I. The goal of systematics:
The diversity of living things presents us with a seemingly infinite variety. The science of systematics is dedicted to identifying and describing the order in the diversity of living things. Typically, systematists employ a taxonomic system in which organisms are classified into groups or taxa (singular: taxon). Many different taxonomic systems are conceivable, but all have the following features:
For example, in our lives, we have all employed the taxonomic system in which animals are classified according to the organizational principle of their utility to humans:
II. Cladograms: Throughout evolutionary history, lineages of interbreeding organisms have evolved through time and occasionally split into separate, reproductively isolated lineages. The result is an evolutionary "tree" with many branches. We represent this tree, or portions of it that we want to talk about, using stick-figure trees called cladograms.
In this cladogram, the organisms A, B, and C at the ends of the branches are known as terminal taxa. The lines themselves represent evolving lineages. Branch points represent lineage splitting events. The point at the fork of each split is called a node, and represents the latest common ancestor of the descendants depicted above it. Time runs from oldest events at the bottom to youngest ones at the top. Thus, in this example, the last common ancestor of A, B, and C occurred earlier in time than the last common ancestor of B anc C.
Note that in a cladogram, it does not matter whether things apear on the left or right. What counts is the sequence of branching events (i.e. which ones appear on top or on the bottom). In the figure above, cladograms 1 and 2 depict exactly the same relationships, whereas cladogram 3 is different.
III. The phylogenetic taxonomic system: Taxonomic groups can be named and defined based on their descent from a common ancestor. The cladogram below shows the real relationships between several major vertebrate groups.
Working from this cladogram, systematists have named the following taxonomic groups:
In this drawing, we have drawn circles around the groups that could be defined by the relationships shown on this cladogram, and indicated their names. Ordinarily, one would simply write the group names next to the node of the last common ancestor:
Thus, the pattern of evolution provides:
V. Monophyletic groups: In phylogenetic systematics, taxonomic groups are defined strictly in terms of the non-arbitrary criterion of descent from a common ancestor. Such taxa are called monophyletic groups.
It is also possible to refer two types of non-monophyletic groups:
Note carefully: Only monophyletic groups are based exclusively on natural, non-arbitrary criteria. When we define a paraphyletic group, we must arbitrarily decide which descendants to exclude. In the case of polyphyletic groups, we must decide which ancestors to leave out.
VI. Closeness of relationships: In phylogenetic systematics, how closely two taxa are related depends entirely on how recent their most recent common ancestor was.
Who is more closely related to taxon B, A or C? To answer this, we must identify the most recent common ancestor of B and A, and of B and C. Having done this, we see that the last common ancestor of B and C lived more recently than the last common ancestor of B and A. Thus, we say that B is more closely related to C than to A.