Κλάδος: Διαφορά μεταξύ των αναθεωρήσεων
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A '''clade'''<ref group=note>(from [[Ancient Greek]] ''{{Polytonic|κλάδος}}'', ''klados'', "branch")</ref> is a group consisting of an organism and all its descendants. In the terms of [[Cladistics|cladistics]], a clade is a single "branch" on the "[[Tree of life (science)|tree of life]]".<ref name=Dupuis>{{cite journal | last=Dupuis | first=Claude | year=1984 | title=Willi Hennig's impact on taxonomic thought | journal=Annual Review of Ecology and Systematics | volume=15 | pages=1–24 | issn=0066-4162}}</ref> The idea that such a "natural group" of organisms should be grouped together and given a [[Taxonomy|taxonomic]] name is central to [[Cladistics|cladistics]]. In [[cladistics]] (which takes its name from the term), clades are the only acceptable units.
The term (which term?, not clade or..?) was coined in 1958 by [[England|English]] biologist [[Julian Huxley]].<ref name=Dupuis/>
==Definitions==
{{main|Phylogenetic nomenclature#Phylogenetic definitions|l1=Phylogenetic definitions}}
===Clade and ancestor===
A clade is termed [[monophyletic]], meaning it contains one ancestor which can be an organism, [[population]], or [[species]] and all its descendants.<ref group=note>, which is clearly inconsistent.
A semantic case has been made that the name should be "holophyletic," but this term has not yet acquired widespread use, although the case is correct. For more information, see ''[[holophyly]]''</ref><ref>{{cite web|title=The PhyloCode, Chapter 1|url=http://www.ohio.edu/phylocode/art1-3.html#chapter1|year=2009|publisher=International Society for Phylogenetic Nomenclature|accessdate=23 January 2010}}</ref><ref>{{cite journal
| doi = 10.1111/j.1095-8312.2008.00984.x
| title = On the difference between mono-, holo-, and paraphyletic groups: a consistent distinction of process and pattern
Γραμμή 19:
| journal = Biological Journal of the Linnaean Society
| volume = 94
| page = 217 }}</ref> The term clade refers to the grouping of the ancestor and its living and/or deceased descendants together. The ancestor can be a theoretical or actual species. This reference, however, being inconsistent, since a descendant can't be distinguished from its ancestor at the same time as being grouped with it consistently, since "single" does not equal "group".
===Clade definition===
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* In stem-based definition, A refers to the ''most inclusive'' clade containing X, Y, etc., and their common ancestor, down to where Z branches off below A. Taxa are included between the node of A and down to (but not including) the branching point to Z; that is, the ''[[Crown group#Stem groups|stem]]'' of A.
* In [[apomorphy]]-based definition, A refers to the clade identified by an apomorphy (a trait) found in X, Y, etc., and their common ancestor.
These three definitions does, however, exclude each other, leaving none of them as a consistent definition of "clade", a goal which is impossible, since the definition of "clade" is inconsistent.
In [[Linnaean taxonomy]], clades are defined by a set of traits (apomorphies) unique to the group. This system is basically similar to the apomorphy-based clades of phylogenetic nomenclature. The difference is one of weight: While phylogenetic nomenclature bases the group on an ancestor with a certain trait, Linnaean taxonomy uses the traits themselves to define the group.
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