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8.6 Evolution and Parasitism |
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The way in which the parasites of warm-blooded vertebrates developed from free-living organisms to endoparasites is open to speculation. A first attempt has been made by comparing the life cycles of saprophytic with intestinal commensal and finally with parasitic nematodes as described in chapter 3.1 pg.175 ff. By arranging the different life cycles accordingly, parasite-host interactions can be shown to have become intensified, the specifications in both partners adapting step by step in a process called cospeciation. For the evolution of parasites, the increasing specifications of both partners are particularly instructive at the time-point when a host changes from a terrestral to a marine life style, e.g. hookworms of seals changed their entry from percutaneous invasion to lactogen transmission: Uncinaria lucasi in Callorhinus ursinus (fig. 8.2, page 283). The entero-somatic invasion route of a directly transmitted intestinal nematode, as e.g. Ascaris spec. exerts, was primarily necessary for intensified tissue contact for immunological modulation of the host. This process of coadaptation of resistance and virulence is a prerequisite of the development of a tissue-dwelling worm by shortening the invasion route ending in the lungs (e.g. Dictyocaulus) instead of returning to the intestine. The cycle becomes cyclical when the embryonated eggs delivered in the lungs, but shed by faeces, are taken up by a snail (e.g. Muellerius, Protostrongylus) or an arthropod in which the larva develops in the fat body to the metacyclic stage, ready to be taken up with the arthropod as food. An example is Diplotriaena agelaius in the Canadian ovenbird Seiurus aurocapillus (and many other birds, such as grackles, red winged blackbirds and corvids) and the grasshopper Cannula pelucida as an intermediate host (fig. 8.10, page 310). The next step, to a filarial worm transmitted by a blood-feeding arthropod, takes place in corvids and many other birds. Cospeciation and coadaptation are thus connected processes acting hand in hand. Another perspective is provided by the influence of parasites on the evolution of their hosts. Balancing the population density of a host stabilizes an ecosystem. Heteroxenic sporozoites of mammals, such as the Sarcosporids, balance the relationship between prey and predator (see fig. 2.58, page 119). At the population level, the influence of parasites on ecosystems leads to the evolution of hosts. The density regulation of host populations by parasites is effected via the fecundity and survival of the host. (Møller 2005). The role of parasites in evolution has been neglected until this century and remains underestimated in palaeontology because of the lack of a fossil record. However, about 50% of all known species, from viruses to metazoans, are parasitic (Price 1980; Toft 1986; cit. Thomas et al. 2005). "…it is hard to imagine what natural communities and ecosystems would be like without shared parasites…they also play … a crucial role in maintaining biodiversity." (Thomas et al 2005, chapter 8.6.1). While acting behind the scenes, pathogens play an important role in maintaining animal communities. Not only geographical isolation, the trophic arms race, symbioses and niche partitioning, but also parasitic infestations have to be taken into account when considering animal (and plant) evolution. Monoxenic parasites control the population density of their host, whereas heteroxenic parasites sustain the balance between predator and prey species. Thereby, they enhance bonds in the ecosystem and protect them against immigrants. Compared with trophic relationships, systems linking eukaryotic parasites and their hosts are immutable. Thus, parasites may be responsible for the stasis and incumbency of ecosystems over geological time, in spite of continuous Darwinian innovation. If this also apply to more rapid changes as in punctuated evolution is open to debate. As heteroxenic parasites address taxonomic levels above that of the species (e.g., families), these taxa also become units of selection in the face of global changes (Thomas et al. 2005; Seilacher et al. in press). References |
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