In January 2009, the popular science magazine New Scientist ran a controversial cover story declaring that “Darwin was wrong,” about the Tree of Life. Darwin famously presented the concept of life branching like a tree in his seminal work and it has been a mainstay, almost an icon, of evolutionary theory ever since. Unsurprisingly, anti-evolutionists aim a lot of their criticisms at this understanding of evolution. It is not uncommon for them to reference the New Scientist article and the evidence it presents. Such an article was doing the rounds recently on Facebook, so I felt the urge to have my say.
The Tree of Life is a model for understanding evolution, but it is not applicable in all cases. Organisms often swap genes through a process called Horizontal Gene Transfer (HGT) which can render the use of a tree ineffective – when genes are hopping about from branch to branch they can no longer be traced in the linear fashion necessary for a tree with dichotomous branching. Whole organisms can also combine, through a process known as endosymbiosis, wherein an organism becomes part of a larger host cell as they become mutually dependent – it’s how eukaryotes have mitochondria. When whole organisms combine, you suddenly have two branches growing into each other. The same problem arises near the tips of branches, where hybridisation amongst closely related species messes with the tree metaphor. All of these things combined make evolution more like a tangled web than a tree of life, so do we need to chop down the tree and find a new metaphor?
There are two ways a model can be used in science which are relevant here. Firstly, the model can be used to literally describe the important features which fit every example of the phenomenon in question. Secondly, the model can be used to give a detailed description of one example, which is used as a basis for understanding more complex examples. The Tree of Life does not fit the first approach very well, so any evolutionary biologist (or critic) looking for it to function this way are not going to find it useful. It does, however, fit the second approach, as multicellular organisms generally do pass on their genes in the linear fashion required for the Tree of Life model to work, so it can be used as a basis for understanding the more complex additions of HGT, endosymbiosis and hybridisation. In this latter sense, it is also useful pedagogically – students learn the basic branching concept of the ToL before moving on to more complex models; that’s how many concepts are taught in science, for example, students learn about electron shells before they learn about how we understand the positions of electrons in light of quantum mechanics.
I like to think of the theory of evolution as a bit like a bungalow. Darwin and Wallace laid the foundations and built the frame of the building, but it needed more to be a home. The Modern Synthesis gave it walls, windows and doors, but it’s not quite the same building we can walk around today. Since then, some walls have been knocked down, some new ones added, rooms redecorated, even a conservatory and a porch built. Darwin and Wallace would possibly not recognise this home at first – they would need a good look around, but they would still recognise it as a bungalow; no extra floors have been added – it certainly isn’t a tower building masquerading as a bungalow. Theories are often relatively simple and that allows them to cover a broad range of phenomena. That’s a strength, not a weakness.