What is Cladistics? 

Cladistics is a technique in the classification of organisms, where they are grouped into different clades. The proof for the relationships that are hypothesized is based on the shared derived characteristics known as synapomorphies. These characteristics do not exist in many of the distant ancestors and groups. The common ancestor along with its descendants is considered to be a part of the clade. The rate of closeness is directly proportional to the cladistic grouping and is useful in analyzing the evolutionary mechanism.

Methodology of Cladistics

Cladistic analysis is carried out by constructing a branching diagram called a phylogenetic tree to depict the hypotheses about evolutionary relationships. Phylogeny is used to relate organisms from different families or species. A cladogram (tree-shaped diagram – dendrogram) is used to show the phylogenetic relationships among organisms. The classification of the organism is based on any specific characteristics. A shared derived character is seen as proof of grouping and can be depicted in a synapomorphy cladogram. In contrast, plesiomorphy is not used to depict a cladogram because it is an ancestral behavior or trait. The synapomorphies of similar characteristics are considered as evidence for the hypothesized relationships between the organisms. These synapomorphies are not detected in distant groups and are only found in close groups of organisms. The possession of the homologous digits with Homo sapiens is an example of synapomorphy within the category of vertebrates.

A character refers to any features that a particular taxon possesses. For example, the prevalence of hair over the body is an important characteristic to distinguish the clade of mammals. It is a distributed derived character that uniquely recognizes mammals. The change in the character plays a key role in evolution. Among vertebrates, the backbone is a partitioned primitive characteristic because it developed in the ancestor common to all vertebrates. 

Methods to construct a cladogram

A cladogram can be based on distinguishing physical data or molecular data.  To create more precise cladograms, researchers often use computer programs. Cladogram can be constructed by considering following steps:

• A taxon or taxa (plural) is a taxonomic unit. It is defined as a unit of any rank that helps in designating an organism or a group of organisms. In this, the organisms are arranged in a hierarchy from kingdom to subspecies.
• Choose the taxa that must be clades. A clade is considered a grouping that normally includes a common ancestor and all their descendants that could be extinct and living.
• Examine the taxa to determine the character. To do this, identify an out-group (species that is not closely related to the clade) or species and group of species that are closely related to the species present in the clade.
• Then differentiate the shared derived characters and not shared ancestral ones.
• Apply parsimony which is used to choose among possible cladograms implying that the least number of changes are the best, and finally, build the cladogram. 

Terminology Based on Traits 

Homoplasy is a trait shared by a collection of species but is not present in the common ancestor. An autapomorphy is a derived trait that is a characteristic feature and is incomparable and distinctive to a given taxon. A plesiomorphy is known as the ancestral trait and is common with other taxa having an earlier common ancestor. Apomorphy was known as a derived trait that is different from the form present in the ancestor. An example of apomorphy is the feathers, which are unique to birds but define all mammals of the class Aves. The terms plesiomorphy and apomorphy are related to each other. 

A synapomorphy is a shared derived characteristic that discriminates a clade from other organisms. Apomorphy can be used to determine a clade. The traits formed from the individual taxa are called autapomorphies, which are found to provide the traits detected in the individual taxa. The synapomorphies are employed for the detection of the clades in an inheritance study.  

A trait is a homoplasy if it is observed to be shared by two or more organisms and not detected in the common ancestor. The convergence development during the course of evolution results in homoplasy. These evolutionary changes lead to the generation of new species. For example, the warm-blooded condition is considered as a separate synapomorphy detected in birds and mammals. The cladograms consisting of diverse quantities of traits are found to be more robust than the ones formulated with the aid of more limited evidence.

The cladogram tree's shape is found to code for the type of the taxa, which is found to differ between mono, para, and polyphyletic.  The shape of the cladogram is useful in the field of taxonomy. The evolutionary changes of a trait or characteristic between organisms are studied by phylogenetic analysis.  The evolutionary changes occur mainly due to the genotypic changes resulting in phenotypic differences. The evolutionary relationship and connection between the organisms are studied under phylogeny. The diversity between the organisms is essential for systematics.

Terminology for Different Taxa 

Three different terminologies are used in the cladogram. A monophyletic group is a valid clade consisting of an ancestral species and all its descendants. Some members have different ancestors, or they may not have a common ancestor and it is called a polyphyletic group. The apomorphias of the monophyletic clade are found to possess one or more apomorphias. The paraphyly is the condition where the assemblage is formed by removing certain smaller clades from a larger clade. In the paraphyletic group, more than one pleiomorphism is detected where the characters are inherited from an ancestor but are not present in all the descendants.  The paraphyletic assemblage is detected to be truncated. The assemblage which is neither monophyletic nor paraphyletic, is grouped as polyphyletic. Polyphyletic is an assemblage consisting of unrelated organisms from different common ancestors. The polyphyletic group is formed based on the homoplasy (the characters or traits observed to be reverted or converged). The systematist does not consider the polyphyletic assemblages as a defined entity.  

Reproval  

The cladistic approach had undergone various criticisms during its employment. They are either employed generally or are used specifically for completing an inheritance study. The techniques were criticized from the time of their discovery.  The decisions regarding the respective trait are found to be a character. Numerous subjective judgments and circular reasons were identified, which restrict the popularity of the technique of cladistics. The year 1970s give rise to the formation of transformed cladistics. This particular technique minimized the problems through the removal of prior assumptions. 

The fossil species are not identified to be the actual ancestors in the analysis of cladistics. They are classified into different extinct branches. There is a chance where the fossil species can be the ancestor of the clade. 

Context and Applications    

This topic is significant in the professional exams for undergraduate and graduate courses specifically related to evolution. Thus, they are also significant in molecular genetics. 

 Related concepts  

• Phylogenetic tree
• Common descent
• Bioinformatics