It is usually believed that all organisms on Earth are involved in the so-called host-parasite interactions, either as a host or as a parasite (see Parasites). This gives an idea of how pervasive and important are these intimate interactions for the ecology and evolution of both partners of the interaction. Parasites are not the only organisms that are intimately associated with another individual for their growth, reproduction, and survival. The evolution of eukar-yotes has been tightly linked with the establishment of permanent and obligate coexistence of genetic entities that were formerly able to live independent from each other. These kind of obligate associations can be classified according to the relative net effect that each partner inflicts on the other. The overall spectrum, therefore, goes from associations where both partners benefit from each other's presence, to the other extreme where the association comes to the exclusive benefit of one partner at the expense of the other. This latter case is what is commonly called a hostparasite interaction. The metabolic resources necessary for the vital functions of the parasite are provided by the host, which in turn receives nothing by the parasite. Thus, this results in a net negative effect of parasitism on host fitness, as the resources consumed by the parasite are no longer available for the growth, reproduction, and survival of the host. It is easy to understand, in this context, that parasites have been and are selected to exploit the host in the most effective way and hosts to limit the negative effect of parasites. The evolution of parasitism and the exploitation strategies that come along, therefore, cannot be easily disentangled from the evolution of host counter-adaptations to resist parasitic attacks. This sets the scene for a coevolu-tionary scenario where hosts and parasites are endlessly selected to respond to the threat provided by the opponent (see Coevolution).
Parasitism has evolved from an ancestral nonparasitic form. Although it is difficult (or impossible) to assess the selective pressures that have promoted the shift from nonparasitic to parasitic life style, the use of modern phylogenetic tools has provided very interesting insights on the evolutionary history of parasitism. Comparison of closely related extant taxa that differ in their lifestyle
(parasitic vs. nonparasitic) also provides an invaluable tool to study the traits (adaptations) that are associated with a parasitic mode of life and to infer the selection pressures that act on these traits.
In this article, we will go through these different aspects of parasite evolution, starting with a snapshot on the impressive diversity of organisms with a parasitic lifestyle.
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