Reference Information1:
How to create a cladogram or phylogenetic tree:
A cladogram groups organisms based on the number of similarities and differences among them. In order to construct a
cladogram (figure 12 - 13), it is important to identify the characteristics those the ancestral population and those of the
descendants. These ancestral traits link the members of related branches to a common ancestor. On the other hand,
characteristics that are found in various evolutionary branches that differ from those of the ancestors are considered derived. In
many cases, a derived characteristic is a unique modification of a shared ancestral characteristic. Derived characteristics or
traits distinguish the members of one evolutionary branch from the members of another branch.
A cladogram is constructed based on the presence of derived traits in two or more related taxa. Ideally, a cladogram
should be based on branches that are defined by a unique derived trait that emerged only once, and are shared by all
subsequent descendents. In this activity, you will need to create a cladogram for the nine different species of mammals.
You can construct a cladogram by following these steps:
1. Identify the taxa. These taxa will be the tips of your tree and must themselves be clades. A clade is a group of
organisms that includes an ancestor and all descendants of that ancestor (see figure 12). In this case, your taxa are the
nine species of mammals.
2. Determine the characters and examine each taxon to determine the character states. In this investigation, you have
access to the first 40 amino acids in the hemoglobin alpha protein as your characters (figure 13). Your character
states, therefore, will be the different types of amino acids at each point in the sequence.
3. Determine the order of appearance of characters. In other words, figure out the order of evolution for each
character. In this investigation, you will need to determine which mutation appeared first, second, and so on in the
alpha globin sequence.
4. Group taxa together by derived or “changed” character states. Do not group the taxa together by the original
character state.
5. Build your tree. To build it correctly, you must follow three rules:
a. All taxa must go on the endpoints of the tree, never at the nodes.
b. All notes must have a shared derived character, which is common to all taxa above the node.
c. All shared derived characters can appear only once on a tree.
Two fundamental principles are used to evaluate a cladogram: parsimony and robusticity. Parsimony is a principle that states
that when there is more than one way to draw a cladogram and when there are no other data that suggest one of these is more
likely than the others, we should choose the one in which derived traits evolve the fewest number of times in the branches of
the cladogram. Robusticity is a principle that states that trees that maintain their basic form, even when different options are
applied to the sequence of
changes in one or more of their
branches are preferable.
Obviously, parsimony and
robusticity are only guiding
principles. As more data is
made available, these principles
are secondary to the picture
afforded by that data in
determining which tree is the
best out of a series of possible
alternatives.
1
Adapted by D. Knuffke from Scientific Argumentation In Biology: 30 Classroom Activities by V. Sampson and S. Schleigh Copyright 2013 by the National Science
Teachers Association. www.nsta.org/store