Outline of an existentialist theory of evolution

In previous posts, I pointed out that there is a critical epistemological difference between the study of biological things and of physical things, due to the fact that living things have a fundamental “teleonomic project”. I described this project as “reproductive invariance”, following the words of Jacques Monod, which is simply to say that what characterizes a living being is that it can reproduce itself. I believe many scientists would propose this definition. However, it is not very satisfying. For example, isn’t a sterile individual alive? One might amend the definition by proposing that life can only be defined at the level of species, but this does not seem very satisfying. Mules for example, are generally infertile, but certainly they are living beings. As a thought experiment, we may imagine a species of immortal humans that do not reproduce. Certainly we would consider them alive too.

So even though reproduction is a salient characteristic of life, it might not be the best way to define it. Here is another possible definition: a living being is something with the teleonomic project of living. That is, a living being is a set of self-sustaining processes, subject to the forces of nature (this is similar to Varela’s autopoiesis). Thus an important feature of living beings is that they could die, which distinguish them from other stable forms of existence such as rocks. The fact that they can die is critical, because it implies that there are constraints on their conditions of existence. For example, when removed from its natural environment (for example put in the void), a living being dies, so a living being is contingent on a given environment. So let us say that a living being is a stable form of existence that requires some energy, specific processes and environmental conditions to sustain itself. This could be called the existentialist view of life.

Many things could be said about this definition (such as the notion of multicellular organism), but this was just to introduce a discussion about evolution. Life is a stable form of existence, but evolution means change. The question of evolution, then, is what changes in forms of existence can occur while yielding stable forms (i.e., not dying). Here I will not discuss evolution from an empirical point of view, but simply develop the existentialist perspective.

Evolution is often presented (at least in popular science writing) as an improvement process. Genetic algorithms follow this view: there is an objective criterion to be maximized, and by mutations and selective reproduction, the criterion increases over generations. One obvious problem is that there is an objective criterion, independent of the organism itself, and defined externally. But in reality, there is no other objective criterion than the existence of the organism, in the environment. The criterion for survival is defined jointly by the organism and its environment, which itself is made partly of living beings. For example the survival of carnivores is contingent on the existence of herbivores (the notion of ecosystem). If carnivores exist, then a criterion for existence of an herbivore is its ability to escape carnivores. This is not an externally defined criterion. The existences of various organisms are interrelated, and the existence of a specific organism is determined by its sustainability as a stable form in the environment (stable at some timescale). Therefore the notion of “fixed point” is a better mathematical analogy than optimization. Potential changes, either external (environment) or internal (mutations), lead either to quick death, or to a new existential fixed point.

Let us start with a fixed, stable environment. Imagine there are only unicellular organisms, and they do not reproduce. It is possible that some of them die, because of radiations for example. Those that do not die are (by definition) robust to these radiations. These cells, perhaps, would live for a very long time – let us say they live eternally. But suppose now that, by some accident, one cell is given the ability to reproduce itself. When this happens, the cell initially multiplies exponentially. But in turn, this implies that the environment for each cell changes as the cells multiply. In particular, since each cell requires energy and the energy supply is limited, the population cannot grow indefinitely. At that saturation point, resources start to get scarce and some cells die. All cells are equally affected by this process, both reproductive and non-reproductive ones. When cells die, there are more resources again and so cells that can reproduce themselves occupy this space. Soon enough, the eternal non-reproductive cells are replaced by short-lived reproductive cells, where reproduction only compensates for deaths. This saturation point is reached extremely fast, because growth is exponential. Thus, the living world evolves from a stable fixed point, the small population of eternal non-reproducing cells, to a new stable fixed point, the short-lived reproducing cells that saturate the environment. This evolution is very quick, and perhaps it can be described as “punctuated equilibria”, as proposed by Stephen Jay Gould.

What is interesting in this example is that the new cells are not replaced by “better ones”, in a teleonomic sense. But simply, non-reproductive cells cannot co-exist with reproductive cells.

Let us consider now the notion of “adaptation”. Assume that sometimes, reproduction is imperfect and the new cell is slightly different from the original cell (a “mutation”). It could be that the cell dies, or cannot reproduce, and then that mutation does not yield a stable form of existence. Or it could be that the cell has a higher chance of survival, or reproduces itself at a higher rate than the other cells. If this is so, this cell and its descendants will quickly occupy (at an exponential rate) the entire environment. It could be said that the “species” adapts to its environment. This is essentially Darwinism, which is compatible with the existentialist view.

What kind of mutations would occur in this situation? At this stage, one may speculate that most mutations lead to death. Therefore, the mutations that are more likely to be reproduced in the long run are those that 1) reduce the rate of mutations, 2) fix all sorts or mutations that would otherwise yield to death or lower reproduction rate, 3) preserve the said mutations from subsequent mutations. Note that there may actually be a trade-off in the rate of mutations, because mutations are (at least initially) necessary to yield the repair mechanisms. Thus cells with a higher rate of mutations would be more fragile but also reach the self-repairing stage faster. These remarks suggest that, in such a stable environment, the population of cells should evolve sophisticated repair mechanisms. They are sophisticated for two reasons: they affect all mechanisms important for the existence and reproduction of the cell, and they are recursive (they affect the repair mechanisms themselves). Thus it could be said that the primary mechanisms developed by cells are homeostatic mechanisms. One very important corollary is that, at this stage, most mutations should now yield functional changes rather than death or dysfunction. This makes the mutation process creative (or without effect) rather than destructive.

An interesting point is that, even though living organisms are defined as fixed points, the evolution process has a direction. That is, it is not possible to evolve “backwards” towards non-reproductive cells. The fundamental reason is that evolution occurred because the two forms of existence were mutually exclusive. That is, reproductive cells and non-reproductive cells cannot co-exist, except for a transient time, because reproductive cells almost immediately occupy the whole environment and alter the conditions of existence of the non-reproductive cells. It is not that reproductive cells are “better” in some teleonomic sense, but simply that the other cells cannot simultaneously exist with them. Note that it is also possible that evolution occurs without the new form of life disappearing – otherwise there would be no unicellular organisms anymore.

It could be opposed that the co-existence of A and B necessarily yielding to the disappearing of A and the existence of B defines an order on the set of living things, and therefore it would be actually right to claim B is “better” than A, with respect to that order relation. But in fact, this is not an order relation in the mathematical sense. First of all, there are things that can co-exist, i.e., that cannot be ordered with respect to each other. Thus, if it is an order, then it is only a partial order. But more importantly, an order is transitive, which means that if B is better than A and C is better than B, then C is better than A. This is certainly not true in general for life. For example, imagine that bacteria B produces substances that are toxic for organism A, so B is “better” than A. But then imagine that some mutation of B can make a new bacteria C that is more efficient than B (e.g. reproduces at a higher rate) while producing different substances that are not toxic for A. Then C is “better” than B, yet C can co-exist with A, so C is not “better” than A. One may also imagine that A can be “better” than C in some way (e.g. eats something that is necessary for C). It follows that the dominance relation that I described above is not an order relation. It implies in particular that it is not possible to define a general external criterion that is maximized through evolution.

But this does not mean that there are no optimality principles in evolution. On the contrary, consider again the previous example of the reproductive cells. Such cells always live under conditions of sparse resources, since they grow as long as there are enough resources. This means that cells die because of the scarcity of resources. Therefore, any mutation that improves the efficiency with which resources are used quickly develops in the population. It follows that resource efficiency is optimal in a stable form of life. Here optimal should not be understood as “best” with respect to a specific criterion, but rather as something that cannot be improved through the evolutionary process – or, to remove any notion of goodness, something that is stable through the evolutionary process. So, processes tend to be “optimal” because non-optimal ones are not stable.

Let us refine this notion of optimality. This is a pervasive notion in neuroscience, in particular. For example, it might be stated that wiring between neurons is optimal in some way. Two remarks are in order. First, this does not mean that, if one were to design a wiring plan from scratch, this is how it would like. Optimality must be understood in the sense of something that cannot be incrementally improved through the process of evolution. For example, there may be all sorts of oddities due to the specific evolutionary history of the species. Second, and this is perhaps more important, this does not mean that the organism has a plan (say, in its DNA), in the way an architect would have a plan, and that this plan is optimal. It only means that there are processes that develop the wiring, and that these processes cannot be improved. These processes do not need a “plan” in the sense of representation. They could take the form of rules of the kind “avoid such molecules” or “follow such gradient”.

This takes us to a refinement of the notion of optimality. Recall the previous remarks about homeostatic mechanisms. I noted that the first mechanisms developed by a cell should be those that minimize the bad impact of mutations, so that mutations are creative (or without effect) rather than destructive. Perhaps the same could be said of efficiency mechanisms. It seems that a general mechanism that regulates processes to make them maximally efficient in their present context (for example, in terms of metabolism) would be much more stable through evolution than specific mechanisms evolving towards maximum efficiency. Therefore, I postulate that there are early mechanisms, shared by all species, which regulate other processes of the organism to have to maximize their efficiency.

This idea could be taken one step further. Such regulation mechanisms can be seen as implementing the creative function of evolution in the time course of a single individual’s life. Clearly such meta-evolutionary mechanisms would be more stable than any particular mechanism (specific of a species) since, by construction, they are robust to all sorts of changes. There is at least one well-known example: the immune system. The immune system develops antibodies for specific foreign objects such bacteria, through what can be described as a Darwinian process (selection and cloning). Obviously this mechanism is much more stable through evolution than any mechanism targeted at a specific antigen. Note how this is “optimal” in the existential sense I discussed above: once it exists, any mechanism for a specific antigen loses its evolutionary value (although this may not be true of serious epidemic diseases). Thus it should be expected that there are such generic mechanisms in any stable form of life, rather than a “plan” for all specific actions that an organism must take. In the same way, it could be argued that the nervous system is such a generic system. In fact the analogy with the immune system was explicitly made by Edelman (a Nobel prize in immunology who then went to neuroscience).

These remarks suggest that some of the first and most universally shared mechanisms possessed by living beings should be meta-evolutionary or adaptive mechanisms by which the organism can adapt and maximize its efficiency in the face of changing conditions both in its environment (environmental change) and in the body itself (evolutionary change). This has an important consequence. When there is a discrepancy between empirical evidence and optimality principles, for example when some mechanism does not seem metabolically efficient, it is sometimes argued that it is not contradictory with the theory of evolution because there may be little evolutionary pressure for that specific mechanism. But implicitly, this argument assumes that evolutionary pressure actually applies to that specific mechanism, while I would argue that there may not be such specific mechanisms. Instead, it is more likely that there are generic efficiency-maximizing mechanisms, and for these mechanisms evolutionary pressure fully applies. Therefore, such discrepancy is more likely an indication that part of the interpretation is wrong, or pieces of the puzzle are missing.

To summarize, the conclusions of this existentialist reasoning about evolution is that all organisms, starting with unicellular organisms, must have developed 1) a large range of sophisticated homeostatic and repair mechanisms, starting with those that make mutations creative rather than destructive, 2) general-purpose regulating mechanisms that maximize the efficiency of other processes, 3) adapting or “meta-evolutionary” mechanisms that adapt the organism’s processes to changes, both external and internal, that occur within the time course of the individual’s life.

Laisser un commentaire

Votre adresse de messagerie ne sera pas publiée. Les champs obligatoires sont indiqués avec *