Introduction to Cladistics.

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:

  • A heirarchy of internested groups

  • An organizational principle such as "evolutionary history," or "utility to humans."

    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:

    Animals
    ____________ |_____________
    Pets...............Vermin...............Livestock

    Generally, there is little ambiguity. Cows are livestock, cats are pets, cockroaches are vermin, etc. Nevertheless, the criteria that we use to classify animals according to this system are arbitrary and subjective. A reptile enthusiast might classify a boa constrictor as a pet, where a person who was terrified of snakes would call it vermin, and an entrepeneur who raises boas for the pet trade would view it as livestock. Ideally, we would like to have some non-arbitrary, natural organizing principle for a taxonomic system that natural scientists can use. Such a principle is provided by the pattern of evolution. In order to understand it, you must first understand the conventions for graphically displaying the pattern of evolution.

    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.

    cladogram 1
    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.

    cladogram 1
    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.

    cladogram 1
    Working from this cladogram, systematists have named the following taxonomic groups:

    cladogram 1
    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:

    cladogram 1
    Thus, the pattern of evolution provides:

  • A heirarchy of internested groups, with those descended from more recent common ancestors being nested within those descended from more distant ones. For instance, "Tetrapoda", the common ancestor of land vertebrates and its descendants, is nested within "Choanata", the common ancestor of vertebrates with choanae and all of its descendants.

  • An organizing principle, the branching pattern of evolution itself.

    IV. Definitions:

  • Phylogeny: The branching evolutionary pattern of ancestry and descent.

  • Phylogenetic systematics: The science of reconstructing phylogeny and developing a taxonomic system based upon it.

    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.

  • Memorize this definition: A monophyletic group is an ancestor and all of its descendants.

    It is also possible to refer two types of non-monophyletic groups:

  • Paraphyletic groups: An ancestor and some but not all of its descendants.

  • Polyphyletic groups: A group of organisms which fails to include at least some of their common ancestors.

    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.

    cladogram 1
    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.