<sod>  [[image:cladogram-example1.png|framed|This cladogram shows the relationship among various insect groups. In some cladograms, the length of the horizontal lines indicates time elapsed since the last common ancestor.]]
 
'''Cladistics''' (or '''phylogenetic systematics''') is a branch of [[biology]] that seeks to erect classification groups for all organisms, based solely on their evolutionary relationships.  Cladistics differs from [[phenetics]], which groups organisms based on ''overall'' similarity (morphological approach), and from more traditional approaches based on "key characters".  In cladistics, key characters or overall similarity are seen as useful only insofar as they point toward the correct evolutionary history, whatever that may be.  Thus subjective criteria &mdash; such as the relative "importance" for classification of various types of characters &mdash; are discarded in favor of a single objective (but inferred) historical criterion: the shared-derived character (or synapomorphy). Thus, cladistic groups must contain a common ancestor and ''all'' of its descendants, regardless of the evolutionary changes they have undergone. For example, if birds are descended from dinosaurian reptiles, then order Dinosauria must include all birds.  If the class Reptilia excludes birds, then it is rejected as an artificially constructed group because it does not reflect natural (evolutionary) history.  The term Sauria, which includes both Reptilia and the birds, is preferred. [[Willi Hennig]] is widely regarded as the founder of cladistics.
 
 ==Introduction==
 
 
 ==Cladistic classification==
 
A recent trend in biology since the [[1960s]], called '''cladism''' or '''cladistic taxonomy''', is to require taxa to be clades. In other words, cladists argue the classification system should be reformed to eliminate all non-clades (paraphyletic and polyphyletic groups). In fact, some cladists have argued for entirely abandoning the [[Linnaean taxonomy|Linnaean]] system of ranked taxa in favor of clades. A formal code of phylogenetic nomenclature, the [[PhyloCode]] [http://www.ohiou.edu/phylocode/], is currently under development for a cladistic taxonomy that abandons the Linnaean structure.
 
A true clade is considered to be ''monophyletic'', or containing one (and only one) complete evolutionary grouping deriving from one common ancestor.  When a named group is found to contain more than one evolutionary line, it is termed ''polyphyletic''.  For example, the once-recognized group Pachydermata was found to be polyphyletic because an [[elephant]] and a [[rhinoceros]] were each found to be more closely related to non-pachyderms than either to each other.  Biologists consider groups that turn out to be polyphyletic to be errors in classification, often occurring because [[convergent evolution|convergence]] or other [[homoplasy]] was misinterpreted as [[homology (biology)|homology]].
 
If a named group is found to include some but not all of the descendants of the ancestor on which the group is based, it is termed [[paraphyletic]].  Paraphyletic groups are usually created when organisms are groups on the basis of plesiomorphies instead of apomorphies.  Classic examples of paraphyly include Pisces (fishes), whose descendants include [[tetrapod]]s (amphibians, reptiles, birds, and mammals), and Reptilia, whose descendants include birds; however, neither tetrapods nor birds are included in the groups named Pisces and Reptilia, respectively.  Most paraphyletics groups, however, were erected at the genus level, because species were grouped on overall similarity prior to a cladistic analysis.
 
The denial of recognition to paraphyletic groups has been very controversial in biology and remains so among a number of more traditional [[evolutionary taxonomy|evolutionary taxonomists]].  They feel that abandonment of paraphyly leads to loss of information in the classification system about significant changes in organisms' morphology, ecology, or life history.  Accordingly, they argue that the notions of clade and taxon should be kept distinct, and that paraphyletic taxa are necessary if every group is going to be broken down completely into subgroups.  In fact, evolutionary taxomonists such as Peter Ashlock include paraphyly under the term ''monophyletic'', reserving the term ''holophyletic'' for the strict sense of ''monophyletic.''
 
Cladists counter that "significant changes" recognized by evolutionary taxonomists are often too subjective to be a basis for classification
 
 == Cladistic parsimony ==
 
 <eod> <sod> [[image:cladogram-example1.png|framed|This cladogram shows the relationship among various insect groups. In some cladograms, the length of the horizontal lines indicates time elapsed since the last common ancestor.]]
 
'''Cladistics''' or '''phylogenetic systematics''' is a branch of [[biology]] that determines the [[evolution]]ary relationships of living things based on derived similarities.  It forms the basis for most modern systems of classification, which seek to group organisms by evolutionary relationships.  In contrast, [[phenetics]] groups organisms based on their ''overall'' similarity, while more traditional approaches tend to rely on key characters.  [[Willi Hennig]] is widely regarded as the founder of cladistics. 
 
 ==Introduction==
 
 
 ==Cladistic classification==
 
A recent trend in biology since the 1960s, called '''cladism''' or '''cladistic taxonomy''', has been to require taxa to be clades.  In other words cladists argue that the classification system should be reformed to eliminate all non-clades.  In contrast, other evolutionary taxonomists insist that groups reflect phylogenies and often make use of cladistic techniques, but allow both monophyletic and paraphyletic groups as taxa.
 
A ''[[monophyletic]]'' group is a clade, comprising an ancestral form and all of its descendants, and so forming one (and only one) evolutionary group.  A ''[[paraphyletic]]'' 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 developed from the ancestral reptile.  
 
A group with members from separate evolutionary lines is called ''[[polyphyletic]]''.  For instance, the once-recognized Pachydermata was found to be polyphyletic because elephants and rhinoceroses arose separately from non-pachyderms.  Evolutionary taxonomists consider polyphyletic groups to be errors in classification, often occurring because because [[convergent evolution|convergence]] or other [[homoplasy]] was misinterpreted as [[homology (biology)|homology]].
 
Whether or not paraphyletic groups are valid taxa remains controversial.  Cladists argue that deciding the relative importance for classification of various types of characters is inherently subjective and so not a valid basis for taxonomy, and that it should be replaced with a single objective (though inferred) criterion, the presence of synapomorphies.  Some cladists have also argued that ranks are too subjective to present meaningful information, and so have moved away from Linnaean taxonomy towards a simple hierarchy of clades.  A formal code of phylogenetic nomenclature, the [[Phylocode]], is currently under development for such cladistic taxonomy.
 
Other evolutionary taxonomists argue that all taxa are inherently subjective, even when they reflect objective relationships.  For instance, at some point ''Homo sapiens'' must have had an ancestor that was a different species, although any such separation is artificial and immediately renders the ancestral species paraphyletic.  Thus it is impossible to create a complete taxonomy including ancestors without using paraphyletic taxa.  They also argue that paraphyletic taxa provide information about significant changes in organisms' morphology, ecology, or life history.  Many use the term ''monophyly'' in its older sense, where it includes paraphyly, and use the alternate term ''holophyletic'' to describe clades.
 
 == Cladistic parsimony ==
 
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