A field guide for the new Tree of Life
If you are a science news junkie, perhaps you saw it while scrolling through your news feed: the new Tree of Life. Last month, researchers announced that they'd used genetic sequences to build a much more inclusive picture of the Tree of Life. The tree is unique because it includes many microbial species that researchers have never even seen, let alone figured out how to grow in a lab. We know these lineages exclusively from broken up bits of DNA that the researchers culled from a wide variety of environments — from California meadows to geyser systems. Some of the DNA was even collected from the inside of dolphins' mouths! The researchers fit those DNA bits together like a puzzle until they got the complete genomes of these mystery organisms and used that information to locate their branches on the tree. All of that is fascinating — but when you dig down and start looking at this new tree, you wouldn't be faulted for wondering, "Uh, where's the tree here?" The new Tree of Life, in fact, looks more like an exploding firework than an oak or an elm. Here, we'll explore a tool that can help you interpret different styles of evolutionary trees (i.e., phylogenies) that you might see online, in textbooks, or in museum exhibits.
Where's the evolution?
A glance at this new tree is likely to inspire many questions. Where's the root of the tree? Why are some branches growing downwards? Why all those different colors? How come some branches are longer than others? What do the red dots mean? And most importantly, how can I tell which lineages are most closely related to one another?
The Field Guide to Evolutionary Trees is an interactive feature that can help make sense of confusing trees. It's part of Understanding Evolution's new area devoted to phylogenetics called The Tree Room. Let's use the Field Guide to interpret this new tree. You can follow along using the Field Guide for yourself if you like.
To use the Field Guide, first you'll have to decide which tree style most closely resembles the new Tree of Life. All of the trees in the Field Guide (shown below at left) depict the same thing — evolutionary relationships — but they use different styles of line to do that, often resulting in trees that look quite different from one another but show the same information. The starburst-shaped tree (circled below) looks very similar to the new Tree of Life.
Start by selecting the tree from the Field Guide that most closely resembles the tree you are interested in. The circled tree is the one that is most similar to the new Tree of Life.
Clicking on the starburst tree leads users to this page of the Field Guide that explains the basic features of the starburst tree.
Clicking on the starburst tree leads you to a simple version of this tree type (above right) that shows just four taxa. By clicking the buttons and question marks on this page, you can answer some important questions about the new Tree of Life, including:
In starburst style trees, like the new Tree of Life, time generally flows from the center of the diagram outwards in all directions.
This branch of the new Tree of Life has been labeled to show how to interpret closeness of evolutionary relationships on a starburst style tree. The Gammaproteobacteria and Betaproteobacteria are more closely related to each other than either is to the Acidithiobacillia.
To answer the other questions about this tree, you'll need to click on the "advanced tree features" button in the Field Guide. Then, by clicking on the question marks on this more complex tree, you can answer other questions about the new Tree of Life, including:
Animals, fungi, and plants represent a tiny fraction of the diversity shown on the new Tree of Life.
In the new Tree of Life, branch length indicates the number of nucleotide substitutions that have accumulated along that branch.
Tree diagrams sometimes add labels or symbols to highlight particular features of the tree, in this case red dots. The caption for the tree in the original scientific paper describes what the red dots mean. Each of these dots represents a lineage that no scientist has ever actually seen and that is known only from DNA. In fact, the scientists working on this study discovered 1011 separate organisms this way! Many of these newly discovered lineages are thought to be symbionts — members of other species' microbiomes. While this is impressive, it almost certainly represents a small portion of the microbial diversity still waiting to be discovered. For this study, DNA was collected from six very different environments, but this variation doesn't come close to encompassing the full range of environments on Earth, nearly all of which are inhabited by different sorts of microbes. As scientists use genetic tools to dig deeper and deeper into the full scope of life's diversity, they will surely be adding new explosions to this already lavish fireworks display! And as these new branches are grafted onto the tree and this information is displayed in different ways, you'll be able to return to the Field Guide and Tree Room for help figuring out what it all means.Primary literature
Discussion and extension questions
Use the Field Guide to answer questions about this tree, which shows the same relationships depicted in the new Tree of Life, but in a different format.
View this article online at:
Understanding Evolution © 2020 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California