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.
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.
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.
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
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.
An Example of the Phylogenetic Tree of life.
Key differences between Phylogeny and Cladistics
