XIX.1 Host-parasite interactions seem to be extremely important in the evolution of organisms
During evolution, the individual species adapt to the conditions in their environment and to changes in these conditions.This adaptation is reflected in the anagenesis of organisms and is manifested in adaptive changes in the morphological and functional structures of individual species.Changes in the external conditions, manifested simultaneously over a larger area or even on a global scale, mostly occur slowly, so that the individual species are usually able to gradually adapt to them evolutionarily.Of course, drastic and rapid changes in the quality of the environment also occur during development of life on Earth, amongst other things as a consequence of global catastrophes caused, for example, by the impact of large meteorites, comets or small planets on the surface of the Earth (see XXII.5.3.2).These drastic, but basically temporary changes, which frequently led to the extinction of a major portion of the species occurring on the Earth at the particular time, tend, however, to affect macroevolution rather than microevolution.During their existence (i.e. during a period of usually several million years), most species never encounter such rapid changes in their environment (and if they do encounter them, they mostly become extinct).
The above is true only for changes in the abiotic factors in the environment. Biotic factorsfollowing from mutual interactions amongst the representatives of different species constantly change during the existence of the individual species, roughly at a rate to which the individual species are capable of adapting evolutionarily.The interaction of a predator with its prey or a parasite with its host, competition amongst various species of predators or parasites and the constant changes in the traits of the individual interacting members that occur as a consequence of microevolution of all or some species exert a constant pressure on mutual adaptation to the changing biotic factors in the environment.A single change in one of these factors, for example, the eradication of a certain type of prey, the extinction of a certain predator or the formation of resistance of a certain species against a parasite can simultaneously lead to an avalanche of changes in the entire ecosystem and create direct or indirect selection pressure on evolutionary changes in a large number of species of organisms.
It can be justifiably assumed that interactions amongst various species of organisms and selection pressures following from these interactions constitute the main driving force for biological evolution.The phenomenon of parasitism is very widespread and a substantial portion of all the organisms on the Earth consists of various species of parasites (Price 1980).Simultaneously, the evolution of a parasite and its host are very closely connected and “arms races” between the two actors in co-evolution are generally very intense.It can thus be anticipated that a major portion of biological evolution and a large percentage of adaptive traits in the framework of biological evolution are in some way connected with the phenomenon of parasitism.According to some concepts, the two most conspicuous evolutionary phenomena, sexuality andspeciation, could have emerged as a consequence of selection pressure on the part of parasites (see XIII.3.4.2, XIX.2.1 and XXI.5.4).