Jan Scholten

0.3.4 Phylogeny, Classes, Cladistics

There are several kinds of groups in taxonomy. The naming can be ambiguous. That is why in taxonomy one has adopted the name “clade” for any group that belongs together. A clade then can be a Kingdom, Phylum, Class, Order, Suborder, Family, Subfamily, Tribe or Genus. The science of grouping or taxonomy has thus been named cladistics. One reason for using the word clade is that there are no good objective reasons why a clade should be a Family, Superfamily, Subfamily or Order. It is difficult to ascertain the level of a clade.

Phylogenetics is the study of evolutionary relatedness among groups of organisms. This is done these days through molecular sequencing data and morphological data matrices. The term phylogenetics derives from the Greek terms phyle, phylon, denoting “tribe” and “race”.

A very important trait of groups is that they should be monophyletic. A group, clade or taxon is called monophyletic if it contains all the descendants of their common ancestor and no other organisms. A basic principle of phylogenetics is that clades should be monophyletic. Monophyletic groups are often characterised by shared characteristics, called synapomorphies. This is sometimes not the case anymore since the DNA analyses were introduced. For instance, the order Malpighiales has hardly any synapomorphies.Paraphyly
A clade is said to be paraphyletic if the group consists of most but not all the descendants of a common ancestor. The clade is not complete, so to say. For instance the clade Gymnospermae is paraphyletic as the Angiospermae have the same common ancestor but don’t belong to the clade Gymnospermae.

A group is called polyphyletic if the common ancestor is not a member of the group. An example is the group of carnivorous plants. They belong to several clades, families. One can say that polyphyly is an extreme example of paraphyly.

Some clades that are paraphyletic still form a well-defined clade, like Gymnospermae.Gymnospermae are paraphyletic because the Angiospermae have developed out of them. Side branches in the tree of evolution are monophyletic, but the main trunk becomes paraphyletic due to new evolutions.
I introduce the term prephyletic for such clades like Gymnospermae. A clade can be called prephyletic if it has been monophyletic but due to new evolution
has become paraphyletic and
if it can be defined by clear
typical characteristics.