18. The boundary for natural selection

We need to keep our language clear. Without complete clarity, we cannot find a way to test Darwin’s ideas as listed above (Hanson, 1981). Competition is, of course, the issue.

Before delving into what competition might be, it is again good to consider the etymological history of terms. There is an important point of similarity between Darwin and Joule. When Joule conducted his experiments, science was at a crossroads. He undertook his experiment because he wanted to test a very specific idea. Scientists were, in general, being forced to re-examine core ideas and Joule contributed signally to that re-examination.

Scientists of Joule’s era hypothesized that gases had some kind of internal pressure, πT, which “glued” them together, and accounted for their differences in behaviour. Joule’s view was that if such an intrinsic force existed, then it would have to do work—i.e. show itself externally—to maintain the gas’ properties when forced to expand. That work would need energy. But by the principle of the conservation of energy, that energy could only come extrinsically from the surrounding water bath. He ought therefore to be able to measure this collision of the intrinsic and the extrinsic, and so measure the intrinsic externally, through a change in the water’s temperature.

The result of Joule’s experiment teaches us the following:

  1. it is intrinsic to gases to expand their volumes indefinitely because by Newton’s first law their molecules seek to move rectilinearly and without hindrance until confronted by some obstacle in the shape of either the system walls or another molecule; and
  2. gases have extrinsic features imposed on them, by the environment, consequent to their being confined to some container with a specified volume, V, and simply on account of their mass and inertia; and with
  3. the intrinsic and the extrinsic being related through their pressure, P, and temperature, T, within that volume, V, and which are properties of their interactions.

In other words, molecules have an intrinsic mode of rushing out into the environment; an extrinsic mode of the environment’s molecules rushing in to them; and an interaction between those two modes. The Joule experiment was therefore an investigation into the intrinsic and the extrinsic properties of a system through the observed constraint of Boyle’s law, PV = T. Thermodynamic laws are the result of efforts to determine the effects of the one upon the other through the third.

Joule attempted to determine what other factors, besides a tendency to increase specific volume, might determine gas behaviour. He was much less interested in volume expansion, ΔV, because he was trying to demonstrate that even though the air expanded, other important properties—in this case temperature—remained constant. Since he measured no change in temperature, he concluded that contemporary theories were “corrupt” or erroneous, and that there was no such thing as internal pressure … a proposed intrinsic property. But Joule’s experiment tells us that the intrinsic molecular can be separated from the extrinsic molecular.

As Darwin knew, competition has two important aspects. He may have moved the balance of the debate away from struggles between species and towards struggles within them, but he still recognized both kinds of struggle. What his new understanding allowed was the introduction of discrete populations as the basis of competition and evolution, along with the explicit study and analysis of their descent:

can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and procreating their kind? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favourable individual differences and variations, and the destruction of those which are injurious, I have called Natural Selection, or the Survival of the Fittest. Variations neither useful nor injurious would not be affected by natural selection, and would be left either a fluctuating element, as perhaps we see in certain polymorphic species, or would ultimately become fixed, owing to the nature of the organism and the nature of the conditions (Darwin, 1872, p. 90).

Darwin’s point is well taken, but there is surely an important conceptual distinction between:

  1. the intrinsic: what biological populations do to and through their members as if of their own natures qua biological entities, and that therefore holds even when they are free from all possible constraints; and
  2. the extrinsic: what biological populations do in so far as they do those things only because the environment is indeed imposing constraints upon them and so on their members; and
  3. the inter-related: what biological populations do in so far as they are due to inter-relations and responses on a continuum between the above two.

As to how to identify and isolate core issues, Turchin points out, in his influential review, that “the environment does not have to be constant” in order for the exponential law that he seeks to elevate to a law to hold true:

My formulation of this principle is as follows: “a population will grow (or decline) exponentially as long as the environment experienced by all individuals in the population remains constant”. Environment here refers to all environmental influences affecting vital rates of individuals, including abiotic factors, the degree of intraspecific crowding, and density of all species in the community that could interact with the focal species (Turchin, 2001).

The question is whether or not the exponential law provides the initial assumptions we need. Those who propose it as determinative also propose that it would hold if there were no constraints. But freedom from constraints is obviously difficult, if not impossible, to test.

Testing for freedom from constraints does not in fact matter, because it is also not the real issue. We can isolate our three core issues—the intrinsic, the extrinsic and their relations—by putting into practice what we have just learned from Joule. The close similarity between Darwin and Joule is that they both demand that some internal commodity—either natural selection or an internal pressure—be identified and quantified. This can, in both cases, only be done by taking measurements in the environment. The difficulty is that Darwin gives no way to quantify, whereas Joule does.

In spite of the metrological difficulty Darwin presents, great thinkers such as he had an uncanny knack for sticking to the essentials of a problem and ignoring all side issues: in his case natural selection, variation, competition, inheritance. So also, when the German astronomer Heinrich Olbers wrote to Karl Friedrich Gauss and asked him why he was not devoting time to Fermat’s Last Theorem, especially since there was a prize attached, Gauss responded and said:

I am very much obliged for your news concerning the Paris prize. But I confess that Fermat’s Last Theorem as an isolated proposition has very little interest for me, for I could easily lay down a multitude of such propositions, which one could neither prove nor disprove (Singh, 1997, p. 105).

Like Darwin, Gauss knew how to stick to simplicities and essentials. Fermat’s Last Theorem was not resolved until 1993, using mathematical techniques that had not even been dreamed of in his day. Darwin showed similar perspicacity.

What Darwin showed was that a very careful selection of an initial approach, and rules to work by, is of the essence in unravelling core concepts. He admittedly erred somewhat in assuming an age-structure for his populations. His hypothesis was that older individuals perish to be substituted for by younger ones, with the age of the population then oscillating. But he was trying to establish a general case. By adopting his approach, he beautifully evaded many unnecessarily complex issues. The later discovery of non-age-structured populations does not detract from the fundamental evolutionary issues Darwin noted and addressed: the consequences of competition on heredity, inheritance, and the origin of species. Whether age-structured or non-age-structured, populations must compete. The latter reproductive strategy seems to be a direct response to the increase in population numbers as the environment’s carrying capacity is approached. It is in other words a competitive response in the best Darwinian style. Non-aging populations simply transfer over to an indefinite generation length. Pushing intrinsic mortality out to more advanced ages by a natural selection that favours early reproduction is also certainly a strategy. An alternative is a reproductive strategy favouring many offspring at an early age. But this differentially accumulates pleiotropic mutations which has an adverse effect on mortality, and encourages the loss of vital functions as age increases. Species that can have apparently indefinite age lengths—i.e. non-aging species—do seem to bypass these well-known perils of aging … but only at a cost (MacLean, Bell, Rainey, 2004). The lowering of intrinsic mortality seems to reduce net fertility. The net rate achieved by whatever response is ultimately selected thus depends upon both the carrying capacity and the number density. The adoption of a non-aging strategy is merely one way of responding to environmental stress, and the existence of such species does not disprove Darwin’s central assertion just because he did not direct himself towards them when establishing the basics. The basics remain the same: the young outnumber the old and many of those young do not therefore survive to reproduce. That is the essential issue in debating variation and descent.

For these various reasons—and including the three core issues we have highlighted—not all ecologists agree that the exponential law should be elevated to prime governance. Robert Leo Smith in his classic text book Elements of Ecology expressed the issues as follows:

No population continues to grow indefinitely. Even those exhibiting exponential growth … ultimately confront the limits of the environment. Most populations, however, do not behave in an exponential fashion. As the density of the population increases, interactions among members of the population and the availability of the resources result in an increased mortality, reduced natality, or both. If the population drops below the density the environment is able to support, mortality decreases, natality increases, and the population grows.

Population regulation, in part, involves competition among individuals of the same species for environmental resources. Competition results only when a needed resource is in short supply relative to the number seeking it. As long as resources are abundant enough to allow each individual a sufficient amount for survival and reproduction, no competition exists. When resources are insufficient to satisfy adequately the needs of all individuals, the means by which they are allocated has a marked influence on the welfare of the population.

Because the intensity of intraspecific competition is density-dependent or density-proportional, evidence of its happening comes slowly. It involves no sudden thresholds; it increases gradually, affecting largely the quality of life rather than the survival of individuals, although through time its effects become accentuated, ultimately affecting individual survival and reproduction. (Smith, 1986).

This is all very well, but we can look again at the Joule experiment. It makes issues clear in a way that these ecological perspectives do not. In spite of the apparent complexity of the surrounding world, the molecular theory combines with thermodynamics to reduce us to these three—and only three—options:

  1. a molecule can move freely through its system’s interior;
  2. a molecule can collide with another molecule within the system;
  3. a molecule can collide with its system’s wall or cross its boundary.

By being able to quantify and then cogently analyse these three, thermodynamicists were gradually able to separate out the intrinsic from the extrinsic and to produce powerful explanations for physical reality. If we can identify a suitable boundary, we can do the same for biology and ecology.