Please wait while we load your article...

Home > Cladistics

Learn more about "Cladistics"

Cladistics

Cladistics (ancient Greek: '''', ''klados'', "branch") is a form of biological systematics that classifies species of organisms into hierarchical monophyletic groups. It can be distinguished from other taxonomic systems, such as phenetics, by its focus on shared derived characters (Synapomorphy|synapomorphies). Previous systems usually employed overall morphological similarity to group species into genera, families and other higher level classification; cladistic classifications (usually trees called cladograms) are intended to reflect the relative recency of common ancestry or the sharing of Homology (biology)|homologous features. Cladistics is also distinguished by its emphasis on maximum parsimony | parsimony and hypothesis testing (particularly falsificationism), rather than subjective decisions that some other taxonomic systems rely upon. Cladistics originated in the work of the German entomologist Willi Hennig, who referred to it as "phylogenetic systematics" (also the name of his 1966 book); the use of the terms "cladistics" and "clade" was popularized by other researchers. The technique and sometimes the name have been successfully applied in other disciplines: for example, to determine the relationships between the surviving manuscripts of the ''Canterbury Tales''. Cladists use ''cladograms'', diagrams which show ancestral relations between species, to represent the monophyletic relationships of species, termed sister-group relationships. This is interpreted as representing phylogeny, or evolutionary relationships. Although traditionally such cladograms were generated largely on the basis of morphological characters, genetic sequencing data and computational phylogenetics are now very commonly used in the generation of cladograms.

History of cladistics

The term ''clade'' was introduced in 1958 by Julian Huxley, ''cladistic'' by Cain and Harrison in 1960, and ''cladist'' (for an adherent of Hennig's school) by Mayr in 1965. Hennig referred to his own approach as ''phylogenetic systematics''. From the time of his original formulation until the end of the 1980s cladistics remained a minority approach to classification. However in the 1990s it rapidly became the dominant method of classification in evolutionary biology. Computers made it possible to process large quantities of data about organisms and their characteristics. At about the same time the development of effective polymerase chain reaction techniques made it possible to apply cladistic methods of analysis to biochemical and molecular genetics|molecular genetic features of organisms as well as to anatomical ones.

Cladistics as a successor to phenetics

For some decades in the mid to late twentieth century, a commonly used methodology was phenetics ("numerical taxonomy"). This can be seen as a predecessor to some methods of today's cladistics (namely Distance matrices in phylogeny|distance matrix methods such as neighbor-joining), but made no attempt to resolve phylogeny, only similarities.

Clades

The clade is the topic of interest to cladistics. A clade is a group of taxon|taxa consisting of an ancestor taxon and all of its descendant taxa. In the diagram provided, it is hypothesized that the first vertebrate species is the common ancestor of all vertebrate species, including fishes (''Pisces''). The first tetrapod is the ancestor of all tetrapods, including amphibians, reptiles, mammals and birds. The tetrapod ancestor was a descendant of the original vertebrate ancestor, but is not an ancestor of any fish. The vertebrate clade therefore consists of a fish clade (emcompassing all fish) and a tetrapod clade (consisting of all the tetrapods) and so on up the tree. An important caution is that any cladogram is a provisional hypothesis. In a hypothetical example, further genetic or morphological evidence might suggest that fish and amphibians share a common ancestor that was not an ancestor of the other tetrapods. The new information would cause us to define a fish-and-amphibian clade, altering the cladogram.

Terminology

The following terms are used to identify shared or distinct characters among groups:
- ''Plesiomorphy'' ("close form") or ''ancestral state'', also ''symplesiomorphy'' ("shared plesiomorphy"), is a characteristic that is present at the base of the tree. For example, in the tree shown, the presence of a backbone (shared by all vertebrates) can be hypothesized to have existed in the common vertebrate ancestor.
- ''Apomorphy'' ("separate form") or ''derived state'' is a characteristic believed to have evolved within the tree. For example, all tetrapods have four limbs; thus, having four limbs is an apomorphy for vertebrates but a plesiomorphy for tetrapods.
- ''Synapomorphy'' ("shared apomorphy") is an ''apomorphy'' which is shared between taxa. The application of the above terms to a group depends on one's perspective in the tree. They are thus relative terms. For example, an apomorphy of one clade is the plesiomorphy of another contained within it. These terms are equivalent to but more precise than the homology, allowing one to express the hierarchical relationships among different homologies. Three main types of groups can be identified by plotting their relationships in cladograms:
- ''Monophyletic groups'' are groups containing only taxa descended from a given ancestor taxon and all those descendants. In the diagram, all vertebrates are monophyletic, since all the taxa are descended from a single ancestor (the common vertebrate species) and there are no others. Monophyly is diagnosed by the synapomorphy relation.
- ''Paraphyletic groups'' are groups excluding one or more descendant taxa of the common ancestor, and thus contain some but not all of its descendants. For instance, excluding birds from the sample cladogram would create a paraphyletic group, ''reptiles''. Paraphyletic groups are typically diagnosed on the basis of shared plesiomorphy and the exclusion of groups that are diagnosable by a synapomorphy. For example, ''birds'' are warm-blooded, excluding them from being ''reptiles'', which because of that exclusion is a paraphyletic group, as birds are also descended from ''amniotes''. The plesiomorphy is the amniotic fluid and the synapomorphy is warm-bloodedness.
- ''Polyphyletic groups'' are groups containing taxa from two or more different monophyletic groups. For instance, a grouping of birds and mammals based on their warm-bloodedness is not monophyletic. In the diagram given (rightly or wrongly) the ''Aves'' ancestor came from the ''Archaeosaurs'', but the ''Mammalian'' ancestor came from the ''Amniotes''. Although there is an ultimate common ancestor among the ''Tetrapods'', the latest common ancestors are not the same and therefore the group is polyphyletic. The warm-bloodedness is not a plesiomorphy but is a homoplasy, or convergence. Polyphyletic groups are recognized by the homoplasy relation (that is, a group is polyphyletic because it is diagnosed by a character that actually forms a homoplasy, see wastebasket taxon). Clades relate to each other in these ways:
- A clade is ''Basal (phylogenetics)|basal'' to another clade if it contains that other clade as a subset within it. In this case, the vertebrate clade is basal to the tetrapod and fish clades. The use of "basal" to mean a clade that is less species-rich than a sister clade, with such a deficit being taken as an indication of primitiveness, is incorrectly applied.
- A clade located within a clade is said to be ''nested'' within that clade. The bird clade is nested within the reptilian clade.

Three definitions of clade

There are three major ways to define a clade for use in a cladistic taxonomy.
- ''Node-based'': the last common ancestor of A and B, and all descendants of that ancestor. Crown groups are a type of node-based clade.
- ''Branch-based'': the first ancestor of A which is not also an ancestor of C, and all descendants of that ancestor. (This type of definition was originally called "stem-based", but this was changed to avoid confusion with the term "stem group".) Total groups are a type of branch-based clade.
- ''Apomorphy-based'': the first ancestor of A to possess derived trait M Homology (biology)|homologously (that is, synapomorphy|synapomorphically) with that trait in A, and all descendants of that ancestor. The process of identifying and naming groups based on apomorphies is the method that most resembles classical systematics, with the proviso that cladistic taxa always denote a clade.

Cladograms

Cladists use ''cladograms'', diagrams which show ancestral relations between taxa, to represent the evolutionary tree of life (science)|tree of life. Although traditionally such cladograms were generated largely on the basis of morphological characters, molecular sequencing data and computational phylogenetics are now very commonly used in the generation of cladograms. The starting point of cladistic analysis is a group of species and molecular, morphological, or other data characterizing those species. The end result is a Tree (graph theory)|tree-like relationship diagram called a cladogram, or sometimes a '''dendrogram''' (Greek for "tree drawing"). The cladogram graphically represents a hypothetical evolutionary process. Cladograms are subject to revision as additional data become available. The terms "evolutionary tree", and sometimes "phylogenetic tree" are often used synonymously with cladogram, but others treat phylogenetic tree as a broader term that includes trees generated with a nonevolutionary emphasis. In cladograms, all species lie at the leaves. The two taxon|taxa on either side of a split, with a common ancestor and no additional descendents, are called "sister taxa" or "sister groups". Each subtree, whether it contains only two or a hundred thousand items, is called a "clade". Many cladists require that all forks in a cladogram be 2-way forks. Some cladograms include 3-way or 4-way forks when there are insufficient data to resolve the forking to a higher level of detail (see under phylogenetic tree). For a given set of species, the number of distinct cladograms that can be drawn (ignoring which cladogram best matches the species characteristics) is:

Application to other disciplines

The comparisons used to acquire data on which cladograms can be based are not limited to the field of biology. Any group of individuals or classes, hypothesized to have a common ancestor, and to which a set of common characteristics may or may not apply, can be compared pairwise. Cladograms can be used to depict the hypothetical descent relationships within groups of items in many different academic realms. The only requirement is that the items have characteristics that can be identified and measured. Recent attempts to use cladistic methods outside of biology address the reconstruction of lineages in:
- Cultural anthropology|Anthropology and archeology. Compares cultures or artifacts using groups of cultural traits or artifact features.
- historical linguistics|Linguistics. Compares languages using groups of linguistic features.
- Textual criticism or Stemmatics. Compares manuscripts of the same work (original lost) using groups of distinctive copying errors.
- Ethology.. Compares animal species using behavioral traits presumed hereditary.

References

Bibliography

-
-
-
-
- Available free online at http://gallica.bnf.fr (No direct URL). This is the paper credited by Hennig (1979) for the first use of the term 'clade'.
-
- The paper to which he was responding is reprinted in .
-
- Translated from manuscript and so never published in German.
-
-
- Reissued 1997 in paperback. Includes a reprint of Mayr's 1974 anti-cladistics paper at pp. 433–476, "Cladistic analysis or cladistic classification." This is the paper to which is a response.
-
-
- .
- .
- Schuh, Randall T. and Andrew V. Z. Brower. 2009. ''Biological Systematics: principles and applications (2nd edn.)'' Cornell University Press, ISBN 978-0-8014-4799-0

External links

-
-
-
-
-
-
Category:Phylogenetics

Related Images

- The three ways to define a clade

Sources: StartLearningNow, Wikipedia | Usage license: GNU FDL

Welcome to Start Learning Now. Explore to your heart's content, and we hope you enjoy reading the material we have assembled for you here!

Related News

Further Resources


	home lucky pick

Related Resources