(previous in thread, caro) : ~
~ The human immune system is poorly designed, as are the defense
~ systems of poisonous plants. Poison ivy and related plants cause
~ allergic skin reactions. They are painful, itchy, and impossible to
~ ignore. However, the skin reaction doesn't occur in the presence of
~ the plant, but several hours to days after exposure, once the
~ animal has surely forgotten what it touched. I'm not sure how this
~ is supposed to protect the plant, but a simpler solution would be
thorns, which many a better-designed plant employs.
I believe the answer to this is that the skin reaction we experience is incidental. Plants like poison ivy are protecting themselves from bugs and molds and bacteria by being so noxious, not from big creatures. We just happen to respond badly to their defenses.
Histamine reactions (though not allergic ones) are like fevers--it would be nice to believe they served some purpose, but it sure isn't clear what it is.
Despite all the hype about genetic algorithms and the like, evolution is a grossly inefficient method for producing barely adequate solutions, although given that all it had to work with to start with was a bit of dirty water, it's done a pretty amazing job with the materials at hand.