Κλάδος: Διαφορά μεταξύ των αναθεωρήσεων

[[File:Monophyletic.svg|thumb|334px|A vertical cladogram showing one clade, ''vertebrata'', in which are nested others. Shown in blue is the clade ''reptilia''. As the traditional Linnaean taxonomic class [[Reptilia]] does not include ''Aves'' (birds), that class does not form a clade.]]

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==Definitions==

A clade is termed monophyletic, meaning it contains one ancestor, all its descendants, and only its descendants.<ref group=note>

A semantic case has been made that the name should be "holophyletic," but this term has not yet acquired widespread use. For more information, see ''[[Holophyletic]]''</ref><ref>{{cite journal

| page = 217 }}</ref> In principle, the ancestor is an individual, but in [[Sexual reproduction|sexually reproducing]] species, there will always be at least two ancestors, and in virtually all instances there will be an influx of genes from the rest of the [[gene pool]] in the descendant lines. The individual ancestor is thus often a theoretical construct, and in practical terms the ancestor of a clade of sexual organisms is a [[species]] or in the best cases a [[population]]. The common ancestor of any group of reasonable size and most of that ancestor's descendants have typically been long extinct.<ref group=note>This may not be true in very small and recent clades, where the common ancestor and all descendants are still living.</ref> It is not necessary for a clade to contain any living representatives.

 

In cladistics, the clade is a hypothetical construct based on experimental data. Clades are found using multiple (sometimes hundreds) of traits from a number of species (or specimens) and [[analysis|analysing]] them [[statistics|statistically]] to find the most likely phylogenetic tree for the group. Although similar in some ways to a [[biological classification]] of species, the method is statistical and more open to scrutiny than traditional methods. Although taxonomists use clades as a tool in classification where feasible the taxonomic [[tree of life]] is not the same as the cladistic. The traditional genus, family, etc. names are not necessarily clades; though they are likely to be.

 

==Taxonomy and systematics==

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 the [[vertebrate]]s) represent clades. With the publication of Darwin's [[Evolution|theory of evolution]] in 1859, taxonomy gained a theoretical basis, and the idea that systematic units represent branches on the evolutionary [[Tree of Life]] was born. In the century and a half since then, taxonomists have worked to make the taxonomic system reflect evolution. However, as the Tree of Life branches rather unevenly, the [[hierarchy]] of the Linnaean system does not always lend itself well to represent clades. When it comes to [[Nomenclature#Biology|naming]], [[cladistics]] and [[Linnaean taxonomy]] are not always compatible. Particularly higher level taxons in Linnaean taxonomy often represent [[evolutionary grade]]s rather than clades, i.e. clades where one or two sub-branches have been excluded. Typical examples include [[Osteichthyes|bony fishes]], who include the ancestor of [[tetrapoda|tetrapods]], and [[reptiles]], ancestral to both [[bird]]s and [[mammal]]s.<ref group=note>The term "reptile" is here to be understood as traditionally defined, e.g. Romer & Parson (1985): ''The Vertebrate Body.'' (6th ed.) Saunders, Philadelphia. There are other (cladistic) definitions of "reptile" that exclude the first [[amniote]]s and the [[synapsid]] line, see [[Sauropsida]].</ref>

 

 

In most instances the two systems are not at odds, however. The cladistic statement, that the clade Lobopodia contains (among others) the Arthropoda, Tardigrada and Onychophora, is factually identical to the Linnaean evolutionary statement that the group Lobopodia is ancestral to the phyla Arthropoda, Tardigrada and Onychophora. The difference is one of semantics rather than phylogeny.

 

== See also ==