The (process) metaphysics of evolution

Heraclitus, by Hendrick ter Brugghen

Metaphysics isn’t exactly the first thing that comes to mind when someone is thinking about the theory of evolution, especially if that someone is an evolutionary biologist who is reasonably skeptical of any metaphysical claim — like yours truly.

Nonetheless, my Exeter University colleague John Dupré has published a stimulating paper that seeks to apply so-called process metaphysics to modern debates in evolutionary biology (full paper here). The result is intriguing, though that doesn’t mean I’m completely on board with what John writes.

Let’s start with the basics. Dupré defines metaphysics as the branch of philosophy that aspires to provide the most general description of reality. You would think that nowadays that aspiration falls squarely within fundamental physics, except for the fact that fundamental physics — as interesting as it is — is largely irrelevant to most of the other sciences, and “the most general description of reality” can’t be just the description of whatever is at the bottom of reality.

John also argues, and I think he is completely right about it, that:

“Though they may sincerely deny it, scientists are almost inevitably committed to metaphysical opinions. … Metaphysics can be ignored but not escaped.”

If that’s true, then we (philosophers) better get our metaphysics straight, and we (biologist) better pay attention to the consequences of our own assumptions on the matter, regardless of whether these assumptions are explicitly stated or not (in fact, particularly if they are not explicitly stated).

The way Dupré goes about this is by applying some philosophical reflection to the scientific work done by biologists, to see if certain metaphysical commitments made by scientists don’t turn out to be incorrect in the light of the scientists’ own work. Specifically, in this paper he explores a very old question in metaphysics: whether the world is composed of things or processes. The first position goes back at least to the Ancient Greek atomists, like Leucippus and Democritus (or, in India, the Jain, Ajivika and Carvaka schools, possibly dating back to the 4th century BCE). The second position was espoused by Heraclitus, and made famous by the Latin version of his dictum, panta rhei, “everything flows.” Nowadays, they are known in philosophy respectively as substance and process metaphysics.

Substance metaphysics implies an ontology of things (as opposed to an ontology of processes), and it has been dominant since the beginning of the scientific revolution in the 17th century. It is connected to the mechanicist approach, where everything is made of things with particular functions, and those things constitute the mechanisms that explain how the world works. Mechanicism in turn implies reductionism: things are made more and more complex from the bottom up, and the causal story is unidirectional.

Contrast this with the more fluid (ah!) process ontology, which rejects both mechanicism and reductionism: what maintains patterns of stability in the world isn’t just the behavior of individual entities, but also the network of relations among patterns and between patterns and the environment in which they are situated. I must confess that I started out, decades ago, as a mechanicist who believed in an ontology of things, just like most scientists do (especially physicists). But it was my own scientific research in evolutionary biology (particularly writing this book) that gradually interested me more and more in an ontology of processes and a less reductionist view of things. Only I didn’t know (until I turned professionally to philosophy) that this was called process ontology, and that it was a well worked out position among metaphysicians.

In his essay, Dupré applies this debate to the nature of biological species (see this paper of mine) and to our conception of evolutionary processes in general. As he clarifies, of course substance ontologists recognize that evolution is a process, but they think it is made possible by the more fundamental existence of things. For process ontologists, by contrast, everything — including living organisms — is a process. There is nothing stable. Panta rhei.

John explains very nicely the contrast between substance and process ontology. Consider the difference between a mountain and a storm. For a substance ontologist, the first one is a stable object, the second a process. But the process ontologist sees both as processes, only at very different time scales: the mountain changes continuously, but it takes millions of years for the change to be noticeable by the human eye. The storm also changes, obviously, but much more rapidly.

Then again, some storms are remarkably persistent, though in a dynamic fashion. Think of the famous Red Spot on Jupiter, which has been observed now for hundreds of years. Process ontologists think that a dynamic storm is a better paradigm for living organisms than a mountain. After all, when living beings achieve a stationary state we call them dead. As my professor of biophysics back in college, Mario Ageno (a student of Fermi, article in Italian), used to say: death is a sudden increase in entropy.

Why does it matter to think of organisms as processes rather than things? Dupré Suggests two reasons:

“The first is that it motivates a significant shift in emphasis with respect to what stands in need of explanation. The traditional concern for thing-centred ontology is change. I do not expect an explanation of why my desk is very much as I left it when I was last in my office. For a process, on the other hand, persistence requires explanation. Physiology is largely concerned with understanding the multitude of internal processes that enable an organism to stay alive, to maintain its thermodynamic disequilibrium with its environment. … The second reason why the processual status of organisms is important is that it places in the proper perspective the search for mechanistic explanation that is often alleged to be central to the contemporary life sciences. I take a mechanistic explanation to be, very roughly, one that involves identifying a set of constituents of a phenomenon and showing how their actions and interactions combine to generate the phenomenon. There is no doubt that this has been an enormously productive scientific strategy. Nonetheless, from a process perspective the mechanisms postulated by such explanations must always be abstractions from the wider biological context, and this always poses potential limits on their application.”

As a result, the organism should be seen not as a set of interlocking things (cells, sub-cellular components, individual molecules), but as a hierarchy of processes acting at different levels (molecular, physiological, and so forth).

If all of this sounds a bit abstract, that’s okay, it’s metaphysics, after all. To make things more concrete, John goes through a fairly in-depth analysis of the question of what evolves, i.e., what is the unit of evolution, in the process making some important clarifications on the nature of biological species.

It is an accepted truism in biology that individuals do not evolve, populations do. Then again, what counts as an individual? This is a very broad, and controversial question, but let’s focus on just one aspect of it: are species themselves “kinds” or individuals? Most biologists, I’m guessing, would answer that of course species are kinds, meaning categories to which a number of individual things belong, if these things satisfy certain criteria. For instance, Homo sapiens is a kind, to which all organisms that are statistically characterized by a certain type of genome, certain physiological, anatomical, developmental, and even cultural attributes, belong. Pan troglodites, a species of chimpanzee, is an evolutionarily related kind, to which all organisms satisfying certain other characteristics belong. And so forth.

However, most philosophers of biology accept some classical arguments put forth by Michael Ghiselin and David Hull, according to which species are not kinds, but rather individuals. If one accepts modern cladistic systematics, species are individuals identified by branches on a phylogenetic tree. This possibility was actually first brought up by a paleontologist, Steven Stanley, in a paper entitled “A theory of evolution above the species level” (pdf here).

Dupré accepts Ghiselin and Hull’s account, with some provisos inspired by his endorsement of process ontology. The most important of these provisos is that a branch of a phylogenetic tree is, of course, a process, not a thing. This neatly resolves a long standing problem for the species-as-individuals view: species change over time, but we said above that individuals do not evolve, populations do, so what gives? The answer is that species are individuals in the same sense that organisms are, even though organisms develop and change in the course of their short lifetime, while species change over longer evolutionary times. In both cases, we are talking about dynamic processes, not static things. That also explains why species typically have fuzzy boundaries: do you expect a thunderstorm to have a sharp and neat boundary, an exact point beyond which it is no longer a storm?

It is important to note, as John does in his paper, that not all processes generate individuals. Geological erosion is a process, but it doesn’t turn up anything like biological species. In order to do that, one needs a stabilizing process. In biology a major, though not the only, stabilizing process is natural selection, which tends to keep variants that are (locally, both spatially and temporally) sufficiently suited to the environment.

One consequence of this view of species as organisms that result from the stabilization of processes is that for most of the evolution of life on Earth there were no such thing as species-as-individuals. Bacteria are not stabilized in the required fashion, because their lineages do not actually identify species at all. And they don’t because they lack sexual reproduction, another major source of stabilization in multicellular species. John sees, correctly I think, the emergence of sex as making possible the emergence of species as individuals.

There is a lot more in the paper, where the author touches on a panoply of fascinating ideas in modern biology, from niche construction to epigenetic inheritance, from parental care to the evolution of culture. John’s treatment of the subject has wide ranging consequences, as he summarizes near the end of the paper:

“If species are what evolve, we should not, for this reason, expect quite general accounts of evolution. The Modern Synthesis [i.e., the increasingly less dominant standard model in biology], specifically, may be more or less true for some kinds of species, but quite inadequate for others. If species have evolved new forms of evolvability [because of the invention of sex], this is surely to be expected. Evolvability of many populations may just be a summative property of organism properties, but as species become integrated processes it is plausible that evolvability might emerge as a specific capacity of lineages.”

Evolutionary mechanisms themselves, in other words, evolve. And they do so because everything in biology is a process, not a thing.