Class Notes‎ > ‎Evolution‎ > ‎

Cladistics and Phylogenetics


What are cladistics?

Cladistics are a way of organizing evolutionary relationships between organisms based on their derived characters.  This relies on the basic assumption that all members of a group share a common ancestry, although some members of a group are more closely related to some, than they are to others organisms.

 Image result for cladistics
An example of a cladogram.

How are cladistics determined?

This is determined by the  said derived characteristics. There are two types, autopomorphies and synomorphies. Autopomorphies are the characters specific to one group, versus the synomorphic which are the derived characters that all the related groups have in common. This can be noted in the cladogram, which is the diagram created based on the information used to create it.

Cladistic History

Cladistics was created by the German taxonomist Willi Hennig who developed cladistics in 1950, but his work was not widely known until it was translated into English in 1966. After scientists began using molecular data in classification, Hennig's cladistics became increasingly adopted. 

Image result for german taxonomist willi hennig
A portrait of Willi Hennig.

Monophyletic, Polyphyletic, and Paraphyletic Groups

When looking at Cladistics, it has taxa, which are groups of organisms, arranged into clades, a group of related organisms being compared. Taxa or any branch that are each other's closest relatives are called sister taxa. All clades are made monophyletic, meaning that all members share a common ancestor and all the descendants of that ancestor are included. The other two types of groups, which are not considered complete cladograms are the polyphyletic group in which the members of the group come from more than one ancestor. This can be near the bottom of a cladogram where the higher ancestor that relates the species is not shown. Lastly the other group is the paraphyletic group in which not all of the descendants of a common ancestor are shown.

Image result for different types of cladistic trees
A cladogram that is divided into the three examples of cladistic groups.

Synapomorphies and Homology

Cladistics are based on shared derived traits that are the reason for two species to be placed next to each other. These traits do not have to be physical, as more commonly it is based on DNA and nucleotide sequence. This type of arrangement is known as synapomorphies. Synapomorphies makes the assumption that homology is the complete basis of the cladogram. Homology is the similarity of features of organisms due to shared ancestry. However sometimes the process of convergence, a phenomenon where two distant lineages gain similar features due to similar roles in their lives. The last reason that non homologous character states can have similar is a reversal in which the new species has a mutation causing the derived state to go back to its ancestral form.

Image result for homology

An example of homology in bone structure between a human arm, fish limb and bird wing.

In cladistics it is most common to look for the maximum parsimony which is the most simple explanation for the tree.


Phylogenetics is the study of evolutionary relationships between groups of organisms. This often consists of analyzing hemoglobin or protein sequence to create a model called a phylogeny. Phylogeny explains how the sequences have evolved to be and is often show in the form a tree. Due to phylogenetics, we have became able gain an understanding how species evolved. Now we can predict how the sequences may change in the future. 

Image result for phylogenetic tree
An Example of the Phylogenetic Tree of life.

Key differences between Phylogeny and Cladistics

Although the purpose of the phylogenetic tree and the cladistic tree is the same, to both show the evolutionary similarity between two separate species, they both differ in the details of how the physical representation of them are and the requirements for each. In a cladogram the branches do not show the period of time that it took to change from one species to the next but rather uses common or derived traits to show the relationship between two different species evolutionary history. In a phylogenetic tree, the branches can have meaning for their length and can represent time, or another factor. In addition the branches are usually based on genetic data, which shows similarities between two organisms based on the net mutations in genetic code versus another species.