What is odd about the “theory of evolution”?
I write this piece with a bit of trepidation. That feeling is not enough to stop me but it did cause me to hesitate.
I am about to discuss some odd features of the theory of evolution today. Much past experience, however, shows that whenever an evolutionist has any criticisms of his field, the statements tend to be seized on by anti-evolution ideologues as evidence that the idea of evolution is false. That inference is itself false but, once stated, it has to be rebutted, however much that feels like a waste of time and energy.
Yet, the points I want to make I feel are worth considering and have a larger context, also worth considering. If scientific thinking aims to develop a clearer picture of our world, there has to be honest discussion and evaluation of its guiding ideas.
The starting point is what we mean by “scientific theory.” This seems straightforward: a scientific theory is an explanation of how something takes place or happened in the past, and which is consistent with prior knowledge though initially proposing something new. The prior knowledge is based either on experiments or abductive reasoning1 or usually both2. A theory is not simply a wild speculation or a guess, without any evidence (such as most political conspiracy “theories”), nor is it just a hypothesis, which may be logical and well-grounded but which so far lacks support. Nor do theories have the solidity of “facts”, which are ideas universally regarded as true. Examples of the latter are: rocks are dense, hard objects from non-living sources; Earth’s atmosphere is made up primarily of nitrogen and oxygen; all animals and plants are composed of cells; DNA is the hereditary material for most forms of life on our planet; the Earth revolves around the Sun, not the reverse. Ideas accepted as facts today often began as theories or hypotheses, before they gained universal acceptance.
If one were to order these categories of ideas in terms of increasing certainty, the sequence would be: (wild) guess, hypothesis, theory, and fact. In contrast to facts, scientific theories have a large measure of support, but not universal, from scientists. There is often some degree of dissent from some individuals who are knowledgeable, rational, respected by their colleagues, etc. That disagreement is fruitful, prompting further thinking.
What makes for a good theory? The criteria are: clarity, brevity, consistency with the known relevant facts, and some degree of novelty. Brevity is a requirement because if one’s statement goes on for hundreds or thousands of words, one is adding on all sorts of elements to the basic idea. The likely degree of truth of the theory then becomes hostage to the validity of the various add-ons. Further, the longer the statement of the theory, the more probable that its clarity will be reduced. As for novelty and consistency with known facts, most new theories have those qualities or they would get little attention.
Certainly, the requirements of clarity and brevity have been met by many theories in the past, both successful and unsuccessful. Hence, the accepted theories of Newton’s laws of motion, special relativity, general relativity, quantum mechanics, the idea that DNA is the hereditary material, the idea that cells are the basic building blocks of animals and plants, the chemiosmotic theory of mitochondria, met these requirements. Other theories that were later invalidated also often had clarity and brevity. Examples are: the idea that light travelled through a substance called “the ether”, the phlogiston theory of how things burn, and the spermist or ovist theories of heredity (respectively that biological heredity was carried exclusively by sperm or by the egg). They fell because they were found to be incompatible with new facts that had emerged.
How does the theory of evolution fare with respect to the criteria of clarity and brevity? Sadly, it does not entirely make the cut. It actually consists of two parts, which can and should be treated separately. Indeed, they had very different histories, beginning with Darwin’s The Origin of Species, first published in 1859. The first part is the assertion that all forms of life on Earth are the product of evolutionary change and are related through evolution. At this point, it is regarded as true by the overwhelming majority of biologists but that was not always so and the idea had a pretty rocky road for a long time. However, by the early 20th century, most biologists believed it, though without really strong proof until the second half of the 20th century3. Of course, there are still people who reject it, on the grounds of their religious beliefs but for people who take scientific evidence seriously, the idea that evolutionary processes are responsible for the diversity of life on Earth is universally agreed. (For the reasons, see Substack no. 3.)
It is the second part of the theory of evolution – the questions about the mechanisms of evolution – where both clarity and brevity fall away and, along with them, universal agreement. One might object, however: did not Darwin argue that the source of evolutionary change was “natural selection” and isn’t that a core belief in evolutionary theory today? The answer is “Well, yes and no” or “It isn’t quite that simple…”.
The problem is that natural selection is not one thing or process but several distinguishable processes. Each has its own properties. Some forms promote stability and little change while others provoke change but not always in the way that Darwin posited, namely promoting better adaptation and fitness. Darwin himself realised this and discussed it. He did not reduce evolution to being driven exclusively by natural selection or claim that all properties of living things were its product, as often believed today.
He was focussed on what we call “directional selection”, namely the favouring of some genetic change over time that changes an organism’s characteristics, to make it better adapted. He was fully aware that many genetic changes produce poorer adaptation or fitness and that these tend to be selected out, perpetuating sameness in the population. This form of selection is called “purifying selection”.
Other forms of selection are named “stabilizing selection” (which promotes retention of certain gene combinations), “frequency-dependent selection (which can yield a regular alteration of forms) and “sexual selection” (discovered and named by Darwin), which promotes mating success, not better adaptation per se. Hence, to say that the mechanism of evolutionary change is change by natural selection greatly over-simplifies. However, a more accurate statement about natural selection cannot be done with brevity.
For several reasons, Darwin’s explanation of evolutionary change was disbelieved by many people for about 50-60 years and heavily debated by others who did not wholly reject it but who were unhappy with it. One problem was that his proposed mechanism of heredity – an essential element in any theory of evolution – was rejected by most biologists, from the moment it was first proposed by him in 1872. (He did not spell out his hereditary hypothesis in The Origin of Species in 1859, but waited another dozen years to do so, in a later book.) Nor when the foundations of modern genetics were agreed upon in the early 1900s, based on the prior work of Gregor Mendel, did Darwin’s theory of evolution immediately benefit. (In an ironic turn of events, it was taken up as evidence against Darwin’s evolutionary idea for nearly 20 years.) Genetics only came to the rescue of Darwinian evolution in the 1920s and ‘30s.
Yet, more was needed to create an acceptable set of ideas about evolution. In particular findings from paleontology, plant science specifically, and from systematics (the study and taxonomic classification of all living things) were needed. This work involved a heroic effort by a large cohort of eminent scientists. By the 1940s and 1950s, there was essentially agreement that there was now a unitary “theory of evolution”. It was called the “Evolutionary Synthesis” or the “Modern Synthesis”. (The latter was the title of a book, published in 1942, by Sir Julian Huxley, a grandson of Thomas Huxley, one of Darwin’s great friends and colleagues).
It is often said, tongue-in-cheek, that failure is always an orphan (no one wants to admit responsibility) but that success tends to have many fathers (many people want to claim credit for a success). No literally truer example exists than the Evolutionary Synthesis, which indeed had many fathers. (Had the social structure of science been different, and healthier, there would undoubtedly have been founding mothers too but large contributions by women to evolutionary biology had to wait until the later 20th century.)
Did this success, which required nearly a century, produce a nice succinct formulation of the theory of evolution? Alas, no. It still required book-length treatments or, at least, long articles, to explain. Furthermore, soon many biologists were disputing various aspects of the Synthesis. The more trenchant critics even argued that it left out so much that it was not really a good explanation of how evolution occurs. These disputes went on for the remaining decades of the 20th century and into the 21st. Then, in the second decade of this century, a number of biologists from different disciplines came together and argued not that the Evolutionary Synthesis was false but that it was so incomplete that it should be replaced by what they called “The Extended Evolutionary Synthesis”. I will not give those ideas in detail here but we will return to this subject later.
The key point is that it is in error to speak of “the theory of evolution” (and indeed most biologists do not). There has never been just one version, except perhaps for the first few years after Darwin published The Origin of Species. Rather, there has been a series of such theories, each one built upon a previous version, with each addition adding new material that had to be considered if one wants to understand the evolutionary process as it occurs in Nature. The Modern Synthesis version of the 1940s was a real achievement but, of course, it had been formulated in ignorance of so much that was discovered later but which was highly relevant. To name the most obvious gap or hole in this respect: the nature of the genetic material, which was only first understood in the Watson-Crick model, published in 1953. In a sense, the Modern Synthesis marked a pause or even a truce in the long heated discussions about evolution that have marked biological science for the past 165 years. There is still no simple and succinct statement of “the theory of evolution”, unlike so many theories in physics, and probably there never will be4.
Yet, no one reading this article should take as its message that the idea of evolution is false or that our understanding today, however incomplete, is without value. On the contrary, our understanding is much richer than it ever was before and the field remains vital and exciting. The evidence that evolution has shaped life on this planet is overwhelming and we understand far better than earlier the multiple, varied factors that have driven it.
The current debates about evolution touch on two more fundamental problems in science. The first is: how do we think clearly about anything in the world? Second: when can we say that an idea has migrated from the realm of “theory” to become a “fact”? Underlying both questions is the presupposition that there is a “scientific method”, with prescribed rules that, when followed, ensure that true views of the subject will emerge. That method, as described in the textbooks, begins with observations, leads to hypotheses, which lead to experiments to test the idea. Ultimately, in this view, false ideas are winnowed out and good ones emerge triumphant. Would that it were that simple! Something like it is possible with phenomena that take place in the present or are made to take place at the will of the investigating scientists. Unfortunately, there are many phenomena that cannot be explored this way, chief amongst these being things that occurred in the past, either once or rarely. One can use logic and abductive reasoning to make “retrodictions”, that is predictions of what would have happened if one’s hypothesis were true, and then ask if the data about that past event bear them out. If the retrodictions hold up, one has more confidence in them, but still one waits for more evidence.
Perhaps one constructive step in this process would be to accept that there might not be neat “theories” for all the sciences. If theories in physics test, almost to the point of destruction, our capacity for understanding at the level of the ultra-small (sub-atomic phenomena) and the unimaginably large (the universe), biology is the realm of endless complexity and interconnectedness, with its own distinct challenges. The greater the elements in a complex phenomenon or situation, the harder it is to formulate neat explanations involving cause and effect, which essentially is what theories aim to be.
Even in physics, however, theories also change and evolve with time and even the undeniable successful ones have to cope with the fact of incompleteness. (Think of the endless failed attempts to unite relativity and quantum theory.) Perhaps the notion of “theory”, at least in biology, should be put into semi-retirement and used less. Instead, where there are some explanatory ideas but not a neatly stated theory, we could speak instead of a “theoretical framework” or “conceptual framework”. I, for one, would be perfectly happy to refer to current ideas about evolution as the “the general conceptual framework of evolutionary ideas today” without referring to the Modern Synthesis (too old-fashioned!), the “Evolutionary Synthesis” (ditto!) or “the Extended Evolutionary Synthesis” (which is still not universally accepted as the big replacement idea).
In closing, let me sum up with a general thought. The messiness of evolutionary theory today is not some unique flaw of this set of ideas but a reflection of the limits of scientific investigation. Certainty, in the form of indisputable proof, is not something that Science, as we know it, can deliver. It is enough that we do our best, with our clearest thinking, and then work honestly to validate or refute those ideas. That is what makes for good science. It should not aim for a final goal of definitive knowledge but be accepted as a Sisyphean process of endless effort. Like Sisyphus, with his task, we must be happy simply with the process, because it is the right thing to do5.
Supplementary reading
Mayr, E., Provine, W.B., eds. (1980). The Evolutionary Synthesis: perspectives on the unification of biology. Cambridge: Harvard University Press
This is the definitive, multi-author history of how the Evolutionary Synthesis, with its multiple sources came into being. Ernst Mayr was a major figure, probably the major figure in this effort, and W.B. Provine was an eminent historian of science.
Wilkins, A.S. (2008). NeoDarwinism. In Icons of Evolution, vol. 2 (ed. B. Regal). Westport: Greenwood Press.
This is a shorter (one chapter) history of the Synthesis and a more recent one than the Mayr-Provine opus. Its conclusion deals with how much was left out of the Evolutionary Synthesis of the 1940s and ‘50s.
Laland, K.N., Uller, T., Feldman, M.W. (2015). The extended evolutionary synthesis: its structure, assumptions and predictions. Proc. Roy. Soc. B. 282: 20152019.
A critique of the Evolutionary Synthesis, discussing what it did not include and how it needs to be expanded and updated to be an adequate explanation of evolutionary change on our planet. (It is a thoughtful and thought-provoking but needless to say, it has prompted counter-criticism.)
Abductive reasoning is the process of logical reference that seeks the simplest and most likely conclusions from a set of relationships; it employs generalization from inductive thinking and the process of logic. We owe this concept to Charles Pierce, the brilliant 19th century philosopher who first developed the philosophy of “pragmatism”.
One definition of the term “scientific theory”, adapted from Wikipedia, is as follows: a scientific theory is an explanation of an aspect of the natural world that can, in principle, be tested by means of experiments, in accordance with the rules of scientific investigation (“the scientific method”), using standard means of observation, measurement, and evaluation of the results. When experimental testing is not possible, theories are evaluated through abductive reasoning. Established scientific theories are those that have withstood rigorous scrutiny by these various means and embody accepted scientific knowledge.
Perhaps there will be found organisms on Earth that do not have the kinds of biomolecules that all cellular organisms on our planet possess, e.g. DNA and ATP, and hence would not have arisen from evolution on Earth. But we can say, with confidence. that the overwhelming majority of organisms, and probably all, did arise as products of evolution on our planet.
I think that the word “synthesis” in the terms “the Modern Synthesis” and the “Extended Evolutionary Synthesis” was a de facto recognition that the ideas were not a “theory” in the conventional sense but rather an acceptable intellectual framework, even if a somewhat baggy one, for thinking critically about evolution.
In “The Myth of Sisyphus”, Albert Camus discussed this situation, one where a desirable end point is never reached – Sisyphus ceaselessly rolling his rock up the hill – and where one has to be content with the process of doing the right thing. His book concludes with: “The struggle itself towards the heights is enough to fill a man’s head. One must imagine Sisyphus is happy.” Camus was not talking about science but his words have application to it.