Anagenesis in parasites
Parasites can be divided into ectoparasites and endoparasites. Ectoparasites live on the surface of the bodies of the host or only in their “dwellings” (nidocolous parasites). In contrast, endoparasites live directly in the organs or tissues of the host species. A conspicuous anagenetic tendency is sometimes observed for endoparasites, i.e. a trend towards simplification of the body structure and individual physiological functions. This trend sometimes even goes so far that an originally multicellular organism becomes a more or less unicellular organism. A typical example is Myxozoa, a group of organisms parasitizing mostly on fish, in which the methods of molecular taxonomy have recently shown that these are not protozoa, but extremely reduced metazoa (Smothers et al. 1994; Siddall et al. 1995). A great many kinds of parasitic crustaceans have comparably greatly reduced body structure. Viruses are located at the other end of the scale of organismal complexity and they could well provide us with similar surprises in the future. It cannot be excluded that it will be found in time that some viruses are actually extremely reduced bacteria. (Well, probably not, but it would be nice, wouldn’t it?)
The reason for the simplification of the body structure of endoparasites is primarily the fact that a great many physiological functions related, for example, to maintenance of homeostasis, are left to the host organism by the parasite. The internal environment of the organism is relatively stable and rich in some easily processed nutrients. Consequently, a great many functions that are vital for freely living organisms are superfluous for a parasite and it can gradually get rid of them during evolution.
It is even evolutionarily advantageous for a parasite if it manages to simplify its body structure as much as possible. The more differentiated tissues and organs its body contains, the greater the number of kinds of proteins that it must synthesize and the more easily can the immune system of the host recognize its presence and foreign nature.
The specific cause of the reduction of the nervous system lies in the extreme predictability and stability of the environment in which the parasite infrapopulation lives. As a result of this predictability, the body of a host is sometimes designated as a third type of environment (in addition to aquatic and terrestrial environments) (Sukhdeo 2000). While every individual of nonparasitic species lives under quite unique conditions, parasites in the bodies of organisms are always in exactly the same, although highly heterogeneous environment. The distribution of the individual organs in the body of the host is almost identical in all individuals of a particular species. Consequently, while the nervous system and learning mechanisms are necessarily of great importance in controlling behavior in nonparasitic organisms, in parasitic organisms, these functions can be taken over by inborn fixed patterns of behavior, initiated without regard to stimuli from the environment (see XVI.2).
In the past, the existence of predominating trends towards reducing complexity in parasitic organisms was considered to be a quite obvious fact (Poulin 1995b). As was later properly emphasized by a great many authors, the reason for general acceptance of this fact was an elementary methodical mistake. Instead of comparing the complexity of parasites in a certain phylogenetic branch with the complexity of nonparasitic organisms of the sister phylogenetic branch, the complexity of parasites was frequently compared with that of their hosts. It is obvious that, when a cow is compared to its tapeworm, the cow must appear more complex. However, when the complexity of parasitic and nonparasitic flatworms were compared, the result was far less obvious.
At the present time, the opinion pendulum has swung to the other extreme. To the contrary, especially amongst parasitologists and cladists, there is tendency towards throwing into doubt the trend associated with simplification of parasites (Brooks & McLennan 1993). Parastiologists have a quite understandable tendency to “favor” the objects of their study, so that they quite willingly accept the opinion that Sacculina barnacles, a crustacean that is most reminiscent of the mycellium of honey funguses, are, in fact, just as complicated as river crayfish; while it is true that they are lacking some organs, they quite certainly have a large number of evolutionarily new features, such as various types of receptors, at a microscopic or ultra-microscopic level. Support for this opinion of parasitologists by cladists then follows from their programmed lack of interest in similarity, and thus in the appearance of studied organisms. Cladists attempt to reconstruct the cladogenesis of organisms entirely on the basis of their mutual relatedness, where they estimate this relatedness wholly on the basis of sharing of new evolutionary features, synapomorphies (see XXIII.6). A new evolutionary feature could consist in both the acquisition and the loss of a certain body structure. Simultaneously, the complexity of the particular structures is of no importance in the formation of evolutionary hypotheses on the relatedness of organisms. If the particular evolutionary change occurred in one stage, then the loss of a complex structure, such as an eye, is equally important as the one-stage loss or acquisition of any other trait. Thus, if cladists attempt to compare the character of the evolution of nonparasitic and parasitic organisms, they first count the number of new evolutionary features that arose independently in a certain time interval. In this comparison, there understandably need not be any difference between parasites and nonparasitic species. When a cladist goes into greater detail and compares the number of new evolutionary features in both organisms, characterized by the loss of an organ and acquisition of a new organ, they do not in any way differentiate between the complexity of the lost and newly acquired organs. Simply on the basis of mechanical comparison of the matrices of evolutionary changes, they then come to the conclusion that tapeworms are an extraordinarily rapidly and progressively developing group. While they have lost a few organs, such as eyes and a digestive system, they have, on the other hand, acquired a large number of organs (mostly various types of chemoreceptors). Thus, only 6% of evolutionary changes here corresponded to the loss of organs (Brooks & McLennan 1993).
Because of the difficulty of understanding the concept of complexity (see I.3), it will still long not be possible to exactly decide to what degree and whether trends towards a decrease in complexity predominate overall in parasites. It is, however, obvious that parasites provide us with a great many examples of secondary reduction of organs that have lost their original function in their new environment. In any case, the character of anagenesis of parasitic animals, i.e. at the very least the occasional trend towards simplified body structure, and thus towards reduction of the overall complexity of the organism, indicates that biological evolution need not always be connected with an increase in the complexity of biological systems. As was already pointed out in Sect. I.2, an increase in the complexity accompanies biological evolution; however, in a number of cases this parameter is quite independent of biological evolution. The only trait that differentiates biological evolution from other types of evolution is thus the formation of adaptive traits.