What is a cladogram, anyway?

Jonathan R. Wagner

Betty Cunningham wrote:

After hearing on the news of the recent camel/llama cross, I'm suddenly wondering how cladistics would show such an animal if it could have offspring.

It is important to bear in mind what a cladogram is and is not.

A cladogram is a branching diagram representing the most parsimonious distribution of derived characters within a set of taxa. We may use this distribution as an hypothesis of the evolutionary propinquity [(kinship)] of those organisms. That is, we can use it to formulate hypotheses based on the question "which taxa are more closely related to each other than to other taxa." While this is an added layer of interpretation, it is consistent with the principle of parsimony.

A cladogram is most emphatically not a description of the pattern that evolution took in producing the taxa under study. Within the confines of "A is more closely related to B than to C" there are a number of possible actual evolutionary sequences which may have occurred (George Olshevsky once counted six for [the Dinosaur Mailing List]). A cladogram does not directly address any of these options, although ancestral character-state information derived from a cladistic analysis (an additional level of inference) may be combined with other data (such as stratigraphic, paleoecological, etc.) to formulate an hypothesis. This is an even more derived level of inference, and the limited terrestrial vertebrate fossil record may impose severe restrictions on our ability to evaluate such hypotheses. Indeed, it may not be possible to test some or all of them.

It is therefore important for everyone to bear in mind that the branching pattern of a cladogram is intended to show the relative relationships among taxa, it is not a true "evolutionary tree" of how those relationships came to be (although in many cases it may indeed reflect the course of evolution). This is not to say that, [for example], birds could have evolved from crocodyles, even though they are closer to non-avian dinosaurs on a cladogram. The tree structure (crocs, (dinos, birds)) simply does not rule out the possibility that birds evolved from dinosaurs, or that the common ancestor of birds and dinosaurs evolved from crocodyles.

Therefore, in your camellamma example [(a hybrid from the crossbreeding of a camel and llama, announced in the news recently)], the tree structure would (probably) look like this: (camel, camellamma, llama), a trichotomous relationship. Why? Because the camellamma should be equally closely related to both parent species. In reality, who knows? It is quite possible that genetic and evolutionary considerations would lead those characteristics of the hybrid which were coded for analysis to more closely resemble one or the other parent. Hybrids are a complex problem. I am extremely glad I don't work on plants...