GEOL 104 Dinosaurs: A Natural History

Fall Semester 2006
Systematics III: Cladistics

Cladograms represent a way of graphically portraying the evolutionary relationships of organisms. How are these relationships discovered?

Anatomical (and behavioral, and genetic) features that are similar between taxa are said to be shared characters. Three different reasons these features are shared:

Cladograms are constructed by sorting out the simplest (most parsimonious) arrangement of shared derived characters. For example, below is a cladogram of living terrestrial vertebrates, and a list of characters above. In that list, a red "X" represents the presence of that feature in the taxon, and a black "0" represents the absence of that feature.

So, in the diagram above:

Or, to put it another way:

  • "Limbs with digits" is a synapomorphy for Tetrapoda, but a symplesiomorphy for all its subclades (e.g., Amphibia, Amniota, Mammalia, Reptilia, etc.)
  • "Eggs with amnion" is a synapomorphy for Amniota, but a symplesiomorphy for all of its subclades (e.g., Mammalia, Reptilia, Testudines, etc.)
  • "Toothless beaks" is a homoplasy (i.e., a convergence) between Testudines and Aves, as the forms intermediate between them (lepidosaurs, crocodilians) retain the primitive condition of having teeth. Another particular kind of character is an autapomorphy, a feature found in only one of the terminal taxa in an analysis.

    Another, simpler way of drawing the above cladogram is to indicate the characters on the braches where they would have originated:

    In the cladogram above, the characters evolved on the branch where the red line indicate:

  • "Limbs with digits" evolved in the common ancestor of all tetrapods, and is a synapomorphy for Tetrapoda;
  • "Eggs with amnion" evolved in the common ancestor of all amniotes, and is a synapomorphy for Amniota;
  • Etc. Note that even the synapomorphies shown here can be modified through additional evolution at any point further up in the cladogram:

    Fun with Cladograms!
    In a cladogram, it is the branching relationships which are important, not the "right to left"/"top to bottom" order. As long as two cladograms contain the same branching relationships, and do not have any contradictory branching relationships, they are equivalent:

    In the above cases,cladograms 1-3 are all equivalent: they represent the same information. However, cladogram 4 is not equivalent to the other three:

    The below pictures are all equivalent to cladogram 1 above, but just drawn in different ways:

    In a cladogram, terminal taxa can be expanded to show the relationships within them, and nodes can be collapsed if we aren't interested in the details within that group.

    This cladogram was modified from the previous one by:

    The cladogram is equivalent to the previous one insofar as it doesn't violate any of the branching relationships, but it:

    Another name for a monophyletic group is a clade; modern systematics is thus called cladistics.

    Diagnosis of a taxon: the list of features that are synapomorphies at that node (or autapomorphies, if a terminal taxon).
    Definition of a taxon (in cladistics): a statement of common ancestry which describes a particular clade.

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    Last modified: 14 July 2006