Evolutionary arms races

The role of antiproliferative effects of an activated defense reducing the next generation of herbivores compared with the straightforward action of toxins or feeding deterrent metabolites in the evolutionary arms race is still under discussion. The proposed mechanism suggests natural selection at the group level,... 

In general, we find a series of related compounds in each plant often a few major metabolites and several minor components which differ in the position of their substituents (Fig. 9.4). Sparteine and lupanine only differ by a single keto function, but their modes of action differ substantially (Fig. 9.2). The profile usually varies between plant organs, within developmental periods, and sometimes even diurnally (e g., in lupin alkaloids60). Also marked differences can usually be seen between individual plants of a single population, even more so between members of different populations. This variation, that is part of the apparent evolutionary arms race between plants and herbivores, makes adaptations by herbivores more difficult, since even small changes in chemistry can be the base for new pharmacological activities. 

Thus male mate-rivals mimic their predators and predators mimic mate-rivals, each to its own benefit. The current reciprocal mimicry situation seems to be the result of a long co-evolutionary arms race involving mate-rivals and the predators, begun by mate-rivals mimicking and making use of flash errors predators made when attracting them (see Fig. 10.3). At the moment there are no alternative explanations for the behavior shown by these species, and field experiments are now being made to test the reciprocal mimicry explanation (Lloyd, personal communication). 

Fig. 10.3 Model of an evolutionary arms race (conflict co-evolution) leading to reciprocal mimicry of Photinus macdermotti males and their predators, Photuris females. A number of variants and origins are possible (from Lloyd, 1981, with permission).

Insects have been challenged with chemical pesticides for centuries, but over the course of the last 100 years innovations in the development of insecticides have placed humans in a direct evolutionary arms race with many insect species. And, in that battle, it would be hard to conclude that the insects are losing, for during the last century, over 500 insect species have generated resistance to one or more pesticides. 

Evolutionaiy co-adaptation of predator and prey is a common occurrence in the history of life. This can lead to evolutionary arms races in which the evolution of better predation ability exerts selective pressure on prey for better escape, which exerts selection on the predator for higher abilities, etc. Similar evolutionary arms races have played out between disease microorganisms and human antibiotic use. In these cases, powerfiil antibiotics are invented and used widely, leading to selection on bacterial disease populations to adapt to them. The failure of antibiotics then leads to a search for a new antibiotic, which, when used widely, generates new adaptation, etc. 

The establishment of plant integrated defenses involves the preferential evolutionary retention and production of those SCs exerting synergistic toxic effects and is possible only if a diversification of secondary metabolism in a given plant has previously occurred. This preliminary diversification of secondary metabolism could be mediated via the classical reciprocal co-evolutionary interactions between a host plant and its major pests, as predicted by the chemical arms race model (Beren-baum and Zangerl, 1996). The PICD hypothesis is consequently not an exclusive evolutionary hypothesis because it is compatible with and dependent on other evolutionary processes. The contribution of the PICD hypothesis is to provide both a functional explanation for the diversity of SCs within plants (Romeo et al, 1996) and a reconciliation between different evolutionary models. 

One fascinating aspect of hydrocarbon evolution as semiochemicals lies in the documented chemical mimicry systems between parasite and host (Chapter 14, this book). Some systems operate by camouflage and passive transport, others by loss of parasite specific compounds, some use de novo synthesis of the same host mixture or part of the mixture, and others are as yet unrevealed. To date we are mostly limited to the description of the host-parasite chemical coevolution (or arms race), but we hope in the near future to be able to associate biosynthetic pathways as well as gene expression with such evolutionary processes. 

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