Population Thinking and Why It Matters

According to the foremost philosopher of biology, the late Ernst Mayr, one of the most important advances in biological science has been population thinking. That is, we now understand that all the members of a population (or any other set of individuals) have individual variability. They are not all the same. Without this understanding, nobody could grasp the concept of natural selection. How could some individuals in a population (of bacteria, trees, or humans) have more offspring than others (aside from sheer luck) if they were all the same?

Here is one simple example. In this photo, I hold in my hand acorns from two post oak trees. The acorns from one tree are larger than those from another tree. I show that this difference in size is greater between trees than it is among the acorns from a single tree. The two on the left are from one tree, the two on the right from another, of the same species.

Darwin based his theory of natural selection on the work of Thomas Robert Malthus, the economist who said that there were always more people than resources, at least most of the time. Suffering and privation were, therefore, inevitable. Darwin did not simply apply this idea to plants and animals, as some people think. Darwin had to make the transition to population thinking. Even Malthus did not have population thinking; to him, all members of a human population were the same: they all worked, ate, had babies, and died, pretty much the same way.

The lack of population thinking is also why early experiments did not have replication. Bacon used only two chickens in his freezing experiment, and Redi had only two flasks for his spontaneous generation experiment. If all chickens, and all flasks with meat in them, were the same, then replication is not necessary.

Common sense, they say, is neither common nor sense. Many people, when they see an example of something, assume that it represents the entire picture. If some Republicans commit an act of domestic terrorism, as happened this last January 6, then all of them will, if given the opportunity. This is not true (I think). Because of individual differences, you need a sample. When I studied how much garbage was left along Oklahoma roadsides, I counted the garbage from many different stretches of roadside, and not just the ones that looked trashy. If I simply looked at the trashiest roadside, and assumed it was typical, I would have reached the incorrect conclusion that Oklahoma highways have over 300 pieces of garbage per mile. (Actually, it is “only” 100.)

How do we know that we have looked at enough samples of people, things, or places to draw a legitimate conclusion about them? That, my friend, is what the science of statistics is all about. And that is another story, though not one you will probably read in this blog.

How Biology Is, Or Is Not, Different: Thoughts from Ernst Mayr

Ernst Mayr was one of the leading figures in modern biology. He was the last surviving architect of The New Synthesis of evolution. And he kept writing books until he died at age 100 in 2005. Because of his age, his writing is fairly clear: he knew he did not have time to go off onto tangents. He had to get to the point, since he knew any sentence might be his last.

One of his main points (expressed in two books, This is Biology and What Makes Biology Unique?, which are very similar but not quite the same) is that biology cannot be judged by the same standards of scientific rigor as the physical sciences. Yes, we all know that biological systems (such as organisms) follow natural laws. But each biological phenomenon is the result of such a prodigious number of interacting natural laws that you can never exactly predict what is going to happen. The best example is evolution. Immanuel Kant said that there would never be a Newton for a blade of grass. Several writers have noted that Darwin became that very person.

But Darwin was a “Newton for a blade of grass” because he changed our view of biology the way Newton changed the view of physics. What Darwin did not do was to establish a system by which the exact course of evolution could be predicted. One reason for this is that each organism is unique, while each electron is the same as every other electron. It is true that the molecules in a glass of water are different from one another; each has its own kinetic energy, and some of them have hydrogen and/or oxygen atoms with extra neutrons. Although one could say that a glass of water has a “population” of molecules, they do not differ from one another in the extreme way that organisms in a population do.

I will let you read Mayr’s books, if you wish. But I want to remind all of us that there are some laws of nature that biological systems always follow. They include:

• The rate of diffusion (of molecules, heat, electrons, etc.) is proportional to the concentration or energy status divided by the resistance. One of the components of resistance is distance; it takes a molecule four times as long to diffuse twice as far. This is why diffusion is rapid over short distances, such as a synapse, and slow over long distances, such as a room. This is true everywhere in biology. This is why leaves and animal tissues both have numerous, tiny vessels. I am aware of no exceptions.
• A related concept is that an increased surface-to-volume ratio increases chemical activity. This is why kindling burns faster than a log, and why bacteria can metabolize so quickly. Any exceptions?
• A third example is from fluid dynamics. The rate of fluid flow is proportional to the fourth power of the diameter of the vessel. This law is always true for laminar flow, such as in blood vessels and xylem. Any exceptions? For larger things, such as water pipes, gas pipes, and rivers, it is almost true, but turbulent flow (when the fluid starts roiling around) slows the fluid down.

Most scientists agree with Mayr. This is the main reason that I am seldom interested in mathematical models of biological phenomena. A few decades ago, various botanists figured out equations for how much transpiration was needed to cool a leaf off, and how big or small a leaf should be to keep from overheating, and other such things. The equations gave a verisimilitude of precision. Actual leaves may or may not follow these equations precisely. They are good generalizations, but no more than that.

I recommend the writings of Ernst Mayr, even if you are not a professional scientist. Even at age 100 he had an all-encompassing mind.

New Wine in Old Wineskins

In their 1981 book The Liberation of Life, Charles Birch and John B. Cobb wrote, “It is remarkable how much new wine we can put into old wineskins.” Making reference to the metaphor used by Jesus, they meant that humans have shown limitless creativity when it comes to cramming new discoveries into old world views.

This book, rather tedious and not much read today, a theologian and a biologist joined forces to examine how a spiritual view of life—not a view based on religious doctrine, but on spiritual sensitivity—might transform both religion and science. I’m afraid I missed most of their points, even when I re-read the marginalia I wrote back when I originally read it. But I want to share a couple of insights that these authors presented.

One of these ideas, which sounds like something from the writings of Ernst Mayr, is that it was not so much the idea of evolution by which science changed our view of the world as it was population biology. In the earliest days of science, the “balance of nature” view prevailed. In this view, providence maintained populations and species by creating them with different birth rates. Balance was maintained because prey reproduced rapidly and predators reproduced slowly. This was a view expressed by Linnaeus in the eighteenth century. Today we understand that there is a struggle for existence, as Darwin called it, and that the rapid reproductive rate of many prey species has evolved as a response to predators. That is, from the pre-Darwinian view, Darwin not only disrupted belief in the orderly realm of supernatural creation by saying that life evolved, but he upset belief in the balance of nature by writing chapter 3 of the Origin: The Struggle for Existence. Darwin’s opponents tried and failed to put new ideas, about the struggle for existence, into old balance-of-nature world view.

Another example is one that remains with us today: the idea of limitless growth. We talk as if we believe that the world economy, and the economy of each country, should grow forever without limit. The alternative gets labeled “stagnation” rather than “equilibrium.” But, as Birch and Cobb pointed out, nobody really believes this. We all know we live on a planet of limited resources. In some cases technology can raise the limits, as with breeding and the invention of fertilizers that boosted crop production. But we all know that we have to make the transition to a sustainable economy—as a whole planet, and as separate countries. The consequences are sobering: the economic and political leaders of the world are lying to us and they know it. And we progressives play right along with it when we say that a sustainable economy, based on solar energy, will continue to allow unlimited growth. To paraphrase Kenneth Boulding, anyone who believes that unlimited growth is possible on a finite planet is either a madman or an economist.

The old wineskin of the balance of nature was born from a view of the Earth as a peaceable garden, like Gilbert White’s garden (The Natural History of Selbourne), and the old wineskin of unlimited growth was born from the period when empires were expanding. Empires could expand only because the cultures with powerful military forces conquered and usually killed the people who had less military strength. We need new wineskins (new concepts) in our world. Actually, they are no longer new, but as 2018 begins, we find that they remain largely unaccepted.