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==Taxonomy and systematics==
[[File:Haeckel arbol bn.png|thumb|right|Early phylogenetic tree by [[Ernst Haeckel|Haeckel]], 1866]]
The idea of "clade" did not exist in pre-[[Charles Darwin|Darwinian]] [[Linnaean taxonomy]], which was based only on [[morphology (biology)|morphological]] similarities between organisms, though many of the better known animal groups in Linnaeus' original [[Systema Naturae]] (notably among the [[vertebrate]]
In [[phylogenetic nomenclature]], clades can be nested at any level, and do not have to be slotted into a [[ranking|rank]] in an overall hierarchy. In contrast, the Linnaean units of "[[order (biology)|order]]," "[[class (biology)|class]]" etc. must be used when naming a new taxon. As there are only seven formal levels to the Linnaean system (whereof [[species]] is the lowest), there is a finite amount of sub- and super-units that can be applied. As taxonomic trees ([[cladogram]]s) become more detailed, some researchers intimately familiar with the topography of the trees they are working with have opted to dispense with the ranks all together, using clade names without Linnaean ranks. The preference of one system over the other is mainly one of application: Cladistics give details, but require intimate knowledge, the Linnaean system gives a well ordered overview, at the expense of details of the phylogenetic tree.
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