Posted by: soniahs | July 13, 2011

Can we talk about evolution without using a tree?

Metaphors in science can be powerful things- they can provide unifying frameworks for thinking about the world, suggest exciting new insights, or at times color our interpretations so that what we see is what we expect to see. Science is communicated to non-scientists largely through metaphors. Sometimes these communication strategies work, and at other times they don’t.

One of the key metaphors used to describe the pattern of descent with modification or evolution over time is the image of a branching tree. I’ve discussed some of the limitations of the tree metaphor in a previous post; essentially, it’s difficult for us to discard the misleading aspects of the tree metaphor while using other associations to communicate about the pattern of evolution. A current PLoS Biology paper by David Penny points out the problems of conflating a branching pattern of evolution in general with cultural associations of a “tree of life” (an image found in varying forms in several cultures), and points out that the tree metaphor only gives us part of the picture.

But do we have to use a tree metaphor at all? Certainly, the tree does a good job of illustrating common descent, and an okay job of showing the formation of new species (species can form through mechanisms like hybridization that the tree isn’t good at depicting). But no metaphor is perfect. Biologists have used other visual metaphors in the past, such as complex systems of symmetry-based relationships, or maps based on ecological affinities of species, but these have their problems as well.

In my graduate work, I’m using digital tools to expand the range of metaphors we have to communicate about evolution, by creating a dynamic evolutionary map. I’m focusing on avian evolution and the pattern of diversification of bird orders over time. I’ll be writing more about this project in the upcoming months, but in this post I want to share the basic draft pattern of the visualization.

The visualization spans a time period from the Cretaceous (in which we see the hypothesized origin of birds) to the present. This series of gifs is the draft version of the evolution of bird orders over time; each dot represents an order (with some exceptions). When the project is finished, viewers will be able to animate the orders forward in time, as well as examine relationships among orders and the evidence for shared descent. I’m already planning some changes near the beginning of the sequence, based on recent molecular studies. The numbers and cross-hairs will also not be in the final version (I’ve been using them to help me keep track of all the orders as I animate it). You should be able to get a sense for how the animation progresses by clicking through this slideshow:

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  1. Soniahs, I agree that metaphors sometimes work and sometimes don’t, and always have limitations. And I applaud your effort. But I also wonder if this visualization, too, uses the tree of life metaphor. Are we not looking at a series of cross-sections showing the different branches, progressively “higher” up the tree? Interesting work and I look forward to seeing it progress! Cheers.

  2. Chris- That’s a good point, and one I’ve been thinking about. Since I’m visualizing a single vertebrate clade, the challenge was to depict the pattern of orders radiating from a single origin without creating a visual that was explicitly tree-like. One of the things I’m interested in looking at is whether viewers with less of a background in biology actually do infer that a tree-like pattern is being depicted, or whether they “see” a map metaphor as they interact with the finished product. Also, does this visualization help eliminate some of the misconceptions from standard tree diagrams, such as the central trunk, or clades that branch from the bottom being more “primitive” than those at the top, etc. I suspect that biologists will probably still see tree-like branching, but those with less phylogenetic training may not. But that answer is still in the future…


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