Host–parasite coevolution

Host–parasite coevolution is a special case of coevolution, which is defined as the reciprocal adaptive genetic change of two antagonists (e.g. different species or genes) through reciprocal selective pressures. In the particular case of host–parasite coevolution the antagonists are different species of host and parasite.

Hosts and parasites exert reciprocal selective pressures on each other, which may lead to rapid reciprocal adaptation. For organisms with short generation times host–parasite coevolution can be observed in comparatively small time periods, making it possible to study evolutionary change in real-time under both field and laboratory conditions. These interactions may thus serve as a counter-example to the common notion that evolution can only be detected across extended time.

The high dynamics associated with these interactions are summarized in the Red Queen hypothesis. It states that "it takes all the running you can do to keep in the same place", i.e. both host and parasite have to change continuously to keep up with each other's adaptations.

Host-parasite coevolution is ubiquitous and of potential importance to all living organisms, including humans, domesticated animals and crops. Major diseases such as malaria, AIDS and influenza are caused by coevolving parasites, and better understanding of coevolutionary adaptations between parasite attack strategies and host immune systems may assist in the development of novel medications and vaccines.

Host–parasite coevolution is characterized by reciprocal genetic change and thus changes in allele frequencies within populations. These changes may be determined by three main types of selection dynamics.

An allele is subject to negative frequency dependent selection if a rare allelic variant has a selective advantage. For example, the parasite should adapt to the most common host genotype, because it can then infect a large number of hosts. In turn, a rare host genotype may then be favored by selection, its frequency will increase and eventually it becomes common. Subsequently the parasite should adapt to the former infrequent genotype.

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