Usually, when someone starts talking about the interface between evolution and physics, it’s a prelude to a terrible argument that attempts to claim that evolution can’t possibly happen. So, biologists tend to be slightly leery of even serious attempts at theorizing about bringing the two fields closer.
Yet this October has seen two papers that claim to describe how a key component of evolutionary theory—selection—fits in with other areas of physics. Both papers are published in prestigious journals (Nature and PNAS), so they can’t be summarily dismissed. But they’re both pretty limited in ways that probably are the product of the interests and biases of their authors. And one of them may be the worst written paper I’ve ever seen in a major journal.
So buckle up, and let’s dive into the world of theoretical biology.
More assembly required
We can start with the terribly written paper. It introduces Assembly Theory, which is a potentially useful way of thinking about natural conditions that can enable combinatorial chemistry, generating a complicated mix of elaborate molecules. But that’s not at all the way the authors, several of whom are chemists, introduce the idea.
The very first sentence of that paper sets up evolution as being difficult to make consistent with physics: “Scientists have grappled with reconciling biological evolution with the immutable laws of the Universe defined by physics.” That’s… not true. Evolution is perfectly compatible with physics, and we’ve known that for quite some time. It’s so impressively untrue that the paper they cite in support of it only mentions physics once, and only to say that people have some misconceptions about it.
It doesn’t get better from there. “These laws [of physics] underpin life’s origin, evolution, and the development of human culture and technology, yet they do not predict the emergence of these phenomena,” they continue. This is true in the sense that any emergent phenomenon is, by definition, difficult to predict based on the behavior of its simpler components. But it doesn’t mean we need a new theory to tie them back to basic physics.
Nevertheless, we get one. “We present Assembly Theory as a framework that does not alter the laws of physics, but redefines the concept of an ‘object’ on which these laws act,” the authors claim. But they never actually do, despite having a section of the paper entitled “Assembly unifies selection with physics.” The objects of Assembly Theory could be things like atoms, which can easily be analyzed using the laws of physics. But they could also be non-physical things like concepts—the researchers mention human languages and memes as likely amenable to Assembly analysis.
So, the whole “unification of selection with physics” is, at best, a distraction and actively interferes with the explanation of Assembly Theory. Accordingly, the paper does a horrible job of explaining it. Somewhat amazingly, however, the theory can easily be explained in a series of less than two dozen tweet-length social media posts, as demonstrated by Carl Bergstrom.
Ignoring the physics
As Bergstrom notes, Assembly Theory works best if you think about it in terms of chemistry. Let’s say you throw a mixture of simple chemicals together and let them react. The potential outcome is a mix of polymers, each assembled from a random combination of the simple chemicals. You’d have lots of molecules, and every one of them would be distinct. But what if that’s not what you see? Instead, you might see a limited number of combinations were highly favored. There’d still be lots of molecules, but all of them would be identical to one of a handful of templates.
That’s basically the situation we see with proteins. With 20 amino acids that can combine with each other in any order, even a population of proteins 50 amino acids long could have a vast number of individual molecules. But the reality is that we only see a tiny fraction of this potential population, because evolution has selected for a limited number of functional proteins.
Assembly Theory posits that any population of molecules can be viewed as a combination of the minimum number of steps needed to assemble it—the history of how it got there—and the number of copies present. The higher this value is, then the stronger the selection that’s needed to produce it.
Like actual evolutionary processes, this recognizes that the end population is a product of the history and contingency involved in the early steps of assembly. And it’s potentially useful in two ways. It provides a way to quantitatively distinguish among mixtures of polymers assembled randomly from different monomers, polymers that are the product of linking lots of copies of a single monomer, and polymers produced by selection. And, as long as the steps and copy number can be quantified, it can measure just how much selection was involved in producing a population of molecules.
What it doesn’t do is unify any of this with physics. And the authors acknowledge that in the body of the paper, writing, “Combinatorial spaces do not play a prominent role in current physics, because their objects are modeled as point particles and not as combinatorial objects.” And “This definition is, in some sense, opposite to standard physics, which treats objects of interest as fundamental and unbreakable.” But none of that stopped them from writing the exact opposite in the abstract.