Cladistics
From Palaeos.org
| SYSTEMATICS |
| Cladistics | Evolutionary systematics | Molecular systematics | Phenetics | Phylogenetic taxonomy | Systematics history |
Cladistics is a method of classification that taxa groups hierarchically into discrete sets and subsets (Kitching et al., 1998). It is also known as phylogenetic systematics in biology, and it hypothesises the evolutionary relationships between organisms based on derived similarities. Cladistics can (and has been) used to organise any comparative data (e.g. linguistics) but is greatest application has been in the field of biological systematics (Kitching et al., 1998). Cladistics is a method for implementing Darwin's concepts of ancestors and descendents (Kitching et al., 1998), using "shared derived traits" (synapomorphies: see below) of the organisms being studied. Cladistic analysis forms the basis for most modern systems of biological classification, which seek to group organisms by evolutionary relationships. In contrast, phenetics (or numercial taxonomy) groups organisms based on their overall similarity, while approaches that are more traditional tend to rely on key characters (morphology). The word cladistics is derived from the ancient Greek κλάδος, klados, "branch."
Introduction
The German entomologist Willi Hennig, in 1950 created a method for explaining his ideas within an evolutionary framework, he wrote about species, speciation and the transformation of morphology through the process of evolution (Kitching et al., 1998). In 1965 and 1966 his work was translated from German into English under the name 'phylogenetic systematics', were it began to become widely known. Hennig's most important contributions were to offer a precise definition of biological relationship and then to suggest how that relationship might be discovered (Kitching et al., 1998). It is from this pioneer that modern cladistic methodology is based, a methodology to reconstruct hypothesised evolutionary relationships between taxa.
Cladistics is considered one of the more useful tools for inferring evolutionary relationships, and its practice has immeasurably advanced our knowledge of the evolutionary history of life. The greatest triumph of cladistics has been the revision of inaccurate systematic treatments of taxa based on the classification system employed by Karl von Linne—and its concomitant typological suppositions. Principle examples include the recognition of paraphyly of Reptilia and Aves in the Linnean sense, and holophyly of Dinosauria (Bakker & Galton 1974, Gauthier 1986, Bakker 1986, Carroll 1988, Dodson et al 1990, Paul 2002).
Despite these achievements, and the usefulness of cladistics notwithstanding, the theory and practice thereof often strike even well educated individuals as esoteric, and uninteresting. One might suppose that this sort of blase attitude owes much to the disrespect systematics has earned from its scientific confreres—being just pigeonholing and abstruse cataloguing (Mayr 1942). This is most unfortunate, however. Cladistics is one of the most robust tools for bringing the profoundly powerful concept of common descent into concrete, graphical terms. To approach evolutionary biology without an understanding and appreciation of the genealogy of life that is entailed by common descent is of course, foolhardy, and thus one must be versed in phylogenetic reconstruction and the business of cladistics. And that is the purpose of this primer: to engender in the reader a sense of the sublimity of systematics— the study of evolution in action—and how it in turn, demonstrates the elegance of evolutionary biology.
Some definitions
A character state that is present in both the outgroups (the nearest relatives of the group, that are not part of the group itself) and in the ancestors is called a plesiomorphy (meaning "close form", also called ancestral state). A character state that occurs only in later descendants is called an apomorphy (meaning "separate form", also called the "derived" state) for that group. The adjectives plesiomorphic and apomorphic are used instead of "primitive" and "advanced" to avoid placing value-judgements on the evolution of the character states, since both may be advantageous in different circumstances. It is not uncommon to informally refer to a collective set of plesiomorphies as a groundplan for the clade(s) they refer to.
Several more terms are defined for the description of cladograms and the positions of items within them. A species or clade is basal to another clade if it holds more plesiomorphic characters than that other clade. Usually a basal group is very species-poor as compared to a more derived group. It is not a requirement that a basal group is present. For example when considering birds and mammals together, neither is basal to the other: both have many derived characters.
A clade or species located within another clade can be described as nested within that clade (i.e. part of a hierarchical succession of nested groups).
Cladistics does not assume any particular theory of evolution, only the background knowledge of descent with modification. Thus, cladistic methods can be, and recently have been, usefully applied to non-biological systems, including determining language families in historical linguistics and the filiation of manuscripts in textual criticism.
Credits
Credits: This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Cladistics"; Introduction section This page incorporates material from EvoWiki and so is licensed under the Attribution-NonCommercial-ShareAlike Creative Commons License). EvoWiki url: http://wiki.cotch.net/index.php/Cladistics Modified by Mark T Young to introduce information from Kitching et al. (1998). Cladistics 2nd Edition. Oxford University Press, Oxford, UK.
