COEVOLUTION WITH PARTICULAR REFERENCE TO HERBIVORY
Of all the extant organisms in the world, it is believed that terrestrial plants and their natural ‘enemies’ constitute more than forty percent. Moreover, plants exhibit a remarkable diversity of supposedly defensive characteristics including trichomes, spines, silica, secondary chemical compounds, temporal avoidance of enemies, and structures along with chemicals that attract predators of their natural enemies. In addition, the exploitation of the plants and their defences is facilitated by a vast number of behavioural, morphological and physiological adaptations by herbivores
Accounting for this diversity has been a major area of research for nearly a century. The seminal article, attributing this diversity to coevolution, was published in 1964 by Ehrlich and Raven. They suggested plants and herbivorous insects evolved reciprocally by the following events: Plants, through occasional mutations and recombinations, produced a series of chemical compounds not directly related to their basic metabolic pathways. Some of these compounds, by chance, serve to reduce or destroy the palatability of the plant in which they are produced. Such a plant, protected from the attack of phytophagous animals, would in a sense have entered a new adaptive zone. Evolutionary radiation of plants might follow.
If a new recombinant or mutant appeared in a population of insects that enabled individuals to feed on some previously protected plant, selection could carry the line into a new adaptive zone. Here it would be free to diversify in the absence of competing herbivores. Ehrlich and Raven (1964) emphasised the importance of the reciprocal selective responses between ecologically linked organisms.
Since 1964, studies have questioned Ehrlich and Ravens postulates. Due to the nature of evolutionary study, ideas are only as strong as the background in the literature; that is, acceptance by the scientific community depends upon its knowledge. In time people learn more and previously weak theories become more feasible. Alternatively, and more so in science, accepted work in time becomes disregarded (example; until the 1950’s geologists believed in static continents, now all believe in plate techtonics and continental drift). The significance of this is that any theory published is only speculation of what is happening in these interactions. The knowledge is blind in that historical findings leading to these assumptions are not concrete. What happened in the past might be a different picture to what we have envisaged so far.
Thompson (1999) has proposed that there are crucial components to coevolution. These need to be recognised before we can fully understand coevolution. Firstly, phylogenetic studies are providing five kinds of data important in interpreting the historical context of coevolving interactions. 1) Shared traits. Phylogenetic studies are allowing us to evaluate which traits of interacting species were...