question concerns plant mimicry

Web Sourse: http://druniverse.wsu.edu/QandA.asp?questionID=5301

Dear Dr. Universe,

Question:

My question concerns plant mimicry. How is it that plants, with no visual ability, are able to mimic the forms of insects, animals and other plants? Is it too whacky to assume that through evolutionary time, DNA (deposited through fluids or scales by the object mimiced) has been "read" by the plant? I look forward to your comments.

Answer:

That's a very interesting question. Let's talk about what mimicry is first, to bring everyone up to speed. Mimicry is when a plant or animal resembles another plant or animal and ends up fooling a "third party." For instance, a non-poisonous snake might have the same coloration as a poisonous one, so that its predators won't want to eat it, because they will have learned that this color snake tastes bad and causes upset stomach. Another example is when a plant that doesn't have any delicious fruit to offer resembles a plant that does, so that pollinators will be attracted to it anyway.
I have to emphasize that plants and animals do not do this on purpose. They can't think of this strategy, then change their DNA. It is an evolutionary process that takes generations to accomplish. So it doesn't matter that plants have no visual ability. Insects, which do, do not use their visual abilities to become mimics. They don't say, "Hey, my body looks like the leaves of that tree, so I'll go over there and hide from predators." Instead, a type of insect that likes to live on a particular tree will survive better if it blends in. Then it will become the most populous of all the variations of its type. In fact, there are often episodes of selection that cause a trait to become a mimicking one. Consider a particular type of moth, Biston betularia, which used to be found mostly in white, and seldom in gray/brown. A population of these moths lived near a city while it became industrialized and trees were eventually coated with soot. After this, the dark moths blended in really well, and the white moths stuck out and were obvious targets to their predators. Soon, the white moths were few and the dark ones were in the majority. (Note that there later turned out to be some problems with this study, but the story still illustrates my point.)
Another important point, which may refute what you have suggested, is that the mimic often pre-dates the camouflage. Fossils of insects with leaflike bodies have been found and dated. One might think that they were selected for by living on leafy trees. In fact, these insects lived before deciduous trees ever existed! Only the advent of leafy trees would have protected these insects from predators due to appearance, therefore, they were not mimics at this time. Only selective pressures make mimicry useful and cause its propogation.
One last point, and perhaps the strongest, addresses the idea of DNA copying. Suppose that we are dealing with two plant species where it would be beneficial for one to mimic the other and share its living space. In this best-case scenario, suppose that genes can be transferred between the two types of plants and they can be read by one another's cellular machinery. (For insect-plant matches, it wouldn't work; like popping a Mac-formatted disk into a PC - unreadable.) We have a couple of problems. First, there would have to be enough DNA transferred to cause a dramatic alteration in the physical appearance of the pre-mimic. This is unlikely but not impossible, given the enoromous amount of time it has taken for plants to evolve on this planet. Second, the exchanges would have to occur in the reproductive cells of the plant. If it happened in just any old cell, that cell would not be used to form a brand new plant, and thus the new appearance would not be propogated. The best you would have would be a cell that divides a few times, and a small patch of plant that resembles the other plant, and that would be the end of it - No offspring carrying the mimicking traits. Yes, it could happen in a cell used for reproduction, but there would have to be extensive gene exchanges in a long series of reproductive cells over time. For related mimics of different species, I cannot think of any reasons to call it impossible. However, when you really consider the odds, it is far more likely that the cause is simply providential mutation plus natural selection. The "reading DNA" hypothesis is much more fantastic an explanation than the accepted one. Thanks for giving me such an interesting question to think about!