Please read with some kindness. I wrote this a couple of years ago, and given the time, I think that I would like to fine tune it somewhat. The writing may be sloppy (I make no assertions of literary skill!), but the content is relatively sound.
Entropy in Evolutionary Biology
by
Steve Gallegos
1998
Evolutionary biology has the writings of Charles Darwin as its foundation. The theory of natural selection has stood the test of time and weathered the harshest critics, and has survived to further define the diversity of life we see today. However, this is not to say that evolution has remained stagnant as a science. In fact, it would be more accurate to state that evolution itself has evolved immensely since 1859. As a relatively young science, evolutionary biology has incorporated nearly every branch of science in its defense. This is referred to as the modern synthesis of evolution. While it is apparent to most that sciences such as genetics and chemistry are intricately tied to the support of evolutionary theory, it may not be so clear as to how a science such as physics might be involved. One might argue, however, that physics is the founding science and all other disciplines must be incorporated into it. This paper will attempt to bring together physics and biology. Specifically, the 2nd Law of Thermodynamics, or entropy, and evolutionary biology.
It is necessary at this point to provide some background into evolutionary biology. Evolution may simply be defined as all of the changes that have transformed life on Earth from its earliest beginnings to the diversity that characterizes it today (Campbell, 1996). In more specific terms it may be defined as a change in allele frequency within a population. In 1859, Charles Darwin stunned the non-secular world when he proposed his Theory of Evolution by Means of Natural Selection. He provided a mechanism for a process that many scientists before him had only speculated about. Natural selection is a means by which only those suited for particular abiotic factors will survive to produce viable offspring whose genome is now also similarly well-adapted. This is presently the most widely accepted theory concerning the principal causal mechanism of evolutionary change. The phrase, "Survival of the fittest," is often quoted synonymously with natural selection. Individual organisms do not survive through geological time (unlike some evolutionary lineages), but what they inherit and pass on does, namely genes. The theory of natural selection asserts that the genetic composition of an evolutionary lineage will change through time by non-randomness (selection) due soley to the fact that not all gene combinations are equally suited to a given environment, and that consequently individuals differ in their biological fitness. Given diversity among individuals making up a population, not all individuals in the population at time t0 will contribute equally to the make-up of the population at a subsequent time t1. To the extent that this is due to the effects of heritable differences upon individuals, natural selection has occurred (M. Thain and M. Hickman, 1994).
Since Darwin first proposed this idea in 1859, many additional theories have been proposed. Some have been written in direct support of natural selection, some as an additional mechanism by which evolution may occur, and still others are written in an attempt to discredit the theory. Stephen J. Gould and Niles Eldridge have proposed a theory that deviates from Darwin’s idea somewhat. Natural selection requires extensive periods of time to work. Gould and Eldridge’s idea, termed Punctuated Equilibrium, requires far less time, and has gained much evidential support in recent years. Their idea is that species may have lengthy periods of stasis, interspersed by periods of rapid evolution (thousands of years as opposed to millions- significant in geological terms). Gould and Eldridge are both quick to point out that their theory is merely another means by which evolution may occur, not an argument against natural selection. Both may be necessary to account for the diversity of species seen today. There are other means by which species evolve, i.e. genetic drift, but they will not be addressed here.
Physics is the science of matter and energy and the interactions between the two. Simply put, physics studies the laws which govern the natural world. This implies that physics as a scientific discipline may be the foundation for every other branch of the physical and natural sciences. Indeed, one would be hard pressed to find a scientific discipline where some aspect of physics is not cited. It is the goal of this paper to bring together some important physical laws with the ideas which govern biological evolution.
Thermodynamics is the study of the relationship between heat, work and the associated flow of energy. It may be one of the most far reaching ideas in physics. While the idea of thermodynamics grew out of efforts to build heat engines, the theories behind the concepts have significance far beyond the scope of engines. There are several aspects of thermodynamics which need to be addressed. The laws of thermodynamics apply to a thermodynamic system, which has conditions described in its state, which are acted upon by processes. A system refers to a specific quantity of matter enclosed by boundaries or surfaces, either real or imaginary (Wilson and Buffa, 1997). After many decades of experience with heat phenomena, scientists formulated two fundamental laws as the foundation of thermodynamics. The First Law of Thermodynamics states that energy, which includes heat, is conserved; that is, one form of energy can be converted into another, but energy can neither be created nor destroyed. This implies that the total amount of energy in the universe is constant. Einstein showed us with the theory of relativity that the conservation law must include matter which is convertible to energy. Of course, further discussion into the theory of relativity and thermodynamics is beyond the scope of this paper. The Second Law of Thermodynamics, in its simplest statement, is that spontaneous change in nature occurs from a state of order to disorder(Davidovits, 1975). This, as a very broad definition, is entropy.
Thermodynamics as it relates to living systems was first noted by the German physician Robert Mayer (1814-1878). While studying the effects of fever on his patients, he concluded that in the body there is an exact balance of energy. In an 1842 article he published, Mayer wrote, "Once in existence, force (energy) cannot be annihilated- it can only change its form." This was further confirmation of the earlier writings of the French scientist, Laurent Lavoisier, in which it was suggested that the heat produced by animals is due to the slow combustion of food in their bodies.
Conservation of energy is implicit in all calculations of energy balance in living systems. If the energetics for the functioning of an animal is considered, the ideas of thermodynamics seem obvious. The body of an animal contains internal energy Et, which is the product of the mass and specific heat; and chemical energy Ec, stored in the tissue of the body. The first law allows some conclusions to be drawn regarding the energetics of the animal. For example, if the internal temperature and the weight of the animal are to remain constant (i.e., Ec and Et constant), over a given period of time the energy intake must be exactly equal to the sum of the work done and the heat lost by the body. An imbalance between intake and output energy implies a change in the sum Ec + Et (Davidovits, 1975). This is the First Law of Thermodynamics applied to living systems.
The Second Law of Thermodynamics may be stated as the direction of spontaneous change in a system from an arrangement of lesser probability to an arrangement of greater probability; that is, from order to disorder. This may seem obvious, but once the universal applicability of the second law is recognized, its implications are seen to be enormous. This is precisely where the importance of the idea of entropy and evolutionary biology becomes apparent.
The First Law of Thermodynamics states that energy is conserved. The body does not consume energy, it changes it from one form to another. It may even be arguable, given this definition, that animals should be able to function without a source of external energy. The body takes in energy in the form of chemical bonds of food molecules and converts it to heat. If the weight and the temperature of the body remain constant and if the body performs no external work, the energy input to the body equals exactly the heat energy leaving the body. If the heat outflow could be stopped, perhaps the body could survive without food. This supposition is absolutely wrong, however. The need for energy is made apparent by examining the functioning of the body in light of the Second Law of Thermodynamics.
The body is a highly ordered system. By means of evolutionary mechanisms, life has evolved to a state of great complexity. A single protein molecule in the body may consist of a million atoms bound together in an ordered sequence. Cells are more complex still. Their specialized functions within the body depend on a specific structure and location. It is known from the Second Law of Thermodynamics that such a highly ordered system, left to itself, tends to become disordered, and once it is disordered it ceases to function. Work must be done on the system continuously to prevent it from falling apart. For example, the blood circulating in veins and arteries is subject to friction, which changes kinetic energy to heat and slows the flow of blood. If a force were not applied to the blood, it would stop flowing in a few seconds. The concentration of minerals inside a cell differs from that in the surrounding environment. This represents an ordered arrangement. The natural tendency is toward an equalization with the environment. Work must be done to prevent the contents of the cell from leaking out. Finally, cells that die must be replaced, and if the animal is growing, new tissue must be manufactured. For such replacement and growth, new proteins and other cell constituents must be put together from smaller, relatively more random subcomponents. Thus the process of life consists of building and maintaining ordered structures. In the face of the natural tendency toward disorder, this requires work.
The work necessary to maintain the ordered structures in the body is obtained from the chemical energy in food. Except for the energy utilized in external work done by the muscles, all of the energy provided by food is ultimately converted into heat by friction and other dissipative processes in the body. Once the temperature of the body is at a desired level, all the heat generated by the body must leave through the various cooling mechanisms of the body. The heat must be dissipated because the body does not have the ability to obtain work from heat energy. The second law sets limits that say that even if the body had means of converting heat energy into work, the amount of work obtained would be small. The temperature differences in the body are small- not more than about 7°C between the interior and exterior. With the interior temperature T1 at 310° Kelvin (37° ) and the exterior temperature T2 at 303° Kelvin, the efficiency of heat conversion to work would be (from equation 1) only about 2% (Brooks and Wiley, 1988).
Equation 1
WORK/HEAT INPUT = 1- T2/T1
Of all of the various forms of energy, the body can only utilize the chemical binding energy of the molecules which constitute food. The body does not have a mechanism to convert the other forms of energy into work. Plants, on the other hand, are able to utilize radiant energy, such as that emitted by the sun. As animals use chemical energy, so plants use solar radiation to provide the energy for the ordering processes necessary for life.
It is necessary to point out that in applying the second law, one must take a very broad view. Within a given system, order can be created, restored or even increased through the expenditure of energy (Wilson and Buffa, 1997). The entropy of the universe as a whole, however, is always increasing.
As stated before, entropy is a measure of disorder, or randomness. Every energy transfer or transformation increases the entropy of the universe. There is an unstoppable trend towards randomization. In many cases, increased entropy is evident in the physical disintegration of a system’s organized structure. Much of the increasing entropy of the universe is less apparent, however, because it takes the form of an increasing amount of heat, which is the energy of random molecular motion. Entropy, in its simplest equation is defined by equation 2:
Equation 2
DS = Q/T
Change in Entropy
(constant temperature only)
Free energy is the portion of a system’s energy that can perform work when temperature is uniform throughout the system, as in a living cell. It is called free energy because it is available for work, not because it can be spent without cost to the universe. In fact, organisms can only live at the expense of free energy acquired from the surroundings. Free energy is defined by equation 3:
Equation 3
G = H - TS
where G is free energy,
H is the systems total energy,
S is entropy,
and T stands for absolute temperature(in Kelvin)
Think of free energy as a measure of a systems instability- its tendency to change to a more stable state. Systems that are rich in energy, such as stretched springs or separated charges, are unstable; so are highly ordered systems, such as complex molecules. Thus, those systems that tend to change spontaneously to a more stable state are those that have high energy, low entropy, or both. The free energy equation weighs these two factors, which are consolidated in the system’s G content. In any spontaneous process, the free energy of a system decreases (Campbell, 1996).
DG = GFINAL STATE - GSTARTING STATE
or
DG = DH - TDS
An example of decreasing free energy, an exergonic chemical reaction such as cellular respiration provides a clear example:
C6H12O6 + 6 O2 ® 6 CO2 + 6 H2O
DG = -686 kcal/mol (-2870 kJ/mol)
For each mole (180g) of glucose broken down by respiration, 686 kilocalories (or 2870 kilojoules) of energy are made available for work. Because energy must be conserved, the chemical products of respiration store 686 kcal less free energy than the reactants. The products are, in a sense, the spent exhaust of a process that tapped most of the free energy stored in the sugar molecules.
Are living systems exempt from the second law? Those who would dispute the ideas put forth in evolutionary biology might argue that the order displayed in living systems and the apparent "order from disorder" picture that the evolution of life portrays is indeed a violation of the second law. Since living systems create order out of relative disorder (for example, by synthesizing large complex molecules out of randomly arranged subunits), it may appear that they do violate the Second Law of Thermodynamics- but this is not the case. If one examines and understands the mechanisms involved in the maintenance of life, it is clear that evolutionary biology does not violate thermodynamic law. The second law only requires that processes increase the entropy of the universe. Open systems can increase their order at the expense of the order of their surroundings. The total process of life, therefore, obeys the second law.
Therefore, living systems are continually contributing to the flow towards disorder. They keep themselves ordered for a while at the expense of the environment. This is a difficult task requiring the most complex mechanisms found in nature. When these mechanisms fail, as they eventually must, the order falls apart and the organism dies. This is the contribution of living systems to the ever increasing entropy of the universe.