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Cladistics ( // , from Greek κλάδος, kládos, "branch") is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived characteristics (synapomorphies ) that are not present in more distant groups and ancestors. Theoretically, a common ancestor and all its descendants are part of the clade, however, from an empirical perspective, common ancestors are inferences based on a cladistic hypothesis of relationships of taxa whose character states can be observed. Importantly, all descendants stay in their overarching ancestral clade. For example, if within a strict cladistic framework the terms worms or fishes were used, these terms would include humans. Many of these terms are normally used paraphyletically, outside of cladistics, e.g. as a 'grade'. Radiation results in the generation of new subclades by bifurcation, but in practice sexual hybridization may blur very closely related groupings.
The techniques and nomenclature of cladistics have been applied to disciplines other than biology. (See phylogenetic nomenclature.)
Cladistics is now the most commonly used method to classify organisms.
The original methods used in cladistic analysis and the school of taxonomy derived from the work of the German entomologist Willi Hennig, who referred to it as phylogenetic systematics (also the title of his 1966 book); the terms "cladistics" and "clade" were popularized by other researchers. Cladistics in the original sense refers to a particular set of methods used in phylogenetic analysis, although it is now sometimes used to refer to the whole field.
What is now called the cladistic method appeared as early as 1901 with a work by Peter Chalmers Mitchell for birdsand subsequently by Robert John Tillyard (for insects) in 1921, and W. Zimmermann (for plants) in 1943. The term "clade" was introduced in 1958 by Julian Huxley after having been coined by Lucien Cuénot in 1940, "cladogenesis" in 1958, "cladistic" by Arthur Cain and Harrison in 1960, "cladist" (for an adherent of Hennig's school) by Ernst Mayr in 1965, and "cladistics" in 1966. Hennig referred to his own approach as "phylogenetic systematics". From the time of his original formulation until the end of the 1970s, cladistics competed as an analytical and philosophical approach to systematics with phenetics and so-called evolutionary taxonomy. Phenetics was championed at this time by the numerical taxonomists Peter Sneath and Robert Sokal, and evolutionary taxonomy by Ernst Mayr.
Originally conceived, if only in essence, by Willi Hennig in a book published in 1950, cladistics did not flourish until its translation into English in 1966 (Lewin 1997). Today, cladistics is the most popular method for inferring phylogenetic trees from morphological data.
In the 1990s, the development of effective polymerase chain reaction techniques allowed the application of cladistic methods to biochemical and molecular genetic traits of organisms, vastly expanding the amount of data available for phylogenetics. At the same time, cladistics rapidly became popular in evolutionary biology, because computers made it possible to process large quantities of data about organisms and their characteristics.
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The cladistic method interprets each shared character state transformation as a potential piece of evidence for grouping. Synapomorphies (shared, derived character states) are viewed as evidence of grouping, while symplesiomorphies (shared ancestral character states) are not. The outcome of a cladistic analysis is a cladogram – a tree-shaped diagram (dendrogram)that is interpreted to represent the best hypothesis of phylogenetic relationships. Although traditionally such cladograms were generated largely on the basis of morphological characters and originally calculated by hand, genetic sequencing data and computational phylogenetics are now commonly used in phylogenetic analyses, and the parsimony criterion has been abandoned by many phylogeneticists in favor of more "sophisticated" but less parsimonious evolutionary models of character state transformation. Cladists contend that these models are unjustified because there is no evidence that they recover more "true" or "correct" results from actual empirical data sets
Every cladogram is based on a particular dataset analyzed with a particular method. Datasets are tables consisting of molecular, morphological, ethologicaland/or other characters and a list of operational taxonomic units (OTUs), which may be genes, individuals, populations, species, or larger taxa that are presumed to be monophyletic and therefore to form, all together, one large clade; phylogenetic analysis infers the branching pattern within that clade. Different datasets and different methods, not to mention violations of the mentioned assumptions, often result in different cladograms. Only scientific investigation can show which is more likely to be correct.
Until recently, for example, cladograms like the following have generally been accepted as accurate representations of the ancestral relations among turtles, lizards, crocodilians, and birds:
If this phylogenetic hypothesis is correct, then the last common ancestor of turtles and birds, at the branch near the ▼ lived earlier than the last common ancestor of lizards and birds, near the ♦. Most molecular evidence, however, produces cladograms more like this:
If this is accurate, then the last common ancestor of turtles and birds lived later than the last common ancestor of lizards and birds. Since the cladograms show two mutually exclusive hypotheses to describe the evolutionary history, at most one of them is correct.
The cladogram to the right represents the current universally accepted hypothesis that all primates, including strepsirrhines like the lemurs and lorises, had a common ancestor all of whose descendants were primates, and so form a clade; the name Primates is therefore recognized for this clade. Within the primates, all anthropoids (monkeys, apes and humans) are hypothesized to have had a common ancestor all of whose descendants were anthropoids, so they form the clade called Anthropoidea. The "prosimians", on the other hand, form a paraphyletic taxon. The name Prosimii is not used in phylogenetic nomenclature, which names only clades; the "prosimians" are instead divided between the clades Strepsirhini and Haplorhini, where the latter contains Tarsiiformes and Anthropoidea.
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The following terms, coined by Hennig, are used to identify shared or distinct character states among groups:
The terms plesiomorphy and apomorphy are relative; their application depends on the position of a group within a tree. For example, when trying to decide whether the tetrapods form a clade, an important question is whether having four limbs is a synapomorphy of the earliest taxa to be included within Tetrapoda: did all the earliest members of the Tetrapoda inherit four limbs from a common ancestor, whereas all other vertebrates did not, or at least not homologously? By contrast, for a group within the tetrapods, such as birds, having four limbs is a plesiomorphy. Using these two terms allows a greater precision in the discussion of homology, in particular allowing clear expression of the hierarchical relationships among different homologous features.
It can be difficult to decide whether a character state is in fact the same and thus can be classified as a synapomorphy, which may identify a monophyletic group, or whether it only appears to be the same and is thus a homoplasy, which cannot identify such a group. There is a danger of circular reasoning: assumptions about the shape of a phylogenetic tree are used to justify decisions about character states, which are then used as evidence for the shape of the tree.Phylogenetics uses various forms of parsimony to decide such questions; the conclusions reached often depend on the dataset and the methods. Such is the nature of empirical science, and for this reason, most cladists refer to their cladograms as hypotheses of relationship. Cladograms that are supported by a large number and variety of different kinds of characters are viewed as more robust than those based on more limited evidence.
Mono-, para- and polyphyletic taxa can be understood based on the shape of the tree (as done above), as well as based on their character states.These are compared in the table below.
|Term||Node-based definition||Character-based definition|
|Monophyly||A clade, a monophyletic taxon, is a taxon that includes all descendants of an inferred ancestor.||A clade is characterized by one or more apomorphies: derived character states present in the first member of the taxon, inherited by its descendants (unless secondarily lost), and not inherited by any other taxa.|
|Paraphyly||A paraphyletic assemblage is one that is constructed by taking a clade and removing one or more smaller clades. (Removing one clade produces a singly paraphyletic assemblage, removing two produces a doubly paraphylectic assemblage, and so on.)||A paraphyletic assemblage is characterized by one or more plesiomorphies: character states inherited from ancestors but not present in all of their descendants. As a consequence, a paraphyletic assemblage is truncated, in that it excludes one or more clades from an otherwise monophyletic taxon. An alternative name is evolutionary grade , referring to an ancestral character state within the group. While paraphyletic assemblages are popular among paleontologists and evolutionary taxonomists, cladists do not recognize paraphyletic assemblages as having any formal information content – they are merely parts of clades.|
|Polyphyly||A polyphyletic assemblage is one which is neither monophyletic nor paraphyletic.||A polyphyletic assemblage is characterized by one or more homoplasies : character states which have converged or reverted so as to be the same but which have not been inherited from a common ancestor. No systematist recognizes polyphyletic assemblages as taxonomically meaningful entities, although ecologists sometimes consider them meaningful labels for functional participants in ecological communities (e. g., primary producers, detritivores, etc.).|
Cladistics, either generally or in specific applications, has been criticized from its beginnings. Decisions as to whether particular character states are homologous, a precondition of their being synapomorphies, have been challenged as involving circular reasoning and subjective judgements.Of course, the potential unreliability of evidence is a problem for any systematic method, or for that matter, for any empirical scientific endeavor at all.
Transformed cladistics arose in the late 1970sin an attempt to resolve some of these problems by removing a priori assumptions about phylogeny from cladistic analysis, but it has remained unpopular.
The cladistic method does not identify fossil species as actual ancestors of a clade.Instead, fossil taxa are identified as belonging to separate extinct branches. While a fossil species could be the actual ancestor of a clade, there is no way to know that. Therefore, a more conservative hypothesis is that the fossil taxon is related to other fossil and extant taxa, as implied by the pattern of shared apomorphic features.
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 that are 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.
Anthropology and archaeology:Cladistic methods have been used to reconstruct the development of cultures or artifacts using groups of cultural traits or artifact features.
Comparative mythology and folktale use cladistic methods to reconstruct the protoversion of many myths. Mythological phylogenies constructed with mythemes clearly support low horizontal transmissions (borrowings), historical (sometimes Palaeolithic) diffusions and punctuated evolution.They also are a powerful way to test hypotheses about cross-cultural relationships among folktales.
Literature: Cladistic methods have been used in the classification of the surviving manuscripts of the Canterbury Tales ,and the manuscripts of the Sanskrit Charaka Samhita .
Historical linguistics:Cladistic methods have been used to reconstruct the phylogeny of languages using linguistic features. This is similar to the traditional comparative method of historical linguistics, but is more explicit in its use of parsimony and allows much faster analysis of large datasets (computational phylogenetics).
Textual criticism or stemmatics:Cladistic methods have been used to reconstruct the phylogeny of manuscripts of the same work (and reconstruct the lost original) using distinctive copying errors as apomorphies. This differs from traditional historical-comparative linguistics in enabling the editor to evaluate and place in genetic relationship large groups of manuscripts with large numbers of variants that would be impossible to handle manually. It also enables parsimony analysis of contaminated traditions of transmission that would be impossible to evaluate manually in a reasonable period of time.
Astrophysicsinfers the history of relationships between galaxies to create branching diagram hypotheses of galaxy diversification.
A clade, also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants - on a phylogenetic tree. Rather than the English term, the equivalent Latin term cladus is often used in taxonomical literature.
In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor and all of its lineal descendants. Monophyletic groups are typically characterised by shared derived characteristics (synapomorphies), which distinguish organisms in the clade from other organisms. An equivalent term is holophyly.
In biology, phylogenetics is a part of systematics that addresses the inference of the evolutionary history and relationships among or within groups of organisms. These relationships are hypothesized by phylogenetic inference methods that evaluate observed heritable traits, such as DNA sequences or morphology, often under a specified model of evolution of these traits. The result of such an analysis is a phylogeny —a diagrammatic hypothesis of relationships that reflects the evolutionary history of a group of organisms. The tips of a phylogenetic tree can be living taxa or fossils, and represent the 'end', or the present, in an evolutionary lineage. A phylogenetic diagram can be rooted or unrooted. A rooted tree diagram indicates the hypothetical common ancestor, or ancestral lineage, of the tree. An unrooted tree diagram makes no assumption about the ancestral line, and does not show the origin or "root" of the taxa in question or the direction of inferred evolutionary transformations. In addition to their proper use for inferring phylogenetic patterns among taxa, phylogenetic analyses are often employed to represent relationships among gene copies or individual organisms. Such uses have become central to understanding biodiversity, evolution, ecology, and genomes. In February 2021, scientists reported, for the first time, the sequencing of DNA from animal remains, a mammoth in this instance, over a million years old, the oldest DNA sequenced to date.
In biology, phenetics, also known as taximetrics, is an attempt to classify organisms based on overall similarity, usually in morphology or other observable traits, regardless of their phylogeny or evolutionary relation. It is closely related to numerical taxonomy which is concerned with the use of numerical methods for taxonomic classification. Many people contributed to the development of phenetics, but the most influential were Peter Sneath and Robert R. Sokal. Their books are still primary references for this sub-discipline, although now out of print.
In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and all descendants of that ancestor excluding a few—typically only one or two—monophyletic subgroups. The group is said to be paraphyletic with respect to the excluded subgroups. A paraphyletic group cannot be a clade, or monophyletic group, which is any group of species that includes a common ancestor and all of its descendants. One or more members of a paraphyletic group is more closely related to the excluded group(s) than it is to other members of the paraphyletic group. The term is commonly used in phylogenetics and in linguistics. Paraphyletic groups are identified by a combination of synapomorphies and symplesiomorphies.
Biological systematics is the study of the diversification of living forms, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees. Phylogenies have two components: branching order and branch length. Phylogenetic trees of species and higher taxa are used to study the evolution of traits and the distribution of organisms (biogeography). Systematics, in other words, is used to understand the evolutionary history of life on Earth.
A cladogram is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to descendants, nor does it show how much they have changed, so many differing evolutionary trees can be consistent with the same cladogram. A cladogram uses lines that branch off in different directions ending at a clade, a group of organisms with a last common ancestor. There are many shapes of cladograms but they all have lines that branch off from other lines. The lines can be traced back to where they branch off. These branching off points represent a hypothetical ancestor which can be inferred to exhibit the traits shared among the terminal taxa above it. This hypothetical ancestor might then provide clues about the order of evolution of various features, adaptation, and other evolutionary narratives about ancestors. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational phylogenetics are now very commonly used in the generation of cladograms, either on their own or in combination with morphology.
Phylogenesis is the biological process by which a taxon appears. The science that studies these processes is called phylogenetics.
Evolutionary taxonomy, evolutionary systematics or Darwinian classification is a branch of biological classification that seeks to classify organisms using a combination of phylogenetic relationship, progenitor-descendant relationship, and degree of evolutionary change. This type of taxonomy may consider whole taxa rather than single species, so that groups of species can be inferred as giving rise to new groups. The concept found its most well-known form in the modern evolutionary synthesis of the early 1940s.
In cladistics or phylogenetics, an outgroup is a more distantly related group of organisms that serves as a reference group when determining the evolutionary relationships of the ingroup, the set of organisms under study, and is distinct from sociological outgroups. The outgroup is used as a point of comparison for the ingroup and specifically allows for the phylogeny to be rooted. Because the polarity (direction) of character change can be determined only on a rooted phylogeny, the choice of outgroup is essential for understanding the evolution of traits along a phylogeny.
In phylogenetics, apomorphy and synapomorphy refer to derived characters of a clade: characters or traits that are derived from ancestral characters over evolutionary history. An apomorphy is a character that is different from the form found in an ancestor, i.e., an innovation, that sets the clade apart from other clades. A synapomorphy is a shared apomorphy that distinguishes a clade from other organisms. In other words, it is an apomorphy shared by members of a monophyletic group, and thus assumed to be present in their most recent common ancestor.
In phylogenetics, long branch attraction (LBA) is a form of systematic error whereby distantly related lineages are incorrectly inferred to be closely related. LBA arises when the amount of molecular or morphological change accumulated within a lineage is sufficient to cause that lineage to appear similar to another long-branched lineage, solely because they have both undergone a large amount of change, rather than because they are related by descent. Such bias is more common when the overall divergence of some taxa results in long branches within a phylogeny. Long branches are often attracted to the base of a phylogenetic tree, because the lineage included to represent an outgroup is often also long-branched. The frequency of true LBA is unclear and often debated, and some authors view it as untestable and therefore irrelevant to empirical phylogenetic inference. Although often viewed as a failing of parsimony-based methodology, LBA could in principle result from a variety of scenarios and be inferred under multiple analytical paradigms.
In phylogenetics, a sister group or sister taxon comprises the closest relative(s) of another given unit in an evolutionary tree.
In phylogenetics, a primitive character, trait, or feature of a lineage or taxon is one that is inherited from the common ancestor of a clade and has undergone little change since. Conversely, a trait that appears within the clade group is called advanced or derived. A clade is a group of organisms that consists of a common ancestor and all its lineal descendants.
In alpha taxonomy, a grade is a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a strictly phylogenetic unit.
In phylogenetics, a plesiomorphy is an ancestral character state. A symplesiomorphy is a plesiomorphy shared by two or more taxa. Pseudoplesiomorphy is any trait that can neither be identified as a plesiomorphy nor as an apomorphy.
In phylogenetics, an autapomorphy is a distinctive feature, known as a derived trait, that is unique to a given taxon. That is, it is found only in one taxon, but not found in any others or outgroup taxa, not even those most closely related to the focal taxon. It can therefore be considered an apomorphy in relation to a single taxon. The word autapomorphy, first introduced in 1950 by German entomologist Willi Hennig, is derived from the Greek words αὐτός, aut- = "self"; ἀπό, apo = "away from"; and μορφή, morphḗ = "shape".
Phylogenetic nomenclature is a method of nomenclature for taxa in biology that uses phylogenetic definitions for taxon names as explained below. This contrasts with the traditional approach, in which taxon names are defined by a type, which can be a specimen or a taxon of lower rank, and a description in words. Phylogenetic nomenclature is currently regulated by the International Code of Phylogenetic Nomenclature (PhyloCode).
Suchia is a clade of archosaurs containing the majority of pseudosuchians. It was defined as the least inclusive clade containing Aetosaurus ferratus, Rauisuchus tiradentes, Prestosuchus chiniquensis, and Crocodylus niloticus by Nesbitt (2011). Generally the only pseudosuchian group which is omitted from Suchia is the family Ornithosuchidae, although at least one analysis classifies ornithosuchids as close relatives of erpetosuchids and aetosaurs. Phytosaurs are also excluded from Suchia, although it is unclear whether they qualify as pseudosuchians in the first place.
Transformed cladistics, also known as pattern cladistics is an epistemological approach to the cladistic method of phylogenetic inference and classification that makes no a priori assumptions about common ancestry. It was advocated by Norman Platnick, Colin Patterson, Ronald Brady and others in the 1980s, but has few modern proponents. The book, Foundations of Systematics and Biogeography by David Williams and Malte Ebach provides a thoughtful history of the origins of this point of view.