Cladistics

The cladistics ( gr κλάδος klados " branch ", " branching " ) - also inaccurate often equated with phylogenetic systematics - is a method of biological systematics within evolutionary biology. It was outlined by the German zoologist Willi Hennig in the 1950s in its basic features and described in his textbook Phylogenetic Systematics 1966.

• 4.1 relationships
• 4.2 Important terms

Objective

Phylogenetic systematics seeks to create a system of organisms, which is based solely on phylogenetic relationship. Groups within such a system are often, but not always, monophyletic. A cladistics is a special phylogenetic systematics, in which all groups are monophyletic. A monophyletic group (also called clade ) contains all the descendants of one ancestral species as well as the parent-species itself, but no species that are not descendant of the ancestral species. The characteristic features of the ancestral species corresponds to the during the analysis to be reconstructed pattern. Are the basis for creating monophyletic groups shared derived characteristics, so-called synapomorphies. The basic pattern represents the entirety of the non-derived features ( plesiomorphies ) of the groups. In contrast to the ideal blueprint, which combines the totality of characteristics of a group in itself, that corresponds to the basic plan the physique and the characteristic value of a type which has real exists.

The result of a cladistic analysis is a relationship hypothesis, which is represented as a cladogram. Hennig referred to the cladogram as a " scheme of argumentation of phylogenetic systematics ". Unlike a pedigree the cladogram has only terminal taxa. It can thus not the development of a recent form of another to, or, in other words, no living ( extant ) species can and must be ancestral species to another recent type. Fossil species can be integrated into a cladogram, but they then also form terminal taxa. That is, the assignment of a fossil nature than the actual original species is avoided. Nodes of a Kladogramms represent the ancestral species of the two from her sister resulting selcetion autapomorphies of the respective sister groups are derived features that are common to all taxa that group together and which do not occur in the sister group.

The cladistics contradicts a so-called " progress Prejudice", which means a development "of the invertebrates to the people " determine. A fundamental problem with this view is that this man must be placed at the head. In fact, there is the " tendency " to form vortices, just as a " tendency " towards to stay invertebrate, such as the much greater diversity of invertebrates demonstrated. It is based solely on phylogenetic relationship and deliver its external features, although as an essential data base, but summarizes taxa based not only on prior similarities in shape and body together.

Phylogenetic systematics is a historical science, because you can not watch the phylogeny of organisms, but only reconstruct. Therefore, all the hypotheses relationship will not always be experimentally confirming hypotheses. Phylogenetic systematics attempted to establish consistent hypotheses and relationship hypotheses that are in conflict resolve. The methodology of phylogenetic systematics gives the scientists a set of tools at hand that allows him to explain his reasoning reproducible.

Cladograms

The representation of relationships takes place in so-called Kladogrammen. These differ from evolutionary family trees in the following points:

• In a branch, there are only two branches ( dichotomous branching ).
• The branches are not weighted, so you have no measure of the change to represent it in a cladogram. ( In evolutionary family trees can represent such a measure in different path lengths for branches, see also divergence).
• There is no absolute time axis.
• All Artspaltungsereignisse are shown in the realistischst possible form.

Each branch is due a derived feature. What this feature should be in each is the subject of research. So you can distinguish, for example, placental mammals about their placenta of marsupials, in turn, as compared with placental animals such as a reduction of the formed primary teeth. However, the eponymous bag is not a synapomorphy, but is within the marsupials repeatedly emerged ( convergence ), not all marsupials have a pouch or some have reduced him.

Cladogram of mammals

This relationship will be illustrated by the example of a simplified Kladogrammes of mammals here:

( Marsupials ) ( Plazentaria )                  \ /    (Milk teeth reduced) /                    \ ( Placenta)                     \ /                      \ /     Monotremata Theria            \ /         ( Cloaca ) /              \ ( Teats )               \ /                \ /                /               /          Mammals It is important that all the branches have at least one autapomorphy.

Characteristics of the basic pattern may be lost again within the group. This is an autapomorphy of the taxon concerned. An example of this is the secondary loss of the wings of many flying insects ( pterygota ).

Cladogram " man, gorilla and chimpanzee "

Charles Darwin assumed that between the species listed below the next relationship between gorillas and chimpanzees and humans would have a special status. Stephen Jay Gould provides evidence that chimps and humans are closest and the gorillas split off earlier in the history of development.

Other Apes gorillas chimpanzees People           \ \ \ /            \ \ \ /             \ \ \ /              \ \ \ /               \ \ /                \ \ /                 \ \ /                  \ \ /                   \ /                    \ /                     \ /                      \ /              Cladogram by Mark Abraham Construction of Kladogrammen

In a cladistic analysis of a group of taxa is an attempt to reconstruct their phylogenetic relationships and to map the resulting branching pattern as a cladogram. Starting point of the analysis, it is usually to collect as many meaningful features as possible. Possibly meaningful are all features that occur in a subset of the analyzed taxa, in another part not. Contrary to the previous approach in systematics is dispensed today usually out weight the features. There is some a priori features a higher weight than the other, there is the danger that the prejudices arranger distort the analysis. Useful for analysis are exclusively features representing Syanapomorphien. Together ancestral home group characteristics ( Symplesiomorphien ) are irrelevant to the analysis, namely (Aut ) apomorphies of a single taxon emphasize its independence, but are not enlightening for his relatives. Normally the considered features are processed and represented in the form of a character matrix in which the expression of the characteristic is encoded by a number ( eg " wing exists" = 1, " wingless " = 0).

Must of course have been previously clarified as far as possible ( based on the Remaneschen homology criteria), the homology of the features for the analysis. Common features that have arisen convergently, or features that are based on parallel evolution (which are characteristics that are developed independently from each in closely related species on a comparable genetic basis ), the analysis can distort if left undetected. Are considered features marked by convergences and numerous independent origination and regressions so that they hardly contribute to the analysis, it is called homoplasy. In many cases it is impossible to determine the degree of homoplasy previously. Strong homoplasy of the features analyzed devalues ​​the resulting cladogram, so prone to homoplasy characteristics should be avoided as much as possible.

The main problem with the cladistic analysis are regressions of characteristics. If one considers, for example, the groups of rocks Springer ( Archaeognatha ), mayflies ( Ephemeroptera) and fleas ( Siphonaptera ) on the basis of the characteristic " wing training ," said wings are present only in the ephemera. The absence of the wings, in the case of cliff divers in mind that they are descended primarily from wingless ancestors. The fleas on the other hand had winged ancestors, only the wings are with them was regressed without any rest. In this case, the interactions are due to a number of other features still clearly indetectable, this is by no means numerous other groups with the same security possible. It even happens that individual lineages back form a feature and it is then acquired independently a second time at the individual lines of the offspring. The question whether a particular feature in the home group was present and was so atrophied in the lines that did not have it or if it was missing in the ancestors and was purchased new in one or more lines within the group under consideration, the " polarity " of the feature mentioned. To determine the polarity of so-called out-groups are included in the analysis. An outside group can be any taxon that is related with the analyzed species though, but that is certainly and clearly outside of the observed relationship circle. For obvious reasons, it should be a little specialized taxon with few autapomorphies. The choice of outgroup (s), the analysis can significantly affect.

In the classical approach Hennig a cladogram was constructed by now that you constructed a hypothetical ancestral form by clarifying the polarities and then the taxa considered trying out for so long ruled by single or group -wise addition, until a convincing pedigree revealed. In modern cladistic analysis of this step is performed by a sorting algorithm. For this, the taxa described by the feature matrix are permuted so long until a branching pattern of minimal length is found. The outgroup taxa are included in this analysis. This shortest branch pattern is considered to be the most likely hypothesis of phylogenetic relationships. This is called parsimony ( engl. "greed ", " economy "). The most often used the sort program PAUP ( Phylogenetic Analysis Using Parsimony ), but there are a number of other common programs. If there are two or more different pedigrees of the same length, are the equivalent hypotheses, and the relationship is not decidable. First, however, analysis result is simply a connection graph. To transform this into a cladogram, the polarity must be cleared (that is, it must be clear which took place the first branching ). This can either be derived from the data analysis, or be forced (with good knowledge of the outer groups). This step is called " rooting " (English rooting ) denotes the Kladogramms.

Biological Systematics

The biological system sees itself as a science, the living entities are classified according to their origin. Therefore, the cladistics is one of its working methods.

When creating a Kladogramms properties of the considered living things are compared. There are often, but not exclusively, use morphological characteristics, characteristics of the metabolism and genetic information.

Thereafter, a plurality of Kladogrammen is created. The one with the least number of cladogram necessary changes within the assumed evolution curve is regarded as the most likely. It is often involved in specifying Kladogramms of interest, other cladograms, which are designed with a very similar number of changes to consider also.

Bioinformatics uses for the reconstruction of Kladogrammen various standard software that analyze multiple sequence alignments and the variability of individual residues, such as Phylip.

The traditional naming in biology can not believe the tree-like structure of evolutionary development. Therefore, a phylogenetic nomenclature, PhyloCode is called discussed.

Relationships

The sister groups of Kladogrammen (here, for example, humans and chimpanzees) do not always match the previously common classification units ( taxa ) of biological classification. This is known taxa according to their actual relationship as:

• Monophyletic - the taxon ( group) has a common ancestral form, and includes all subgroups that derive from this ancestral form, and the stem-form itself, but not other groups. The monophyletic group founded by apomorphies of common stock and is also referred to as closed.

• Paraphyletic - the taxon ( group) does have a common ancestral form, but does not contain all taxa, as is the case with the monophyletic. A Paraphylum justified by the plesiomorphies be sister and is also referred to as open.

• Polyphyletic - the group has no common ancestral form.

Important terms

Autapomorphy, synapomorphy, Plesiomorphie, Symplesiomorphie, Crown Group