NationMaster - Encyclopedia: Apomorphy

Cladistics (Greek: klados = branch) is a branch of biology that determines the evolutionary relationships between organisms based on derived similarities. It is the most prominent of several forms of phylogenetic systematics, which study the evolutionary relationships between organisms. Cladistics is a method of rigorous analysis, using "shared derived properties" (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 groups organisms based on their overall similarity, while approaches that are more traditional tend to rely on key characters. Willi Hennig (1913 - 1976) is widely regarded as the founder of cladistics. Image File history File links Typical example of a horizontally-oriented cladogram, with horizontal and vertical lines used to indicate relationships. ... Biology is the branch of science dealing with the study of life. ... A speculative phylogenetic tree of all living things, based on rRNA gene data, showing the separation of the three domains, bacteria, archaea, and eukaryotes. ... A phylogeny (or phylogenesis) is the origin and evolution of a set of organisms, usually of a species. ... In biology, systematics is the study of the diversity of organism characteristics, and especially how they relate evolutionarily. ... In biology, phenetics, also known as numerical taxonomy, is an attempt to classify organisms based on overall similarity, usually in morphology or other observable traits, regardless of their phylogeny or evolutionary relation. ... Willi Hennig (April 20, 1913 - November 5, 1976) was a German biologist and is known as the founder of phylogenetic systematics (cladistics). ...

As the end result of a cladistic analysis, treelike relationship-diagrams called "cladograms" are drawn up to show different hypotheses of relationships. A cladistic analysis can be based on as much or as little information as the researcher selects. Modern systematic research is likely to be based on a wide variety of information, including DNA-sequences (so called "molecular data"), biochemical data and morphological data. A labeled tree with 6 vertices and 5 edges In graph theory, a tree is a graph in which any two vertices are connected by exactly one path. ... Comparative anatomy is the study of similarities and differences in organisms. ...

In a cladogram, all organisms lie at the leaves, and each inner node is ideally binary (two-way). The two taxa on either side of a split are called sister taxa or sister groups. Each subtree, whether it contains one item or a hundred thousand items, is called a clade. A natural group has all the organisms contained in any one clade that share a unique ancestor (one which they do not share with any other organisms on the diagram) for that clade. Each clade is set off by a series of characteristics that appear in its members, but not in the other forms from which it diverged. These identifying characteristics of a clade are called synapomorphies (shared, derived characters). For instance, hardened front wings are a synapomorphy of beetles, while circinate vernation, or the unrolling of new fronds, is a synapomorphy of ferns. Image File history File links Typical example of a vertically oriented cladogram, with diagonal intersecting lines indicating relationships. ... A labeled tree with 6 vertices and 5 edges In graph theory, a tree is a graph in which any two vertices are connected by exactly one path. ... A taxon (plural taxa), or taxonomic unit, is a grouping of organisms (named or unnamed). ... Shared characteristics that define a cladistic grouping. ... A Laughing Gull on the beach in Atlantic City. ... Suborders Adephaga Archostemata Myxophaga Polyphaga See subgroups of the order Coleoptera Beetles are one of the most diverse groups of insects. ... This Australian tree fern is producing a new frond by the process of circinate vernation Vernation (from vernal, since that is when leaves spring forth in Temperate regions) is the formation of new leaves or fronds. ... Classes Marattiopsida Osmundopsida Gleicheniopsida Pteridopsida A fern, or pteridophyte, is any one of a group of about 20,000 species of plants classified in the Division Pteridophyta, formerly known as Filicophyta. ...

Definitions

A character state (see below) 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.

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 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. In phylogenetics, basal members of a group are subgroups that diverged very early from the others. ...

A clade or species located within another clade can be described as nested within that clade.

Cladistic methods

A cladistic analysis is applied to a certain set of information. To organise this information a distinction is made between characters, and character states. Consider the color of feathers, this may be blue in one species but red in another. Thus, "red feathers" and "blue feathers" are two character states of the character "feather-color."

In the old days, the researcher would decide which character states were present before the last common ancestor of the species group (plesiomorphies) and which were present in the last common ancestor (synapomorphies). Usually this is done by considering one or more outgroups (organisms that are considered not to be part of the group in question, but that are related to the group). Only synapomorphies are of any use in characterising cladistic divisions.

Next, different possible cladograms were drawn up and evaluated. Clades should have as many synapomorphies as possible. The hope is that a sufficiently large number of true synapomorphies will be large enough to overwhelm any unintended symplesiomorphies (homoplasies), caused by convergent evolution (i.e. characters that resemble each other because of environmental conditions or function, not because of common ancestry). A well-known example of homoplasy due to convergent evolution is the character wings. Though the wings of birds and insects may superficially resemble one another and serve the same function, each evolved independently. If a bird and an insect are both accidentally scored "POSITIVE" for the character "presence of wings", a homoplasy would be introduced into the dataset, which may cause erroneous results. In evolutionary biology, convergent evolution describes the process whereby organisms not closely related independently acquire similar characteristics while evolving in separate and sometimes varying ecosystems. ...

When equivalent possibilities turn up, one is usually chosen based on the principle of parsimony: the most compact arrangement is likely the best hypothesis of relationship (a variation of Occam's razor). Another approach, particularly useful in molecular evolution, is maximum likelihood, which selects the optimal cladogram that has the highest likelihood based on a specific probability model of changes. William of Ockham Occams Razor (also spelled Ockhams Razor) is a principle attributed to the 14th-century English logician and Franciscan friar William of Ockham. ... Maximum likelihood estimation (MLE) is a popular statistical method used to make inferences about parameters of the underlying probability distribution of a given data set. ...

Of course, it is no longer done this way. Researcher bias is something to be avoided, and these days besides the software to calculate the trees themselves, there is sophisticated statistic software to provide a more objective basis.

Cladistics has taken a while to settle in, and there is still wide debate over how to apply Hennig's ideas in the real world. There is concern that use of widely different data sets (for instance, structural versus genetic characteristics) may produce widely different trees. However, largely, cladistictics has proven useful in resolving phylogenies and has gained widespread support.

As DNA sequencing has become easier, phylogenies are increasingly constructed with the aid of molecular data. Computational systematics allows the use of these large data sets to construct objective phylogenies. These can filter out true synapomorphies from parallel evolution more accurately. DNA sequencing is the process of determining the nucleotide order of a given DNA fragment, called the DNA sequence. ... Molecular systematics is a product of the traditional field of systematics and the growing field of bioinformatics. ...

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. Historical linguistics (also diachronic linguistics or comparative linguistics) is primarily the study of the ways in which languages change over time. ... Paternity is the social and legal acknowledgment of the parental relationship between a father and his child. ... Textual criticism is a branch of philology that examines the extant manuscript copies of an ancient or medieval literary work to produce a text that is as close as possible to the original. ...

Cladistic classification

A recent trend in biology since the 1960s, called cladism or cladistic taxonomy, requires taxa to be clades. In other words, cladists argue that the classification system should be reformed to eliminate all non-clades. In contrast, other taxonomists insist that groups reflect phylogenies and often make use of cladistic techniques, but allow both monophyletic and paraphyletic groups as taxa. Types of biological clade, as used in PhyloCode Diagram by User:Gdr File links The following pages link to this file: Cladistics PhyloCode User:Gdr/Gallery Categories: GFDL images ... This cladogram shows the relationship among various insect groups. ... Evolutionary taxonomy or evolutionary systematics seeks to classify organisms using a combination of phylogenetic relationship and overall similarity. ... In biology, phylogenetics (Greek: phylon = tribe, race and genetikos = relative to birth, from genesis = birth) is the study of evolutionary relatedness among various groups of organisms (e. ... In phylogenetics, a group is monophyletic (Greek: of one stem) if all organisms in that group are known to have developed from a common ancestral form, and all descendants of that form are included in the group. ... Paraphyletic - Wikipedia /**/ @import /skins-1. ... A taxon (plural taxa) is an element of a taxonomy, e. ...

A monophyletic group is a clade, comprising an ancestral form and all of its descendants, and so forming one (and only one) evolutionary group. A paraphyletic group is similar, but excludes some of the descendants that have undergone significant changes. For instance, the traditional class Reptilia excludes birds even though they evolved from the ancestral reptile. Similarly, the traditional Invertebrates are paraphyletic because Vertebrates are excluded, although the latter evolved from an Invertebrate. In phylogenetics, a group is monophyletic (Greek: of one stem) if all organisms in that group are known to have developed from a common ancestral form, and all descendants of that form are included in the group. ... Paraphyletic - Wikipedia /**/ @import /skins-1. ...

A group with members from separate evolutionary lines is called polyphyletic. For instance, the once-recognized Pachydermata was found to be polyphyletic because elephants and rhinoceroses arose from non-pachyderms separately. Evolutionary taxonomists consider polyphyletic groups to be errors in classification, often occurring because convergence or other homoplasy was misinterpreted as homology. In biology, a taxon is polyphyletic if it is descended from more than one root form (in Greek poly = many and phyletic = racial). ... In evolutionary biology, convergent evolution describes the process whereby organisms not closely related independently acquire similar characteristics while evolving in separate and sometimes varying ecosystems. ... In evolutionary biology, convergent evolution describes the process whereby organisms not closely related independently acquire similar characteristics while evolving in separate and sometimes varying ecosystems. ... In biology, two or more structures are said to be homologous if they are alike because of shared ancestry. ...

Following Hennig, cladists argue that paraphyly is as harmful as polyphyly. The idea is that monophyletic groups can be defined objectively, in terms of common ancestors or the presence of synapomorphies. In contrast, paraphyletic and polyphyletic groups are both defined based on key characters, and the decision of which characters are of taxonomic import is inherently subjective. Many argue that they lead to "gradistic" thinking, where groups advance from "lowly" grades to "advanced" grades, which can in turn lead to teleology. Teleology should be avoided as outside the realm of science. Teleology (telos: end, purpose) is the supposition that there is design, purpose, directive principle, or finality in the works and processes of nature, and the philosophical study of that purpose. ...

Going further, some cladists argue that ranks for groups above species are too subjective to present any meaningful information, and so argue that they should be abandoned. Thus they have moved away from Linnaean taxonomy towards a simple hierarchy of clades.

Other evolutionary systematists argue that all taxa are inherently subjective, even when they reflect evolutionary relationships, since living things form an essentially continuous tree. Any dividing line is artificial, and creates both a monophyletic section above and a paraphyletic section below. Paraphyletic taxa are necessary for classifying earlier sections of the tree – for instance, the early vertebrates that would someday evolve into the family Hominidae cannot be placed in any other monophyletic family. They also argue that paraphyletic taxa provide information about significant changes in organisms' morphology, ecology, or life history – in short, that both taxa and clades are valuable but distinct notions, with separate purposes. Many use the term monophyly in its older sense, where it includes paraphyly, and use the alternate term holophyly to describe clades (monophyly in Hennig's sense).

A formal code of phylogenetic nomenclature, the PhyloCode, is currently under development for cladistic taxonomy. It is intended for use by both those who would like to abandon Linnaean taxonomy and those who would like to use taxa and clades side by side. Types of clade PhyloCode is a draft for a formal set of rules governing phylogenetic nomenclature. ...

Contents

Definitions

Cladistic methods

Cladistic classification

See also

References

External links

Contents

Definitions var ACE_AR = {Site: '756743', Size: '300250'};

Cladistic methods

Cladistic classification

See also

References

External links

Definitions