Saturday, December 25, 2010

A preoccupation with phylograms.

Systematists and comparative biologists like, among other things, making trees. A tree (phylogram, cladogram, dendrogram, whatever other names you might want to call it) is a nice summary of relationships (nodes) between lineages (branches). We take great joy in constructing these drawings, find them aesthetically pleasing, use them to store information.

You might even say we like them too much.

The problem with "making trees" is that these phylograms (or cladograms or dendrograms) are not endpoints, but starting points. Even when we are talking about something other than the black box analyses of molecular systematics, this summary is not meant as a be all end all to our research. Even if we had the perfect summary tree of all extant life, our work would not be done, it would be just beginning.

There is this preoccupation with tree making to the exclusion of the potent question: why do we make trees? Yes, of course, to summarize, but that is only the start! Comparative biology is reciprocal illumination, whereby each new piece of evidence is fitted into the whole and shuffled back through an endlessly tightening spiral of work.

My first thought upon viewing a dendrogram (after the obvious "does this make sense?") is: how does this inform me of everything else? What hypotheses about genetics, physiology, morphology, ecology and behavior does this suggest? What further description in what areas will allow me to refine and test our understanding?

I mean, take this recent paper on Arthropod sysematics. What does a taxon Pancrustacea really mean? And I'm not meaning "this taxon includes so and so taxa", I'm asking those above questions. What hypotheses about morphology would Pancrustacea imply? What more can we learn about the Cephalocardia and Remipedia that would enhance our understanding of the relationships? Or this paper: What sort of interesting biological traits does this hypothesized "Simulioidea" have? What can it inform us of the biology of the included families?

A friend of mine said, "A conclusion is simply where you stop thinking." It seems trees in biology can be the same, which is missing the point. One of the other things biologists enjoy making is lists, which are always seen as simple summaries and never conclusions. How is it that trees are any different?

No comments: