Cladistics

Cladistics is an approach to <a href="/facts/Taxonomy_(biology)/Qkg8wuyw">biological classification</a> in which <a href="/facts/Organism/j8JkL09u">organisms</a> are categorized in groups ("<a href="/facts/Clade/XefyYmWn">clades</a>") based on hypotheses of most recent <a href="/facts/Common_ancestry/bwEgMu7F">common ancestry</a>. The evidence for hypothesized relationships is typically shared <a href="/facts/Derived_(phylogenetics)/t21GWRl1">derived</a> characteristics (<a href="/facts/Synapomorphies/t21GWRl1">synapomorphies</a>) that are not present in more distant groups and ancestors. However, from an empirical perspective, common ancestors are inferences based on a cladistic hypothesis of relationships of taxa whose <a href="/facts/Phenotypic_trait/seYSUTul">character states</a> can be observed. Theoretically, a last common ancestor and all its descendants constitute a (minimal) clade. Importantly, all descendants stay in their overarching ancestral clade. For example, if the terms worms or fishes were used within a strict cladistic framework, these terms would include humans. Many of these terms are normally used <a href="/facts/Paraphyly/tmTWjACz">paraphyletically</a>, outside of cladistics, e.g. as a '<a href="/facts/Evolutionary_grade/ssv1Meoz">grade</a>', which are fruitless to precisely delineate, especially when including extinct species. <a href="/facts/Evolutionary_radiation/DDc0R6GS">Radiation</a> results in the generation of new subclades by bifurcation, but in practice sexual hybridization may blur very closely related groupings.
As a hypothesis, a clade can be rejected only if some groupings were explicitly excluded. It may then be found that the excluded group did actually descend from the last common ancestor of the group, and thus emerged within the group. ("Evolved from" is misleading, because in cladistics all descendants stay in the ancestral group). To keep only valid clades, upon finding that the group is paraphyletic this way, either such excluded groups should be granted to the clade, or the group should be abolished.
Branches down to the divergence to the next significant (e.g. extant) sister are considered stem-groupings of the clade, but in principle each level stands on its own, to be assigned a unique name. For a fully bifurcated tree, adding a group to a tree also adds an additional (named) clade, and a new level on that branch. Specifically, also extinct groups are always put on a side-branch, not distinguishing whether an actual ancestor of other groupings was found.
The techniques and nomenclature of cladistics have been applied to disciplines other than biology. (See <a href="/facts/Phylogenetic_nomenclature/VSzWng0P">phylogenetic nomenclature</a>.)
Cladistics findings are posing a difficulty for <a href="/facts/Taxonomy_(biology)/Qkg8wuyw">taxonomy</a>, where the rank and (genus-)naming of established groupings may turn out to be inconsistent.
Cladistics is now the most commonly used method to classify organisms.

Cladistics open-in-new

Cladistics