Articles

The Yucca Moth & the Yucca Plant

by Mark Stewart

There are many binding social relationships in nature which can be explained only with difficulty on the basis of evolution. One of the best known of these is the relationship between the yucca moth and the yucca plant, or Spanish bayonet (Yucca glauca and others). The yucca flowers hang down, and the pistil, that is the female part of the flower, is lower than the stamens, or male part. However, it's impossible for the pollen to fall from the anther or pollen cacs to the stigma, the part of the pistil which receives the pollen, because the stigma is cup-shaped, and the section receptive to the pollen is on the inner surface of the cup. The female of the yucca moth (Pronuba) begins work soon after sundown. She collects a quantity of pollen from the anthers of the yucca plant and holds it in her specially constructed mouth parts. She then usually flies to another yucca flower, pierces the ovary with her ovipositer, and after laying one or more eggs, creeps down the style (the stalk of the pistil) and stuffs a ball of pollen into the stigma. The plant produces a larger number of seeds. Some of these are eatern by the larvae of the moth, and some mature to perpetuate the species.

It's hard to imagine what would cause a moth to collect pollen and to stuff it into a stigma. One hesitates to believe the female knows what the result will be, and it's generally assumed that this is an example of instinctive behavior. Yet this is an obligate relationship, for in the absence of the moth the yucca plant produces no seed, while without the yucca plant the moth cannot complete its life cycle. The moth larvae can feed only on the seeds of the yucca plant. So if the moth should fail to pollinate the yucca, the result would be the eventual extinction of both plant and insect.

How can this phenomenon be explained on the basis of evolution? Which came first, the yucca moth or the yucca plant? At the present time the moth can't live very long without the yucca plant, for it needs the plant to complete its life cycle. The plant cannot live for more than a few years without the moth, for it wouldn't be able to produce seeds and with the death of those plants now alive the species would become extinct. The evolutionist, of course, assumes that this arrangement developed as a result of evolutionary processes, but it's incredible that both the yucca moth and the yucca plant should have reached their present stage of development within a period of just a few years.

Yet to assume that at one time both had different methods of completing their life cycles doesn't solve the problem either, for then the question arises: Why did they give these up? If, for example, the yucca plant at one time was capable of being fertilized by several species of insects, why should it evolve in such a manner as to be dependent on a single species at the present time? It's a remarkable strain on one's credulity to assume that finely balanced systems, such as the vertebrate eye or the feathers of birds, could develop by random mutations. This is even more true for situations such as the yucca moth - yucca plant relationship.