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(klədĭs`tĭks) or

phylogenetic systematics

(fī'lōjənĕt`ĭk), an approach to the classificationclassification,
in biology, the systematic categorization of organisms into a coherent scheme. The original purpose of biological classification, or systematics, was to organize the vast number of known plants and animals into categories that could be named, remembered, and
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 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 DarwinDarwin, Charles Robert,
1809–82, English naturalist, b. Shrewsbury; grandson of Erasmus Darwin and of Josiah Wedgwood. He firmly established the theory of organic evolution known as Darwinism.
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'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 paleontologypaleontology
[Gr.,= study of early beings], science of the life of past geologic periods based on fossil remains. Knowledge of the existence of fossils dates back at least to the ancient Greeks, who appear to have regarded them as the remains of various mythological creatures.
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 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 fossilfossil,
remains or imprints of plants or animals preserved from prehistoric times by the operation of natural conditions. Fossils are found in sedimentary rock, asphalt deposits, and coal and sometimes in amber and certain other materials.
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; datingdating,
the determination of the age of an object, of a natural phenomenon, or of a series of events. There are two basic types of dating methods, relative and absolute.
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Biology a method of grouping animals that makes use of lines of descent rather than structural similarities