CLADOGRAM: WHAT IT IS FOR AND EXAMPLES - BIOLOGY - 2025

A cladogram is a branching diagram or scheme of the characteristics shared by a group of organisms, representing the most likely evolutionary history of the lineage. The reconstruction is carried out following the methodology proposed by the biologist Willi Hennig.

Cladograms are characterized by grouping taxa based on their synapomorphies or derived characters that are shared.

What is it for?

Cladograms allow you to visualize the phylogenetic relationships between a group or groups of organisms of interest.

In evolutionary biology, these diagrams make it possible to elaborate phylogenetic trees and, therefore, to reconstruct the evolutionary history of a group, helping to define its classification and taxonomic ranges.

In addition, it helps to elucidate evolutionary mechanisms by examining the way in which organisms change over time, the direction of this change and the frequency in which they do so.

As elaborated?

One of the main goals of evolutionary biologists is to find the position of species on the "tree of life." To achieve this, they analyze different characteristics in organisms, be they morphological, ecological, ethological, physiological or molecular.

The morphological characteristics of the individuals have been widely used to establish their classification; however, there comes a point where they are not enough to discriminate on specific branches of the tree. In this case, molecular tools help to discern these relationships.

Once the trait is chosen, hypotheses of kinship relationships between the species of interest are constructed and represented schematically.

In this diagram the branches represent hypothetical ancestors where an event of cladogenesis or separation of evolutionary lineages occurred. At the end of each branch are each of the taxa that were included in the initial analysis, whether they are species, genera, among others.

Homologies

In order to establish relationships between a group of organisms, homologous characters must be used; that is, two characteristics that share a common ancestor. A character is considered homologous if they acquired their current state by direct inheritance.

For example, the upper limbs of humans, dogs, birds, and whales are homologous to each other. Although they fulfill different functions and at first glance they look very different, the structural pattern of the bones is the same in the groups: they all have a humerus, followed by the radius and the ulna.

In contrast, the wings of bats and birds (this time based on the structure to fly) are not homologous because they did not acquire these structures by direct inheritance. The common ancestor of these flying vertebrates did not present wings and both groups acquired it in a convergent way.

If we want to deduce phylogenetic relationships, these characters are not useful because, although they are similar, they do not adequately indicate the common ancestry of organisms.

Shared primitive and derived characters

Now, a homologous character of all mammals is the backbone. However, this structure does not serve to differentiate mammals from other taxa, because other groups - such as fish and reptiles - have backbones. In cladistic language this type of character is called a primitive shared character or a simpleiomorphy.

If we want to establish the phylogenetic relationships between mammals using the spinal column as a criterion, we cannot reach any reliable conclusion.

In the case of hair, it is a characteristic shared by all mammals that does not exist in other groups of vertebrates. For this reason, it is a shared derived character - synapomorphy - and it is considered an evolutionary novelty of a specific clade.

To elaborate a cladogram, the phylogenetic systematics proposes the formation of taxonomic groups using shared derived characters.

Classification schools: cladism

To establish the classification and the phylogenetic relationships between organisms, it is necessary to resort to objective norms that use a rigorous method to elucidate these patterns.

In order to avoid subjective criteria, schools of classification emerge: traditional evolutionary taxonomy and cladism.

Cladism (from the Greek clados, meaning "branch") or phylogenetic systematics was developed in 1950 by the German entomologist Willi Hennig, and is widely accepted for its methodological rigor.

Cladists construct cladograms that represent the genealogical relationships between species and other terminal taxa. Similarly, they look for ordered sets of shared derived characters or synapomorphies.

This school does not use shared ancestral characters or simpleiomorphies and only grants validity to monophyletic groups; that is, groupings that include the most recent common ancestor and all descendants.

Paraphyletic groups (groupings of organisms that include the most recent common ancestor, excluding some of their descendants) or polyphetics (groupings of organisms from different ancestors) are not valid for cladists.

Principle of parsimony

It is possible that when making a cladogram, several graphic representations are obtained that show different evolutionary histories of the same group of organisms. In this case, the most “parsimonious” cladogram is chosen, which contains the least number of transformations.

In light of parsimony, the best solution to a problem is the one that requires the fewest assumptions. In the field of biology this is interpreted as fewer evolutionary changes.

Differences between cladograms and phylogenetic trees

Generally, taxonomists tend to establish technical differences between a cladogram and a phylogenetic tree. It is necessary to clarify that a cladogram is not strictly equivalent to a phylogenetic tree.

The branches of a cladogram are a formal way of indicating a nested hierarchy of clades, whereas in a phylogenetic tree the branches are representations of lineages that have occurred in the past. In other words, the cladogram does not imply an evolutionary history.

To obtain a phylogenetic tree it is necessary to add extra information: additional interpretations related to the ancestors, the duration of the lineages in time and the amount of evolutionary changes that have occurred between the studied lineages.

Therefore, cladograms are the first approximations for the final creation of a phylogenetic tree, indicating the possible branching pattern.

Examples

Amniotes

The cladogram of amniotes represents three groups of tetrapod vertebrates: reptiles, birds, and mammals. All of these are characterized by the presence of four layers (chorion, allantois, amnion and yolk sac) in the embryo.

Note that the concept of "reptile" is paraphyletic, since it excludes birds; for this reason it is rejected by the cladists.

Apes

The cladogram of the apes includes the genera: Hylobates, Pongo, Gorilla, Pan, and Homo. Popularly, the concept of ape is paraphyletic, because it excludes the genus Homo (we humans).

References

1. Campbell, NA, & ​​Reece, JB (2007). Biology. Panamerican Medical Ed.
2. Curtis, H., & Schnek, A. (2006). Invitation to Biology. Panamerican Medical Ed.
3. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology. New York: McGraw – Hill.
4. Kardong, KV (2002). Vertebrates: comparative anatomy, function, evolution. McGraw-Hill.
5. Soler, M. (2002). Evolution: the basis of Biology. South Project.