The next three chapters (part two) look at specialization from different perspectives. First from a phylogenetic perspective, arguing that specialization is not necessarily derived and that the evidence for phylogenetic tracking (parallel speciation in different taxa) is weak. Then from the point of view of molecular genetics, with a close look at a few cases where the underlying genetics of specialization are understood. Finally from the point of view of ontogeny, explaining how complex life cycles and ontogenetic compartmentalization make specialization to multiple hosts possible. In all three cases the importance of a geographical perspective is stressed.
Part three looks at specialization in different kinds of interactions. Parasitism (and particularly endosymbiotic parasitism) is conducive to extreme specialization, and specialization is more common in parasitic taxa than once thought, with what were once thought to be single species attacking multiple hosts often turning out to be complexes of sibling species or geographically differentiated populations. This is also true for free-living grazers and predators, though here specialization is less extreme. Specialization is harder for victims, but it is possible for them to maintain defenses to multiple attackers, and feedback between predator prey populations can produce geographical patterns in specialization. Two chapters deal with mutualism. Extreme mutualism is only common in intimate symbiotic interactions, but short-term mutualisms can spread and expand to include many species and to produce complicated geographical structures. A number of things limit mutualist specialization: the tendency to attract new species, the difficulty of "screening" out free-loaders, and interference from other interactions that affect host fitness.
Part four finally brings us to coevolution. Thompson first argues that there is no correlation between the mode of inheritance (gene-for-gene or polygenic) and the existence of coevolution. He then presents the outlines of a geographic mosaic theory of coevolution, contrasting it with approaches that deal only with local populations. (This follows up on the emphasis on a geographical perspective throughout the preceding discussion of specialization.) He then looks at various aspects of coevolution in the light of this theory. Coevolution can play an important role in speciation, especially in pollinated plants and endosymbiont afflicted species. There are connections between coevolution and sex, and it can be hard to define coevolution when one of the species involved is asexual. Asymmetries in evolutionary interactions may result from new species joining interactions or from speciation. There are many ways in which groups of species rather than pairs can coevolve — coevolutionary alternation, successional cycles, coevolutionary turnover — and there is no need to put these in the too hard basket of "diffuse coevolution". The final chapter completes the demolition of "diffuse coevolution" as a useful category by looking at some examples of extremely complicated interactions — such as "escape and radiation" and the interactions of birds with fruit and flowers — and suggesting ways in which concrete hypotheses about coevolution can be formulated and tested even for these.
Thompson covers an extraordinary breadth of material, with examples drawn from an wide range of taxa and based on both laboratory experiments and fieldwork. (The Coevolutionary Process is almost a review of work on specialization and coevolution; over a thousand works are listed in the fifty page bibliography.) Despite this breadth, he manages to include detailed discussion of critical case studies and never allows the volume of empirical data to obscure the theoretical perspective; he manages a very nicely judged balance between the two. Thompson's arguments for the importance of a geographical perspective in the study of specialization and coevolution are compelling, but I suspect The Coevolutionary Process will be better remembered for becoming one of the standard introductions to evolutionary ecology. It assumes a basic knowledge of ecology and of molecular and population genetics, but it should be accessible to anyone with a background in the biological sciences; it is recommended reading for anyone with an interest in evolutionary ecology.