The evolution of a parasite and its host often has the character of an “arms race”, in which the host develops more or less specific mechanisms of defense against parasitization and the parasite, on the other hand, develops mechanisms allowing it to avoid or overcome these defense mechanisms. However, the fact that a parasite adapts perfectly to a certain species of host means that it closes the route to parasitization on other species. Thus, “arms races” frequently lead to very narrow specialization of the parasitic species and often result in narrow host specificity of the parasite. Narrowing of the host spectrum frequently ends with a state where the parasite is capable of completing its life cycle only in the members of a single host species and does not attack even closely related species at all (Fig. XIX.3). This is in sharp contrast with the situation for a number of groups of predators, where narrow specialization of the predator on a single species is certainly not the rule.
For example, molecular mimicry is characteristic for some parasitic microorganisms (including viruses) (Moloo, Kutuza, & Boreham 1980). The parasite adapts the structure of its macromolecules to the structure of the relevant macromolecules of the host organism. If, for example, a certain virus were capable, through gradual accumulation of substitution mutations, of eliminating, from its proteins, all the peptides that are recognized as being foreign by the immune system of the host species, and thus adapt its “peptide vocabulary”, i.e. the set of peptides occurring in its proteins, to the vocabulary of its host, it would quite certainly escape from the reach of the immune system of the host, and could thus spread uncontrollably in the relevant host population. (The fact that it is not an easy matter for the parasite to achieve this final state and the role of MHC-antigens and sexuality in the defense of the host were described in Section VIII.4.3.1.) However, if the peptide vocabularies of two host organisms are very different, and this is highly probable as a result of the relevant selection pressure from parasites (see below), then a parasite cannot simultaneously adapt its vocabulary to two different vocabularies of two host species, except at a cost of drastic limitation of its own peptide vocabulary, which is incompatible with functionality of the proteins. Any form of utilization of the principle of molecular mimicry thus again creates selection pressure for gradual narrowing of the host spectrum of the parasite.