Tree Thinking
Darwin’s first tree
The following post is a crash course in how to interpret evolutionary trees written for the layman. I’ve avoided all the tricky nomenclature (like “phylogeny” and “cladogram”) for clarity. Enjoy!
As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications.
-Charles Darwin
When Darwin presented his theory on the origin of species by means of natural selection, he chose to illustrate the evolution of lineages by means of a tree. It’s a metaphor and a method that has stayed with us since then - for better or for worse. These days, we know that what evolution has produced is more of a sprawling shrub with mostly dead branches, rather than the majestic tree Darwin envisioned, and that the base of the shrub is probably more like a web than a trunk. (As most of you are no doubt aware by now, New Scientist recently had a story on this with a frightfully dumb headline on the cover.) These issues aside, depicting the relationship between species by the use of trees has turned out to be immensely useful.
Evolutionary trees are now ubiquitous; apart from being all over the scientific literature, they also turn up in museum exhibits, newspaper articles and wherever else the relationships of different species are discussed. Trees are a powerful and intuitive graphical representation of evolutionary relationships - but there are pitfalls. There is still a lot of confusion among people in general (and even biologists) when it comes to how these trees should be interpreted. We all have a tendency to read things and interpret images a certain way, and we often imagine an order of progression even when the data doesn’t support it. Many biologists don’t realise how important it is to choose a graphical representation of their data that isn’t just correct, but also doesn’t trick the viewer into believing conclusions that simply aren’t there; conclusions that have nothing to do with biology.
The ape branch
Case in point, take a look at the tree to the left. Humans are clearly at the top of this tree. It supports the idea that we are the pinnacle of evolution, the end result, perhaps even the reason for it all, don’t you think? No. In fact, the tree says that humans are entrenched among the apes, as much a primate as any of them. But since a lot of people haven’t learned “tree thinking”, that’s not what a lot of people see.
Because (at least in the west) we read from left to right, the tree depicted implies an order of progression from gibbons to humans. Some people would go so far as to interpret this tree as saying that gibbons evolved into orangutans, which evolved into gorillas, which evolved into chimps, which finally evolved into humans - the age-old “If humans came from monkeys, why are there still monkeys?” fallacy.
Gratuitous platypus
Even if you don’t fall into the progression trap, the tree still suggests at a glance that gibbons are more “primitive” than the rest. And this fallacy is something that’s not restricted to laymen. Some biologists would refer to the gibbon branch as “basal”, and it’s quite common to talk about “higher” and “lower” forms of life. This is all poor tree thinking. As long as all the species depicted in the tree are alive today, and as long as the assumption is made that life only begun once on earth, it’s important to remember that every single extant lineage has evolved for exactly the same amount of time. The “lowliest” of bacteria have been evolving for the same amount of time as us humans. We may say that some forms of life possess more “primitive characters” than others. For instance, monotremes are mammals that lay eggs, and since we believe that the ancestors of all mammals also laid eggs, we may call this a primitive trait - but that doesn’t mean that the monotremes are in some way less evolved than the rest of the mammals. They simply possess fewer derived traits.
So what can we say about this tree? What it tells us is that humans and chimps share a common ancestor that is more recent than the ancestor of humans, chimps and gorillas. The hypothetical ancestors are represented by the nodes of the tree. Conversely, it tells us that the lineage that would eventually become gibbons split from the rest earlier than the lineage leading to orangutans. It doesn’t say anything about what the hypothetical ancestors were - this tree in no way implies that the common ancestor of humans and chimps looked more like a human or a chimp. All we know is that it shared some traits with the lineage that led to the gorillas, which implies - to us, although the chimps might disagree - that that ancestor might have been more like the other apes than a human. Had the other apes been omitted from this tree and replaced by a fish, it would have been anyone’s guess as to whether chimps or humans possessed the most derived traits.
Finally I’d like to mention the concept of relatedness. When looking at a tree, how can you tell which species are more closely related to another? Firstly I would actually be wary of using the term “related” in this case at all. Rather than saying “this species is more related to that species than the third one”, I would say “these two species share a more recent common ancestor with each other than with this third one”. Because common ancestry is what this is all about. It is easy to look at a tree like the one above and get the idea that orangutans are more closely related to gibbons than to humans - they are adjacent to each other and only one node separates them. This is not correct. The common ancestor of orangutans and humans is more recent than the one of orangutans and gibbons, which means that orangutans and humans are more closely related.
Wrapping up, I present a very simple exercise for the reader! After reading this post you should be able to answer the following questions no problem, and as a little perk, the people who give me three correct answers will get to give me a topic or question of their choice to blog about (anything you like within reason - the more interested I am in the topic the better the post will be - if you ask me about the political situation in Nepal, or anywhere really, I’m afraid I’m stumped).
1. Are the trees below identical? (If not, how are they different?)
2. “In the first tree, species L is more closely related to species S than P.” True or false?
3. “In all trees, S, C and L share a common ancestor.” True or false?
