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cladistics

cladistics (klədĬs´tĬks) or phylogenetic systematics (fī´lōjənĕt´Ĭk), an approach to the classification of living things in which organisms are defined and grouped by the possession of one or more shared characteristics (called characters) that are derived from a common ancestor and that were not present in any ancestral group (as envisioned by Charles Darwin's idea of "descent with modification" ). Developed by Willi Hennig, a German entomologist, in the 1950s, it is a method of reconstructing evolutionary relationships that emphasizes the importance of descent and common ancestry rather than chronology.

Cladistics places species in a group, or clade, based on a shared character. Within a clade, species that share other characters unique to them are grouped together, and so on, until a cladogram (a branching diagram that resembles a family tree) is assembled. For example, all vertebrates make up a clade; all tetrapods (vertebrates that have four limbs with wrists, ankles, toes, and fingers) form their own clade within the vertebrate clade. In this example the vertebrate clade would be considered "primitive" and the tetrapod clade "derived" or "advanced." In living creatures genetic characters or behaviors as well as more obvious anatomical features might be considered in assembling a cladogram. In paleontology the characters are necessarily skeletal.

Cladistics is especially significant in paleontology, as it points out gaps in the fossil evidence. It is also felt to be more objective than fossil study, which of necessity extrapolates from a limited number of finds that may or may not be representative of the whole.

See also fossil; dating.

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cladistics

cladistics A method of classification in which animals and plants are placed into taxonomic groups called clades strictly according to their evolutionary relationships. These relationships are deduced on the basis of certain shared homologous characters (known as synapomorphies; see apomorphy) that are thought to indicate common ancestry (see monophyletic). Implicit in this is the assumption that two new species are formed suddenly, by splitting from a common ancestor, and not by gradual evolutionary change. Also, it requires that truly homologous characters are distinguished from homoplasic features, i.e. ones resulting from convergent evolution (see homoplasy). A diagram indicating these relationships (called a cladogram) therefore consists of a system of dichotomous branches: each point of branching represents divergence from a common ancestor, as shown in the diagram. Thus the species A to F form a clade as they share the common ancestor X, and species A to D form a clade of a different taxonomic rank, sharing the ancestor X2. Species C to F do not form a clade, since the latter must include all the descendants of a common ancestor.

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cladistics

cladistics (cladism, phylogenetic systematics) Special taxonomic system, founded by W. Hennig (1966), and applied to the study of evolutionary relationships. It proposes that common origin can be demonstrated by the shared possession of derived characters, characters in any group being either primitive or derived. In the branching diagrams (cladograms) used to portray these relationships, it is assumed that cladogenesis, or splitting of an evolutionary lineage, always creates two equal daughter taxa: the branching is dichotomous. Thus each pair of daughter taxa constitutes a monophyletic group with a common stem taxon, unique to the group, and a parent taxon always gives rise to two daughter taxa which must be given different names from each other and from the parent, so the parent species ceases to exist. A cladogram is therefore synonymous with a classification. A shortcoming of the method would seem to be that usually it takes no account of the time dimension.

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cladistics

cladistics(cladism, phylogenetic systematics) A special taxonomic system, founded in 1966 by W. Hennig, that is applied to the study of evolutionary relationships. It proposes that common origin can be demonstrated by the shared possession of derived characters, characters in any group being either primitive or derived. In the cladograms used to portray these relationships, cladogenesis always creates two equal sister groups: the branching is dichotomous. Thus each pair of sister groups constitutes a monophyletic group with a common stem taxon, unique to the group, and a parent taxon always gives rise to two daughter taxa which must be given different names from each other and from the parent, so the parent species ceases to exist. Monophyletic groups are deduced by identifying synapomorphic character states.

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cladistics

cladistics The application of phylogenetic systematics to produce a taxonomic system that is applied to the study of evolutionary relationships. In the cladograms used to portray these relationships, cladogenesis always creates two equal sister groups: the branching is dichoto-mous. Thus each pair of sister groups constitutes a monophyletic group with a common stem taxon, unique to the group. Monophyletic groups are deduced by identifying synapomorphic character states.

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cladistics

cladistics A special taxonomic system applied to the study of evolutionary relationships. In the branching diagrams or ‘cladograms’ used to portray these relationships, cladogenesis, or splitting of an evolutionary lineage, always creates two equal sister taxa: the branching is dichotomous. Thus, each pair of sister taxa constitutes a monophyletic group with a common stem taxon, unique to the group. See PHYLOGENETIC SYSTEMATICS.

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