12 February 2009

In praise of evolutionary trees: a personal gallery

Charles Darwin's On the Origin of Species By Means of Natural Selection contains exactly one picture (here).

It's an evolutionary tree, a visual depiction of relationships between organisms. Such a relationship is now known a phylogeny, a word that Darwin coined. The point Darwin is trying to make with it is not the one that most people associate with evolutionary trees; Darwin was trying to emphasize what he called his Principle of Divergence. Nevertheless, trees have become more broadly representative of the entire evolutionary enterprise.

In that vein, and because variation is a key theme of evolution, I thought I would present the trees I've published in my research papers, with some commentary, for Darwin Day. I don't pretend that my trees are anywhere near as impressive as, say, David Hillis's massive, beautiful supertrees, but I present them here nonetheless.

There are two scientific stories being told here. One is about digging (figures 1, 2, 3, 4 and 6), and the other is about escape responses (figures 5, 7, and 8). I've grouped them in chronological order, however.

Figure 1.

This figure represents, well, bafflement. Not on my part, but on the part of the taxonomists. At the time, carcinologist Fred Schram memorably described the phylogeny of this group (Reptantia) as a "morass." Nobody had much of an idea of who was related to what, so instead of nice two way splits... a five way free-for-all. Original dissertation here (be warned, it's big!).

Figure 2.

This tree is nice and simple, and lends itself to using diagonal lines. Original paper here.

Figure 3.

This tree is considerably more complex. I'm trying not only to show the taxonomic groupings, but key features. The angle of the diagonal lines isn't terribly aesthetically pleasing to me, but I was trying to make the figure as compact as possible. Original paper here.

Figure 4.

Getting featured on the cover is always a nice ego boost. On this one, there's an ancestor nestled at the bottom of the tree, the earliest known decapod crustacean. Original paper here.

When I showed this figure to the editor, Glenn Northcutt, when he was visiting our campus, he asked, "Why are the animals pointing to the right?" He explained that in vertebrate anatomy, there was a long tradition of the anterior always being to the left. I'm pleased he published the figure anyway. Still...

Figure 5.

You'll notice everything facing left from here on in.

This one has even more species, with various traits in the branches. It was just easier to make straight lines instead of diagonals. Original paper here.

Figure 6.

That this one is back to diagonals is indicative of how long that paper sat waiting for me to revise and put it into the hands of editors! It was actually created well before the one above. Original paper here.

Figure 7.

Similar to the one before last, I think this one offers a few improvements in presentation. More of the taxa names are properly aligned, for starters. This and the cover are probably my two best looking trees. Original paper here.

Figure 8.

These two trees were the most complicated I've had to make yet. I was trying to show more groups of animals and more features. The size of this thing meant that I had to "chop the tree down" and show it horizontally, and that I couldn't show little icons of the animals, as I had done before. I really like putting in the pictures, because in some of my papers, they're about the only indication of what the organism looks like! But there was no possibility to do that and have any hope that the thing would fit on the page.

The other thing I like about this is that it shows that a tree is really an hypothesis. There are two trees, because there are two major competing hypotheses. There are similarities between them, and it turns out that both of them give the same "punch line" for my story anyway, which is multiple, repeated losses of certain neurons. Original paper here.

One thing I'm rather pleased with is that I've never re-used a tree. I've always redrawn them, always trying to look at each one afresh and make decisions appropriate for that tree. I hope these trees give a glimpse of the importance of evolutionary thinking in the research that I do.

Charles Darwin's first evolutionary tree (pictured) was not the first -- that honour lies with Jean-Baptiste Lamarck. nor was he the first to make the creation of trees rigorous and following explicit, testable rules -- that distinction lies with Willi Hennig. Nevertheless, I think we should celebrate evolutionary trees as part of Darwin Day because they represent one of the most important elements that Darwin brought to biology: history. Or maybe "deep time" is a better term.

Organisms have a past, and that past matters. That ancestry helps explain features of organisms that may otherwise be inexplicable, and it gives us a way to make predictions about organisms that we have not yet studied.

Perhaps more importantly, trees remind us that life is ancient. Organisms have been engaged in the struggle for existence on this planet a long, long time. Even the two centuries since Darwin's birth is a paltry handful of heartbeats in evolutionary time.

Before I go, I have one last tree. I did not create this one; I am grateful to Neurotree for this information. Since today is Darwin Day, an event that bills itself as a celebration of science and humanity, let me add in some personal connections. It's a reminder that the scientific community is, from a certain point of view, very small.

Figure 9.

8 Steps:

Zen Faulkes (The University of Texas-Pan American)
was a post-doc for


David L. Macmillan (University of Melbourne)
who was a grad student for


Graham Hoyle (University of Oregon)
who was a research assistant for


Bernard Katz (University College London)
who was a post-doc for


John Carew Eccles (Australian National University)
who was a grad student for


Charles Scott Sherrington (University of Oxford)
who was a post-doc for


Michael Foster (University of Cambridge)
who was a grad student for


Thomas Henry Huxley
who was a collaborator, friend and colleague of


Charles Robert Darwin (Independent)

Darwin created the foundation for evolutionary biology; Huxley created the biological curriculum for universities of his day; Michael Foster was a pioneer in physiology; Sherrington stood at the precipice between general physiology and neurobiology; Eccles, then Katz, strode into neurobiology; Hoyle stepped into the realm of invertebrate neuroethology using insects; Macmillan, while still a neuroethologist, preferred crustaceans as subjects; and I try to follow as best I can.

Descent with modification, indeed.

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