Thermodynamic Driving Forces

In my previous post on Jeffrey Wicken’s Evolution, Thermodynamics, and Information, I discussed how he distinguishes machines from organisms. Today, I will focus on Chapter 5 of the book: “Thermodynamic Driving Forces”.

 

Wicken doesn’t like the expression “driving forces” because “it suggests some kind of external propulsion – which is not what the teleomatic directive of the Second Law to randomize matter and energy in quantum space is about.” But he admits that “the metaphor has value, since there is an internal propulsion to the emergence and evolution of life”. Note his mention of randomizing matter and energy; I’ll get to that momentarily.

 

As in previous chapters, Wicken is concise in his definitions and tells you where he is going upfront: “Evolution is about variation, the constraints under which it occurs, and its selection according to ecological success. We will be discussing the physical basis for the variation-constraint-selection triad…” So, let’s proceed with some thermodynamics.

 

For anything to proceed according to the Second Law, you need an energy source that can be utilized, and a sink to dissipate the energy (or as Wicken says, “to receive its entropic waste”). For energy to flow from higher to lower potential, there needs to be “[energy] charging of the prebiosphere to higher levels”. Once this charging is achieved, the dissipation of that energy generates molecular complexity. Carbon chemistry is particularly facile for molecular diversity.

 

What’s the best source? Photons. Concentrated packets of energy. Matter (atoms and molecules) can absorb the photon energy and become “energized” so to speak by having an electron excited energetically. Its potential energy has increased. It could release that energy by emitting a photon of the same wavelength, but more interestingly it could dissipate the energy via different pathways. It could dissipate it by making a new chemical bond between two atoms – an exothermic reaction that releases energy as heat (non-utilizable entropic waste). By forming chemical bonds, matter becomes more structured.

 

But Wicken goes further and argues that for this structuring to lead to complexification, requires that “the properties of the elements involved in the structure not be sufficient to determine that structure.” This is particularly interesting because it suggests that for complex matter to form, you need to be able to access multiple diverse structures and that there is uncertainty as to which subset of structures is actually formed. Physics and chemistry constrains those possibilities but if there is still room to play after those constraints, complexification is not only possible but it is expected according to the Second Law. Wicken says “complexity requires structure; but it also requires options”.

 

And now the crux of Wicken’s argument: “The physical basis for information is complexity. Complexity makes available different possibilities for functional interconnections. Organization can’t itself be quantified, since it reflects how well a system operates with respect to some desired output, rather than a physical property. One can, however quantify the complexity of an organization. A major evolutionary trend has been toward increased organizational complexity. The question is, does this trend result simply from selective pressures that provide ecological space for complex systems? Or is there a drive toward complexity operating in evolution independently of natural selection? Both are involved in evolution; but it is in prebiotic evolution that the drive toward complexity can be seen most clearly apart from selection, preparing the scene for life’s emergence. The operation of this drive depended on certain conditions of closure to which the prebiosphere was subject.”

 

Wicken defines information mathematically using a simple equation that mirrors Boltzmann’s entropy equation. He then divides up this “macroscopic” information into three parts: energetic (a function of internal energy), thermal, and configurational. The latter two are the “negative entropy” terms that can be calculated from statistical mechanics. The changes to the thermal contribution relate to structuring (“the movement of thermal energy from practically continuous translational modes to much less densely spaced vibrational modes”) and can be quantified.

 

Now let’s get back to dissipation. Wicken divides these into two categories: “energy-randomization” and “matter randomization”. Energy-randomization is driven by forming chemical bonds. This reduces internal energy (and its informational counterpart). Wicken states: “The result of these associations has been to reduce the number of discrete chemical entities in the biosphere and to increase their average sizes and complexities.” Where does this energy go? It is dissipated thermally.

 

Matter-randomization is the more interesting case. Essentially the more different kinds of molecules you can form, the more you can dissipate configurational information (or produce configurational entropy). Wicken uses some simple kinetic arguments to show that this must be a crucial player in any chemical reaction trying to move towards equilibrium: “Matter-randomizing considerations therefore promote all reactions through some compositional range, and reciprocally assure that no reaction can proceed entirely to completion.”

 

Once he has done this, Wicken can now track the information from all three parts through the various scenarios of prebiotic chemistry. Forming the diverse molecular building blocks of life is possible because this process dissipates configurational information. But after the divergent step comes the convergent step of forming biopolymers and other aggregates driven by the formation of chemical bonds. Wicken then discusses the constraints in play and what drives the process forward in connection with the Second Law. I feel that I’m grasping the mere edges of his argument, but I think he’s on to something compelling. I’m also biased my own research project of mapping the energetic space of proto-metabolism so I’m predisposed to his matter-randomization arguments. But I’ll need to ponder this a little longer before I can translate a Wicken-type analysis to a model system for which I have data.