Coevolution, With Particular Reference To Herbivory

with particular reference to herbivory

BIOL 0106


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 already present in the hosts ancestors. This allows us to determine whether traits are coevolved or merely a trait exhibited as a consequence of the organisms genotype. For example, Yucca plants provide a source of food for host specific Yucca moths, with which they are believed to have coevolved. Examining the phylogenetic trees of these moths elucidated this. Most moths in this family (Prodoxidae) exhibited host specificity (Davis et al 1992). Before this technology, people would have assumed the specificity of the Yucca moth to be a product of the coevolution.
This brings up a useful comment by Vermeij (1994). Almost all inferences about coevolution are derived from the existence of trends in the expression of traits that function during interactions between species. Evolutionary trends have often been found by analysing ancestor-dependant relationships within monophyletic groups, or clades. Although many trends are best sought this way, others cannot in principle be detected within single clades and instead arise when ecologically and functionally comparable clades replace each other through time.
2) Unique traits. The Yucca moths as described above have tentacles on their mouthparts used to hold pollen for later active transfer to floral stigmas. Using phylogenetic studies, it has been found that the ancestors to these moths did not have tentacles, suggesting a coevolutionary adaptation.
3) Relative malleability of traits. Regardless of selection intensities, some traits may be more malleable than others. Recognising this enables us to discriminate between organisms that appear to be evolutionarily constrained (low malleability) and those that appear to be evolutionarily dynamic (high