The term "cladistics" for this method, which is based on the use of closed communities of descent called Klade , was originally introduced by the important evolutionary biologist Ernst Mayr , who wanted to criticize rather than neutrally describe the method Hennig himself called " phylogenetic systematics" . In the German-speaking world in particular, the term cladistics therefore had a negative connotation for a long time and was avoided by the proponents of the method, Hennig himself only used it in quotation marks. Due to the use of cladistics in the English-speaking world without this judgmental coloring, it is mostly used today without judgmental connotation. The Willi Hennig Society publishes its journal under the title Cladistics.
The cladistic method of constructing cladograms on the basis of apomorphies as the basis of biological systematics is now a generally accepted standard in biology; alternative concepts such as phenetics are only of historical interest. Using cladistics as an exclusive taxonomic method is the predominant scientific standard, but it is still criticized by a group of biologists and not practiced by them themselves.
The phylogenetic system aims to create a system of organisms that is based exclusively on phylogenetic relationships. Since all organisms are actually related to one another (descent theory), as convincingly justified by the theory of evolution, there is only one correct system that would represent the evolution that actually took place on earth. But since this natural relationship is based on processes that lie millions of years in the past, it cannot be observed directly, but only inferred from evidence. The task of systematics is to find this natural system. New taxonomic groups, such as new species, have arisen from the fact that existing species have split up, as different populations within a species have diverged (see speciation ). All higher taxonomic units (groups such as vertebrates, mammals, flowering plants) ultimately go back to individual parent species, so that no other system is required for the higher units (macroevolution) than for the lower ones (microevolution).
However, groups within a kinship-based system are not always easily identifiable if the organisms are classified only according to their similarity to one another. A parent group can evolve into two groups of descendants, one of which looks relatively similar to its ancestor, while another can look radically different. For example, the ancestors of the coelacanth , lungfish and terrestrial vertebrates living today form a common family group whose ancestors looked much more like a coelacanth than a human or an elephant. In addition, the coelacanth resembles other fish in general body shape much more closely than the land vertebrates, to which it is more closely related than to these. Since both descend from common (also fish-like) ancestors, they are of course also related to each other, albeit somewhat more distantly. That means: such groups are not always monophyletic . The group of fish, for example, is based on phylogenetic relationships, but nevertheless the fish are not a monophyletic group (fish contain all other vertebrates as descendants).
A cladistics is a special phylogenetic system in which all groups are also monophyletic. A monophyletic group (also called clade ) contains all descendants of a parent species as well as the parent species itself, but no species that are not descendants of this parent species. The features of the parent species correspond to the basic pattern to be reconstructed during the analysis. The basis for the creation of monophyletic groups are common derived characteristics, so-called synapomorphies . The basic pattern represents the totality of the non-derived characteristics ( plesiomorphies ) of the groups. In contrast to the ideal building plan, which unites the totality of all characteristics of a group, the basic plan corresponds to the body structure and the characteristics of a species that actually existed.
The result of a cladistic analysis is a relationship hypothesis, which is represented as a cladogram . Hennig describes the cladogram as the "argumentation scheme of the phylogenetic system". Unlike a family tree , the cladogram only has terminal taxa . It does not allow the development of a recent form from another, or, to put it another way, no living (recent) species can and must be the parent species of another recent species. Fossil species can be integrated into a cladogram, but they then also form terminal taxa. This means that assigning a fossil species as the actual parent species is avoided. Nodes of a cladogram represent the ancestral species of the two sister groups that emerge from it. Autapomorphies of the respective sister groups are derived features that are common to all taxa of this group, and which do not occur in the sister group.
The cladistics contradicts a so-called “progress prejudice”, which means to determine a development “from the invertebrates to the people ” . A fundamental problem with this point of view is that people have to be put at the top. In fact, the "tendency" to form eddies is just as opposed to a "tendency" to remain invertebrates, as demonstrated by the much greater biodiversity of invertebrates. It is based exclusively on phylogenetic relationship and takes external characteristics as an essential data basis, but summarizes taxa not only on the basis of similarities in shape and body structure.
Phylogenetic systematics is a historical science, since the phylogenesis of organisms cannot be observed, but only reconstructed. Therefore, all kinship hypotheses will always remain hypotheses that cannot be experimentally confirmed. The phylogenetic system tries to set up consistent hypotheses and to resolve relationship hypotheses that are in conflict with one another. The method of phylogenetic systematics provides the scientist with an instrument that allows him to present his arguments in a reproducible manner.
The relationships are shown in so-called cladograms. These differ from evolutionary family trees in the following ways:
- With a branch there are always only two branches ( dichotomous branching).
- The branches are not weighted, so you don't have a measure of the change to represent it in a cladogram. (In evolutionary family trees, such a measure can be represented in different route lengths for branches, see also divergence ).
- There is no absolute timeline.
- All species division events are represented as realistically as possible.
Each branch is justified by a derived characteristic. What this characteristic should be in each case is the subject of research. For example, placenta animals can be differentiated from marsupials by their placenta. B. a reduction of the trained milk teeth. However, the eponymous pouch is not a synapomorphism, but has arisen several times within the marsupials (convergence), and not all marsupials have a pouch or some have reduced it.
This relationship is illustrated here using the example of a simplified mammalian cladogram:
It is important that all branches have at least one autapomorphism .
Features of the basic pattern can be lost again within the group. This is then an autapomorphism of the affected taxon. An example of this is the secondary loss of wings in many flying insects (Pterygota).
"Human, gorilla and chimpanzee" cladogram
Charles Darwin assumed that gorillas and chimpanzees are closely related to the species listed below and that humans have a special position. Stephen Jay Gould sees evidence that humans and chimpanzees are closest and that gorillas split off earlier in evolutionary history.
Mark Abraham cladogram
Basically, a family tree can be represented in different ways by exchanging individual branches (mathematically a permutation ) in the case of a branch (or several ). In the above cladograms one could e.g. B. Swap humans and chimpanzees (or Theria and Monotremata). Despite their different appearance, the cladograms each represented the same facts, called topology . A different topology would exist if the chimpanzees branched off first on the line of descent to humans and only later the gorillas.
Construction of cladograms
In a cladistic analysis of a group of taxa, an attempt is made to reconstruct their relationships and to depict the resulting branching pattern as a cladogram. The starting point of the analysis is usually to collect as many meaningful characteristics as possible. Possibly meaningful are all characteristics that occur in one part of the analyzed taxa , but not in another part. Contrary to the earlier procedure in the systematics, one nowadays usually refrains from weighting the characteristics. If some features are given a higher weight a priori than others, there is a risk that prejudices of the processor will distort the analysis. Only features that represent synapomorphies are useful for the analysis. Commonly inherited core group characteristics (symplesiomorphies) are irrelevant for the analysis, (aut-) apomorphies of an individual taxon emphasize its independence, but are not illuminating for its relationship. Normally, the characteristics taken into account are prepared and presented in the form of a character matrix in which the characteristics of the characteristic are coded by a number (e.g. "wing available" = 1, "wingless" = 0).
For the analysis, of course, the homology of the features must have been clarified as far as possible beforehand (based on the Remane homology criteria ) . Common traits that have arisen convergently or traits that are based on parallel evolution (that is, traits that have arisen independently of one another on a comparable genetic basis in closely related species) can skew the analysis if they go undetected. If the characteristics under consideration are so characterized by convergences and numerous independent origins and regressions that they hardly contribute to the analysis, one speaks of homoplasia . In many cases, it is impossible to determine the degree of homoplasia beforehand. Severe homoplasia of the analyzed features devalues the resulting cladogram, which is why features prone to homoplasia should be avoided as far as possible.
The main problem with cladistic analysis is the regressions of features. If one considers z. For example, the groups of rock jumpers (Archaeognatha), mayflies (Ephemeroptera) and fleas (Siphonaptera) based on the characteristic “wing development”, then wings are only present in the mayflies. The lack of wings in the case of the rock jumpers is due to the fact that they descend from primarily wingless ancestors. The fleas, on the other hand, had winged ancestors, only their wings have receded without any residue. If in this case the connections can still be clearly explained due to numerous other characteristics, this is by no means possible with the same degree of certainty for numerous other groups. It even happens that individual stem lines recede a trait and it is then acquired a second time in individual lines of the offspring, independently of one another. The question of whether a certain characteristic was present in the parent group and thus has been regressed in the lines that do not now have it, or whether it was missing in the ancestors and was newly acquired in one or more lines within the considered group, becomes the Called "polarity" of the feature. To determine the polarity, so-called outer groups are included in the analysis. An outgroup can be any taxon that is related to the species analyzed, but that is definitely and clearly outside of the kinship group under consideration. For obvious reasons, it should be a less specialized taxon with few autapomorphies. The choice of outgroup (s) can significantly influence the analysis.
In Hennig's classic approach, a cladogram was constructed by constructing a hypothetical stem form by clarifying the polarities and then arranging the taxa under consideration by adding them individually or in groups until a convincing family tree was found. In modern cladistic analysis, this step is taken over by a sorting algorithm . For this purpose, the taxa described by the feature matrix are permuted until a branching pattern of minimal length is found. The outgroup taxa can be included in this analysis. This shortest branching pattern is then considered to be the most likely hypothesis of the relationships. This is called parsimony (English "avarice", "thrift"). The PAUP (Phylogenetic Analysis Using Parsimony) sorting program is most commonly used, but there are a number of other popular programs. If there are two or more different family trees of the same length, these are equivalent hypotheses and the relationship cannot be determined. The first analysis result, however, is simply a connection graph. In order to turn this into a cladogram, the polarity must be clarified (i.e. it must be clear which of the branches occurred first). This can either be derived from the data analysis or (with good knowledge of the outgroups) forced. This step is (engl. As "rooting" rooting ) denotes the Kladogramms.
The biological systematics sees itself as a science, classified living things according to their origin. Hence, cladistics is one of their working methods.
A large number of cladograms are then created. The cladogram with the lowest number of necessary changes within the assumed evolutionary course is considered the most likely. When specifying a cladogram, it is often of interest to consider other cladograms that are constructed with a very similar number of changes.
Cladistic status of taxa
The traditionally used classification units ( taxa ) of the biological classification above the rank of a species cannot always be easily adopted into a cladistic-based, modern system because of the underlying concepts. In addition, by using molecular instead of morphological data in biology or through new fossil finds and thus an expansion of the existing database in paleontology, new relationships analyzes of groups already examined can lead to new results, with consequences for previously defined taxa. The following cladistic statuses are distinguished for taxa above the rank of a species:
The taxon has a most recent common ancestor (MRCA) and also includes all subgroups that are derived from this stem form, as well as the stem form itself, but no other groups. The monophylum is based on apomorphies of the common stem form and is also referred to as closed .
- Example: The Metazoa include all animal multicellular cells. These have the apomorphic characteristic multicellularity in common.
An alternative name for a monophylum is clade . In the modern system, every taxon is in principle always a monophylum. Traditional taxa that turn out to be monophyletic are also generally recognized in modern systematics.
The taxon has a recent common stem form, but does not contain all subgroups that go back to this stem form, as is the case with the Monophylum. A paraphylum is based on the symplesiomorphies of the taxa it contains and is also referred to as open .
- Example: The reptiles are paraphyletic, since the birds are classically not counted among them, although their last common ancestral species was a dinosaur and thus they have the same stem as all other animal species of the group of reptiles. The taxon of the Sauropsida , which combines the class of reptiles and the class of birds, is, however, monophyletic.
Traditional taxa that prove to be paraphyletic are usually either only used informally or are redefined as monophyla, whereupon they then also contain those groups that they must contain as monophylum, but traditionally did not. In English, a paraphylum is also referred to as "grade", as a counterpart to the "clade", the monophylum.
The taxon does not have a common stem that is younger than the common stem that its sub-taxa have with other taxa.
- Example: The worms ("Vermes") comprise groups that are far apart from each other. Further examples are the warm-blooded animals ("endothermia"), water birds , freshwater fish and trees .
However, such taxa were often recognized as "unnatural groupings" many decades ago, so that they are generally not used as systematic terms today.
- Peter Ax : The phylogenetic system. Systematization of living nature based on its phylogeny. Fischer, Stuttgart a. a. 1984, ISBN 3-437-30450-X .
- Peter Ax: Systematics in Biology. Representation of the phylogenetic order in living nature (= UTB 1502). Fischer, Stuttgart 1988, ISBN 3-437-20419-X .
- T. Ryan Gregory: Understanding Evolutionary Trees. In: Evolution: Education and Outreach. Vol. 1, No. 2, 2008, pp. 121–137, doi: 10.1007 / s12052-008-0035-x (full text freely accessible).
- Willi Hennig : Basics of a theory of phylogenetic systematics. German Central Publishing House, Berlin 1950.
- Willi Hennig: Phylogenetic systematics (= Pareys study texts. No. 34). Paul Parey, Berlin a. a. 1982, ISBN 3-489-60934-4 .
- Willi Hennig: Tasks and problems of genealogical research (= Parey's study texts. No. 35). Paul Parey, Berlin a. a. 1984, ISBN 3-489-61534-4 .
- Olivier Rieppel : Introduction to computer-aided cladistics. Dr. Friedrich Pfeil, Munich 1998, ISBN 3-931516-57-1 .
- Walter Sudhaus , Klaus Rehfeld : Introduction to Phylogenetics and Systematics. Gustav Fischer, Stuttgart a. a. 1992, ISBN 3-437-20475-0 .
- Johann Wolfgang Wägele : Basics of Phylogenetic Systematics. Dr. Friedrich Pfeil, Munich 2000, ISBN 3-931516-73-3 .
- Bernhard Wiesemüller, Hartmut Rothe , Winfried Henke : Phylogenetic systematics. An introduction. Springer, Berlin a. a. 2003, ISBN 3-540-43643-X .
- Rainer Willmann : The kind in space and time. The species concept in biology and paleontology. Paul Parey, Berlin a. a. 2005, ISBN 3-489-62134-4 .
- OneZoom: Tree of Life - Fractal, zoomable family tree of all recent living beings
- Ernst Mayr: Principles of Systematic Zoology. McGraw-Hill, 1969.
- Ernst Mayr (1974): Cladistic analysis or cladistic classification? In: Journal for Zoological Systematics and Evolutionary Research 12: 94–128.
- Willi Hennig (1974): Critical remarks on the question of Cladistic Analysis or Cladistic Classification. In: Journal for Zoological Systematics and Evolutionary Research 12: 279–294, English translation by CD Griffiths: Cladistic Analysis or Cladistic Classification? A Reply to Ernst Mayr. In: Systematic Zoology , 24 (1975): 244-256.
- Willi Hennig Society. Retrieved March 29, 2016.
- phylogenetic systematics - Lexicon of Biology. In: Spektrum.de. Retrieved March 29, 2016 .
- EO Wiley & Bruce S. Lieberman: Phylogenetics. Theory and Practice of Phylogenetic Systematics. A John Wiley & Sons, 2nd edition, 2011. ISBN 978-0-470-90596-8 .