Cladistics
<p ><b>Cladistics</b> (<a href="http://en.wikipedia.org/wiki/Ancient_Greek" title="Ancient Greek" >Ancient Greek</a>: κλάδος, <i>klados</i>, "branch") is a method of classifying species of organisms into groups called <a href="http://en.wikipedia.org/wiki/Clade" title="Clade" >clades</a>, which consist of an ancestor organism and all its descendants (and nothing else). For example, birds, dinosaurs, crocodiles, and all descendants (living or extinct) of their most recent common ancestor form a clade.<sup id="cite_ref-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Cladistics#cite_note-0" >[1]</a></sup> In the terms of <a href="http://en.wikipedia.org/wiki/Systematics" title="Systematics" >biological systematics</a>, a clade is a single "branch" on the "<a href="http://en.wikipedia.org/wiki/Tree_of_life_(science)" title="Tree of life (science)" class="mw-redirect" >tree of life</a>", a<a href="http://en.wikipedia.org/wiki/Monophyletic" title="Monophyletic" class="mw-redirect" >monophyletic</a> group.
Cladistics can be distinguished from other <a href="http://en.wikipedia.org/wiki/Taxonomic" title="Taxonomic" class="mw-redirect" >taxonomic</a> systems, such as morphology-based<a href="http://en.wikipedia.org/wiki/Phenetics" title="Phenetics" >phenetics</a>, by its focus on shared derived characters (<a href="http://en.wikipedia.org/wiki/Synapomorphy" title="Synapomorphy" >synapomorphies</a>). Systems developed earlier usually employed overall morphological similarity to group species into genera, families and other higher level groups (<a href="http://en.wikipedia.org/wiki/Taxon" title="Taxon" >taxa</a>); cladistic classifications (usually in the form of trees called<a href="http://en.wikipedia.org/wiki/Cladogram" title="Cladogram" >cladograms</a>) are intended to reflect the relative recency of common ancestry or the sharing of<a href="http://en.wikipedia.org/wiki/Homology_(biology)" title="Homology (biology)" >homologous</a> features. Cladistics is also distinguished by an emphasis on <a href="http://en.wikipedia.org/wiki/Maximum_parsimony" title="Maximum parsimony" class="mw-redirect" >parsimony</a> and hypothesis testing (particularly <a href="http://en.wikipedia.org/wiki/Falsificationism" title="Falsificationism" class="mw-redirect" >falsificationism</a>), leading to a claim that cladistics is more objective than systems which rely on subjective judgements of relationship based on similarity.<sup id="cite_ref-1" class="reference" ><a href="http://en.wikipedia.org/wiki/Cladistics#cite_note-1" >[2]</a></sup>
Cladistics originated in the work of the German <a href="http://en.wikipedia.org/wiki/Entomologist" title="Entomologist" class="mw-redirect" >entomologist</a> <a href="http://en.wikipedia.org/wiki/Willi_Hennig" title="Willi Hennig" >Willi Hennig</a>, who referred to it as "phylogenetic systematics" (also the name of his 1966 book); the use of the terms "cladistics" and "clade" was popularized by other researchers. The technique and sometimes the name have been successfully applied in other disciplines: for example, to determine the relationships between the surviving manuscripts of the <i><a href="http://en.wikipedia.org/wiki/Canterbury_Tales" title="Canterbury Tales" class="mw-redirect" >Canterbury Tales</a></i><sup id="cite_ref-2" class="reference" ><a href="http://en.wikipedia.org/wiki/Cladistics#cite_note-2" >[3]</a></sup> and 53 manuscripts of the Sanskrit <i><a rel="nofollow" class="external text" href="http://www.istb.univie.ac.at/caraka/" >Carakasaṃhitā</a></i>.<sup id="cite_ref-3" class="reference" ><a href="http://en.wikipedia.org/wiki/Cladistics#cite_note-3" >[4]</a></sup>
Cladists use <i><a href="http://en.wikipedia.org/wiki/Cladogram" title="Cladogram" >cladograms</a></i> – diagrams which show ancestral relations between species – to represent the monophyletic relationships of species, termed sister-group relationships. This is interpreted as representing phylogeny, or evolutionary relationships. Although traditionally such cladograms were generated largely on the basis of morphological characters, <a href="http://en.wikipedia.org/wiki/Genetic_sequencing" title="Genetic sequencing" >genetic sequencing</a> data and <a href="http://en.wikipedia.org/wiki/Computational_phylogenetics" title="Computational phylogenetics" >computational phylogenetics</a>are now very commonly used in the generation of cladograms.
Cladistics, either generally or in specific applications, has been criticized from its beginnings. A decision as to whether a particular character is a synapomorphy or not may be challenged as involving subjective judgements,<sup id="cite_ref-4" class="reference" ><a href="http://en.wikipedia.org/wiki/Cladistics#cite_note-4" >[5]</a></sup> raising the issue of whether cladistics as actually practised is as objective as has been claimed. Formal classifications based on cladistic reasoning are said to emphasize ancestry at the expense of descriptive characteristics, and thus ignore biologically sensible, clearly defined groups which do not fall into clades (e.g. reptiles as traditionally defined or <a href="http://en.wikipedia.org/wiki/Prokaryotes" title="Prokaryotes" class="mw-redirect" >prokaryotes</a>).<sup id="cite_ref-5" class="reference" ><a href="http://en.wikipedia.org/wiki/Cladistics#cite_note-5" >[6]</a></sup></p>