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Taxonomy, nomenclature and concept of species

In 1758 Linnaeus published the tenth edition of "Systema naturae", in which he used the binomial system consistently, also for animals. This work represented a turning point in zoological terminology. It is due to his work that we have a naming system with two parts: first the genus and then the species. E.g. Schistosoma mansoni, Escherichia coli, Aedes aegypti. If there are subspecies (races), a third word is added, e.g. Trypanosoma brucei gambiense. Thus, living organisms are divided into hierarchical groups according to the similarities in their structure. The successive groups are: Kingdom, Phylum, Class, Order, Family, Genus and Species. A mnemonic sentence to help to remember the sequence is "King Phillip Came Over For Good Spaghetti".

Sometimes a subgenus is given and is written between brackets, e.g. Aedes (Stegomyia) aegypti. When there are species complexes, as in Simulium damnosum, reference is often made to S. damnosum s.l. (sensu lato - in the broad sense, i.e. the species complex) or S. damnosum s.s. (sensu stricto - in the narrow sense). Different groups within a complex may exhibit very different patterns of behaviour. Thus, Anopheles gambiae sensu strictu is highly anthrophilic, while the sister species Anopheles quadriannulatus is totally zoophilic and has no medical significance. The presence of the latter in an environment, however, can cause confusion in a control programme.

Example: Order: Diptera
Suborder: Nematocera
Infra-order: Culicomorpha
Superfamily: Chironomoidea
Family: Simuliidae
Subfamily: Simuliinae
Tribe: Simuliini
Genus: Simulium
Subgenus: Erwardsellum
Species: Simulium (Edwardsellum) damnosum

According to the "International Code of Zoological Nomenclature", the genus name is always written with a capital letter and the species name always with a lower case letter (e.g. Glossina tachynoides). This applies even if the name is derived from a proper name, e.g. Culicoides grahamii. In scientific publications, genus and species name are italicised or underlined. Names also never contain an accent, apostrophe or umlaut (thus no Aëdes aegypti or Tipula o'neili). Two words are sufficient, e.g. Mycena luxaeterna for a luminescent mushroom species (instead of Mycena lux aeterna). The name of the genus can be abbreviated, e.g. Anopheles funestus becomes A. funestus if this does not lead to confusion or potential mistakes in the text. Sometimes the generic name is abbreviated to two letters to prevent confusion. Suppose a text contains the mention of Culiseta and Culex. If both are abbreviated to C. then it is no longer possible to know to which this refers. If Culex is identified by Cx. then clarity is restored. Sometimes the name or the initials of the discoverer of the species are included (not italicised), possibly with the year of description: e.g. Enterobius vermicularis (Linnaeus, 1758). This mention of the name, however, is optional and does not form any part of the actual scientific name. In view of the fact that knowledge and opinion are constantly changing, taxonomic classifications (certainly the "middle groups") sometimes differ from author to author and according to the time of publication. There is no such thing as "The One Final Correct Classification".

Conventionally, the species is defined as a population which can reproduce among itself and which is reproductively isolated from other populations. This appears clear when we talk for example of humans, horses, wild ducks or rattlesnakes, but with other organisms it is much less obvious. What is the situation with the taxonomy of extinct species? What about symbiotic organisms, from lichen to protista, which cannot live without their symbiont? Some organisms have no sexual reproduction (for example amoebae). If there are sterile hybrids (e.g. horse x donkey-> mule), then this is an answer. Sometimes however there are fertile hybrids (some animals, many plants). The problem of species definition is central in biology at present. This has practical implications for example for the better understanding of the variability of diseases such as amoebiasis, leishmaniasis or Chagas' disease. Better insights into vector populations also depend on good definitions (some morphologically identical mosquitoes appear genetically to consist of various complexes with, for example, differing biting or reproductive behaviour).

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