Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

The ability of a parasite to survive and reproduce in the host organism and the ability of the parasite population to survive consistently in the population of the host species are a consequence of gradual evolutionary adaptation of the parasite to the particular host species (Kaltz & Shykoff 1998). If the host species comes into contact with a new range of potential parasites, for example if it invades a new territory, it is generally resistant to parasitization by unspecialized parasites. Escaping from the reach of the original parasites is usually considered to be the most important cause of the ecological success of a great many invasive species (Mitchell & Power 2003; Torchin et al. 2003) (Fig. XIX.5). Only after a longer period of time are some parasites that occur in the given territory able to adapt to a new host sufficiently to survive consistently in the population of new hosts and are thus able to bring the spreading of the invasive species under control.
However, in some cases, the pathogenic manifestations of infection by an unadapted parasite are very drastic, so that in a great many cases it ends with the death of the host organism. These cases are apparently far rarer than cases where an unadapted parasite is not capable of reproducing at all and of substantially damaging the new host. However, from the viewpoint of humans, these cases are more obvious and thus they get far more publicity. If the population of hosts survives the meeting with the new parasite, changes gradually occur in the progress of the infection and, understandably, also in the dynamics of its spreading in the host population. Specifically, the pathogenic manifestations of parasitosis are reduced, so that the originally fatal infection is gradually reduced to a milder sickness and, in extreme cases, can even end up practically free of symptoms (asymptomatic) (Combes 1997). The well-documented history of the intentional introduction of the virus of rabbit myxomatosis into Australia (Fig. XIX.6) is a textbook case of reduction of the pathogenicity of infection that has been well documented.
The phenomenon of gradual reduction in the pathogenicity of parasites, described commonly but rather inaccurately as the phenomenon of reduced virulence (see XIX.4.1), is, on the one hand, influenced by evolution of the host organism and selection of individuals that are more resistant to reproduction of the pathogen and to the pathological manifestations of its action in the organism and, on the other hand, selection within the parasitic species is important here, as parasites that do not excessively damage their hosts can produce more infectious stages and infect more hosts during their existence in a single host organism.
However, it should be pointed out that, in a great many cases, the pathogenic processes that accompany parasitosis are directly connected with the reproduction of the parasite – the more invasive stages it forms during the overall time of infection, the more intense are the pathogenic processes and their consequences (Fig. XIX.7). In this case, to the contrary, the pathogenicity gradually increases with gradually increasing adaptation of the parasite. Even cases where the pathogenic processes are part of manipulative behavior of the parasite and participate substantially in the effectiveness of spreading of the parasite in the host population are not exceptional (see below). Also in these cases, the virulence of the parasite is not reduced; to the contrary, the parasite is able to do more damage to the members of species or populations to which it is adapted in the long term (Ebert 1994; Ebert & Herre 1996).