What is a cladogram? Definition and examples

A cladogram is a branched diagram or dendrogram that reflects the evolutionary relationships between sister groups and that is built according to criteria from a cladistic analysis.

In turn, cladistics studies whether the characteristics of a species are exclusive or derived from another; such features are called synapomorphies . By determining whether a characteristic is exclusive or derived, individuals can be grouped together and associated with a common ancestor, since synapomorphies are homologous characters, that is, inherited from the ancestor of all individuals in that group. With this, cladistics proposes the taxonomic classification after studying the distribution of synapomorphies in a phylogenetic tree or cladogram.

Components of a cladogram

From the branches of a cladogram a pair of new branches can originate; all three meet at a point called a node , which indicates the separation of two evolutionary lineages. The represented taxa are located at the end of each branch. Taxa is the plural of taxon , a set of related organisms and grouped in a certain classification. For example, the scientific name for cows is Bos taurus . In this case, the Bos taxon is associated with a category called genus and the taurus taxon with a category called species .

Cladograms may have stem , which indicates the common ancestor of the individuals represented. Each node, together with the species derived from that branch represents a monophyletic group or clade .

Cladograms are useful to identify the synapomorphies that define each of the nodes. Also to determine homoplasies , that is, similarities that do not imply a common ancestor; With this, conclusions can be made about the way such similar traits evolved.

Cladogram classes

The cladograms present monophyletic groups. However, depending on the classification criteria used, the resulting groups may not correspond to sets descended from a common ancestor, giving rise to paraphyletic and polyphyletic groups.

Other types of dendrograms

In addition to cladograms, there are other types of dendrograms useful for establishing evolutionary relationships, such as phenograms and phylograms.

• The phenogram is distributed into nodes indicating the characteristics shared by the taxa descended from that node.

• In the phylogram the sister groups are located at different distances from the common ancestor; also because the degree of divergence is expressed in the angle formed by the branches.

Steps to build a cladogram

When constructing a cladogram it is important to consider the following steps.

1. Delimit the interest or internal group, made up of the groups whose relationships will be analyzed. This group is called a taxonomic sample.
2. Select the external group to the set of the sample of interest, which allows establishing the ancestral state of a given characteristic and its derived states, present in the descendants.
3. Choose the criteria to determine, within the variability of characters, those that will be analyzed in the taxonomic sample.
4. Encode the chosen characters. If they are isolated, the characters can be coded according to their absence or presence; if they are quantitative, they can be indicated in ranges or classes.
5. With the coded characters, build matrices that, from various algorithms hosted in computer programs, will yield the probable cladograms. The resulting cladograms depend on the number of taxa considered: for three taxa, three trees. 
6. Evaluate the trees obtained through a choice criterion, such as parsimony , which chooses the tree that has the least number of changes, that is, that adds the fewest number of steps along all its branches.

Sources

Curtis, H., Barnes, N.S., Schnek, A., Massarini, A. Biology . 7th edition. Editorial Médica Panamericana., Buenos Aires, 2013.

Gelambi, M. Cladogram: what is it, what is it for, examples . lifer.