Expert: Dana Krempels, Ph.D. - 12/18/2009

Question
Hi,

I was wondering why it is that cladograms can show ancestral traits of organisms unless the organism developed from endosymbiosis? for instance, I know that mitochondria and chloroplasts developed in eukaryotic cells through endosymbiosis. Why can't a cladogram illustrate this?

Also, I am studying for my AP biology test I will take in the spring. I am not taking a biology class right now, so this is NOT a homework question.

Thank you!

Answer
Dear Minna,

Cladograms are diagrams showing how hypothetical ancestral taxa (represented by the nodes) branch dichotomously into descendant taxa (represented by the branches).  Derived characters distinguish the branching sister taxa from one another, and are sometimes included with on the diagram along the axis of the taxon's branch. Since each endpoint on a branch of a cladogram represents a single taxon, a cladogram showing the fusion of two taxa to generate a taxon generated by endosymbiosis wouldn't fit the method of showing new taxa as branches derived from ancestral nodes.  

However, if one were to consider the acquisition of an endosymbiont to be a derived character that distinguished a descendant taxon from its sister taxa, I'm not sure why such an acquisition could not be shown on a cladogram as such a character on the axis of the recipient (of the endosymbiont) taxon's branch.

Is the AP biology exam study book posing a question like this?  If so, what is the exact wording of the question?  I want to be sure I'm answering what you're actually trying to find out. :)

Hope this helps, though.

Dana