Plant-Herbivore Interactions

As is the case with most supposedly modern concepts in evolutionary biology, the idea of coevolution, or reciprocal evolutionary change between interacting species, actually goes back to Charles Darwin. In the introduction to The Origin of Species (1859), he wrote: …In considering the Origin of Species, it is quite conceivable that a naturalist, reflecting on the mutual affinities of organic beings, on their embryological relations, their geographical distribution, geological succession, and other such facts, might come to the conclusion that species had not been independently created, but had descended, like varieties, from other species. Nevertheless, such a conclusion, even if wellfounded, would be unsatisfactory, until it could be shown how the innumerable species inhabiting this world have been modified, so as to acquire that perfection of structure and coadaptation which justly excites our admiration. It is, therefore, of the highest importance to gain a clear insight into the means of modification and coadaptation…. Early on, then, Darwin pointed out the importance of interactions among organisms in determining evolutionary change, as opposed to “external conditions such as climate, food,” or even “the volition” of the organism itself. Interactions among organisms, however, take many forms. Antagonistic interactions, in which one species benefits and the other is harmed, are themselves diverse. Among those interactions in which both species are animals, the gamut runs from predation, in which one species kills and consumes several individuals of the other species during its lifetime, to parasitism, in which one species merely saps the “reserves” and rarely kills its host. Intermediate and unique to the phylum Arthropoda is parasitoidism, in which one species kills its prey, as does a predator, but, like a parasite, is normally restricted to a single host individual. A comparable continuum exists for interactions between an animal and a plant species; these associations are usually referred to as forms of herbivory (with parasitoidism akin to internal seed feeders of plants). In mutualistic interactions, both species benefit from the interaction. Mutualisms can involve interactions between animals and plants, generally in which a food reward from the plant is exchanged for mobility provided by the animal partner.