, they say the rash can't spread, but it does. I don't know how, but it does.

(And by the way...I know, I know, "leaves of three, let them be." But I'm so sensitive to this stuff I get it without touching it. I believe one of my children must have been the "carrier" and threats and warnings have already been issued.)

In an attempt to turn a bad situation into an occasion for deep philosophical thought...and I'm not promising this is going to be successful, because I'm having trouble keeping the aforementioned frenzy at bay...let us ponder the existence of poison ivy. Since my kids sometimes read my blog, I will not ask this question with quite the force it deserves. So, let us leave out all extraneous anglo-saxon words, and simply ask: why?

It makes no sense. Let's get off on an erudite foot, and start with Aristotle, who sometimes talks like the whole world fits together like a jigsaw puzzle. Don't think for a second I'm going to get up and go to my bookshelf right now to get the exact quote, but he says something like--Nature makes nothing in vain, so plants exist for the sake of animals, and animals exist for the sake of people. In short, the cow shouldn't feel bad about eating the grass, and you shouldn't feel bad about eating the cow.

Not to take Aristotle too literally, this implies that poison ivy must do people some good. But it doesn't. It really, really doesn't.

With the next flare up just around the corner, let's not belabor the point. Moving right along, there's the biblical creation story. Apart from the fact that I'm just a little skeptical about the deity in question, I really like that story. No, this is not a moment for restraint. I'm going to go for broke and say I love it. I love it because it says God created this and that and the other because they were all simply good. Not good for this or good for that, but just good.

What a beautiful, inspiring story, if people would just extract the moral message in it, instead of regarding this (by an amazing stretch of the imagination) as a serious theory about the origins of the universe. The creation story might then motivate care for the planet, instead of conflict with the best current science. But never mind. The point is--sure, I love the idea that the trees, the land animals, the winged birds, etc., are all good. But what about poison ivy? It isn't.

As all the best current science tells us, posion ivy evolved, and it's got to produce urushiol, the nasty stuff that produces the rash, because of natural selection. In other words, the wimpy ancestors of poison ivy mutated so as to produce urushiol, and the mutants thrived, and mutants among the descendants produced more, until today we've got rampant urushiol-producing poison ivy.

Here's the thing that I just don't understand (she said with gritted teeth). The whole thing has just gone too far. Urushiol supposedly wards off "predators" so that the plant thrives and spreads. But I've read that it doesn't affect animals. If that's true, it's human beings that are the potential predators. But the thing is, urushiol doesn't keep us at bay, it makes us really, really mad.

We have poison ivy in the field and on the banks of the creek by our house. We finally decided to Do Something About It, so we dumped a large volume of poison all over it. Urushiol elicits this kind of all out nuclear attack, so how can it really be life-enhancing for the plants?

In short, by all standards I can think of, poison ivy shouldn't exist. I mean maybe if it were just mildly poisonous, but this is ridiculous.

Well, I managed to distract myself. The itching frenzy has subsided for now. Cased closed, but puzzle not solved.

6 comments:

potentilla said...

There is no particular reason to assume that the characterisitcs of poison ivy as seen today are adaptive today. They could have been selected for because of some adaptive advantage for one of poison ivy's ancestors in the past; and not "adjusted" because there was no strong selective pressure on them to do so.

On a quick Google, I can't tell whether "doesn't affect animals" means "doesn't make animals as uncomfortable as people" or whether some sorts of animals get a mildly unpleasant reaction (grazers perhaps, or animals that would be after the berries but could usefully be discouraged from destroying the whole plant in the process).

The extreme reaction of (some) humans could be an accident of human physiology (just as there is no reason to think that snakes evolved to be deadly to humans because of humans). This hypothesis is somewhat supported by he fact that one has to be sensitised (ie one's immune system learns to react strongly to urushiol) and also that there is individual variation.

Another possibility is that selective pressure from humans was implicated in the development of the poison, but mostly during the long period when humans could not synthesise poisons in factories to get rid of poison ivy (I don't think this one is very likely, because of the timeframes, and because some of poison ivy's relative such as mangoes also produce urushiol).

This is all just speculation; I haven't Googkled anything more scientific than Wikipedia. But in general, evolution is a muddly sort of process.

Jean Kazez said...

I'm sure when I stop itching I'm going to be able to think more clearly about this.

Yes, you're right--the poisons we have now weren't available back when poison ivy was getting "naturally selected." (And by the way, I'm very allergic to mangoes as well..it was sort of a relief when I found out it was all one allergy.)

It is kind of weird though if animals don't react--I read it in one place and want to know more.

It must be because of my present mood that I think the anomaly is the plant, and not my own immune system. This is obviously not an adaptive response. Fortunately, I don't have to go out in the woods and collect berries to survive... because I think I wouldn't.

potentilla said...

This looks as though it would be worth reading:-

AGUILAR-ORTIGOZA, C. & V. SOSA 2004. The evolution of toxic phenolic compounds in a group of Anacardiaceae genera. Taxon 53: 357-364

but I can't find a way to do so without paying.

Extreme immune responses to anything seem a slightly odd phenomenon, now you mention it. Something I was reading the other day (forget what, offhand) opined that our immune system is akin to "another brain" in complexity and subtlety.

Doug.E.Barr said...

Apparently, the plant is spreading. It seems that poison ivy thrives in hot dry conditions. In Canada, scientists are finding poison ivy replacing other flora that used to grow in the no longer moist ditches in the countryside. We only have ourselves to blame.

Ophelia said...

Good, two birds with one stone: I'm about to tag both of you with a tag that includes telling the people you tag that you've tagged them.

Jean Kazez said...

I'm intrigued with the "tagging" business.

Now that I think of it, I did read someplace that global warming was increasing the amount of poison ivy, but I repressed it... THANKS A LOT!