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A recent trend in biology since the 1960s, called cladism or cladistic taxonomy, requires taxa to be clades. In other words, cladists argue that the classification system should be reformed to eliminate all non-clades. In contrast, other insist that groups reflect and often make use of cladistic techniques, but allow both and groups as .
A group is a clade, comprising an ancestral form and all of its descendants, and so forming one (and only one) evolutionary group. A group is similar, but excludes some of the descendants that have undergone significant changes. For instance, the traditional class Reptilia excludes birds even though they evolved from the ancestral reptile. Similarly, the traditional Invertebrates are paraphyletic because Vertebrates are excluded, although the latter evolved from an Invertebrate.
A group with members from separate evolutionary lines is called . For instance, the once-recognized was found to be polyphyletic because elephants and rhinoceroses arose from non-pachyderms separately. Evolutionary taxonomists consider polyphyletic groups to be errors in classification, often occurring because or other was misinterpreted as .
Following Hennig, cladists argue that paraphyly is as harmful as polyphyly. The idea is that monophyletic groups can be defined objectively, in terms of common ancestors or the presence of synapomorphies. In contrast, paraphyletic and polyphyletic groups are both defined based on key characters, and the decision of which characters are of taxonomic import is inherently subjective. Many argue that they lead to "gradistic" thinking, where groups advance from "lowly" grades to "advanced" grades, which can in turn lead to . In evolutionary studies, teleology is usually avoided because it implies a plan that cannot be empirically demonstrated.
Going further, some cladists argue that ranks for groups above species are too subjective to present any meaningful information, and so argue that they should be abandoned. Thus they have moved away from Linnaean taxonomy towards a simple hierarchy of clades. The validity of this argument hinges crucially on how often in evolution near-equilibria are . A quasi-stable state will result in phylogenies which may be all but unmappable onto the Linnean hierarchy, whereas a punctuation event that balances a taxon out of its ecological equilibrium is likely to lead to a split between clades that occurs in comparatively short time and thus lends itself readily for classification according to the Linnean system.
Other evolutionary systematists argue that all taxa are inherently subjective, even when they reflect evolutionary relationships, since living things form an essentially continuous tree. Any dividing line is artificial, and creates both a monophyletic section above and a paraphyletic section below. Paraphyletic taxa are necessary for classifying earlier sections of the tree – for instance, the early vertebrates that would someday evolve into the family Hominidae cannot be placed in any other monophyletic family. They also argue that paraphyletic taxa provide information about significant changes in organisms' morphology, ecology, or life history – in short, that both taxa and clades are valuable but distinct notions, with separate purposes. Many use the term in its older sense, where it includes paraphyly, and use the alternate term to describe clades (monophyly in Hennig's sense). As an unscientific rule of thumb, if a distinct lineage that renders the containing clade paraphyletic has undergone marked and collected many - especially ones that are radical and/or unprecedented -, the paraphyly is usually not considered a sufficient argument to prevent recognition of the lineage as distinct under the Linnean system (but it is by definition sufficient in phylogenetic nomenclature). For example, as touched upon briefly above, the ("reptiles") and the (birds) are both ranked as a Linnean , although the latter are a highly derived offshoot of some forms of the former which themselves were already quite advanced.
A formal code of phylogenetic nomenclature, the , is currently under development for cladistic taxonomy. It is intended for use by both those who would like to abandon Linnaean taxonomy and those who would like to use taxa and clades side by side. In several instances (see for example ) ist has been employed to clarify uncertainties in Linnean systematics so that in combination they yield a taxonomy that is unambiguously placing the group in the evolutionary tree in a way that is consistent with current knowledge.
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