How We Look at the Relationships of Taxa

G.F. Guala

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Systematists: those of us who study the relationships of taxa use a specific methodology for determining those relationships.
As in any science, this methodology has evolved over the years and a few years ago we were in the midst of what Thomas Kuhn would have called "a paradigm shift".

The new paradigm is the cladistic method.  I will explain why after you learn how the process works.

Previous terms: CLADE, CLADOGRAM, PHYLOGENY



 

How do we do it?

  • The entire process consists of recognizing nested clades.
  • You have to start with a clade.
  • The taxonomic units (often called operational taxonomic units or OTUs) that you work with have to be clades (e.g. species).
  • You then proceed to discover the clades (made up of the smallest clades or taxonomic units) within the big clade that you started with.
  • Because clades are defined by characteristics, you first need to examine a bunch of characteristics of your OTUs.
  • Each of the characteristics that you examine will be divided into character states

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    Every clade must be defined by a unique characteristic.  This is called a SYNAPOMORPHY. (a liberal translation would be: syn = shared, apo = derived, morphy = characteristic).  This is exactly what a synapomorphy is - a characteristic that is shared among the members of the clade and which is evolutionarily derived.  For example you have wide leaves and narrow leaves in a group, "wide" and "narrow" are character states.

    Which one is derived?
    To figure this out, you need an OUTGROUP.
    An outgroup is the clade most closely related to but not a part of the clade that you are analyzing.
    The idea is that you have a group that you are analyzing and its closest relative.

    Because the group that your clade shares a common ancestor with the outgroup, the character state in that ancestor must be the primitive state.  Thus, if there are two states in your clade and one of those is in the outgroup, that must be the primitive or basal state within your clade.


     


  • You must list all of the character states for each OTU.
  • Then it is simply an optimizing process to find the simplest cladogram that explains the data.
  • We have complex computer programs that do this but for a 3 taxon statement, you can easily do it in your head.


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