| |
Parasitism (from Greek parasitas, ‘eater at another\'s table’), in the life sciences, describes the association between two organisms where one benefits at the expense of the other. Parasitic organisms are found in most parts of the living kingdom, from the viruses, which are all obligate parasites, to the vampire bat, a parasitic mammal, and parasitic plants such as dodder. Some authorities exclude viruses and pathogenic bacteria from the category of parasites, but this is not justified given the nature of their association with the individuals which they affect. The relationship between parasite and host (the affected individual on which the parasite is metabolically dependent) is usually highly specific and can be damaging by affecting growth rate, reproductive capacity and survival ability; however, it is disadvantageous to the parasite to cause the death of its host (see parasitoidism). The host-parasite relationship is highly dynamic in an evolutionary sense, as the host is under intense adaptive pressure to avoid parasitism while the parasite must continually keep up with the host\'s evolution in order to perpetuate its life cycle. It was suggested by J.B.S. Haldane (1892 - 1964) that evolutionary pressure imposed by parasites may be a major driving force in evolution, and Richard Southwood has recently proposed that parasitic tropical diseases may have been instrumental in driving the development of early man and his migration out of the tropics.
Parasites may live within the body, either inside or in between cells, in the gut, or on the surface of the body; they may themselves bear parasites (hyperparasitism). There also exist various forms of behavioural parasitism, such as brood parasitism (as seen in the cuckoo) and social parasitism (where one species enslaves another). Many parasites have evolved specialized systems of transmission, for instance the use of insect vectors by microparasites such as the protozoan which causes malaria, or complex life cycles involving several host stages. In general, the complexity of the parasite is reflected in the complexity of its association with its host: protozoan and helminth (see helminthology) parasites are often implicated in highly adaptive strategies for evasion of the immune response and modification of host behaviour. RB
See also amensalism; parasitology; symbiosis; virology.Further reading Philip Whitfield, The Biology of Parasitism. |
|
