divulgatum está dedicado a la difusión de conocimiento científico en español e inglés, mediante artículos que tratan en detalle toda clase de temas fascinantes pero poco conocidos.
divulgatum is devoted to the dissemination of scientific knowledge in Spanish and English, through articles that go into the details of all sorts of fascinating, if not
widely known, subjects.

Thursday, October 3, 2019

Evolution in evolution

The story of how we came to understand evolution is a fascinating example of the real character of scientific revolutions.

The first edition of On the Origin of Species, published by Charles Darwin in 1859.
(Credit: Scott Thomas Photography.)

IT IS NATURAL to imagine scientific revolutions as events that unfold in the blink of an eye, like thunderbolts of blinding truth. We tend to depict the likes of Galileo, Newton, Darwin and Einstein as bold figures who utterly and single-handedly transformed our understanding of the world, to everyone’s shock and awe. It is rather hard to find, however, quite so many examples of such breathtaking shifts of paradigm developing nowadays. This is not, of course, because scientific research has in any way stalled; in fact, its progress is now faster and more impressive than ever before. The true reason that we cannot recall too many contemporary scientific revolutions is that these are not actually revolutions as such — sudden, dramatic transformations — but rather long processes which normally evolve and develop over decades. While popular narratives like to depict key scientific discoveries in an overly dramatic atmosphere of climax, enlightenment and triumph, the reality is that, because scientists are sceptical by nature, every great change of paradigm has taken many years to become accepted. All ideas in science, even the most transformative ones, are subjected to a considerably slow-paced process of academic scrutiny, and, if possible, experimental validation, until they gradually become part of the established scientific canon. The double-helical structure of DNA, for instance, despite being so highly praised today, was regarded as little more than a possibility for years after it was first proposed by Watson and Crick. Even Newton, the archetype of scientific genius and achievement, had to go through decades of bitter intellectual competition before his law of universal gravitation became widely recognised outside of England.

Among the many possible narratives of gradually unfolding scientific revolutions, one is arguably as fascinating as it is obscure: this is the story of how Charles Darwin’s famed theory of evolution by natural selection became the supreme dogma of biology (a story brilliantly recounted by Ernst Mayr in the 1980 book The Evolutionary Synthesis). Contrary to popular belief, this occurred not as a swift change of paradigm, but as a protracted process of fierce scholarly debate which would not come to an end until the mid-1940s, almost a century after Darwin first published his theory in 1859. During this time, the forking of biology into new fields of knowledge prompted an irreconcilable divide between classically trained naturalists and experimental biologists, leading to a pervasive chasm in evolutionary thought that would only be bridged with the forging of a new unified theory of evolution. This theory, which we know today as the modern evolutionary synthesis, states that the gradual evolution of species can be explained in terms of the accumulation of small genetic alterations, recombination (the shuffling of the genetic material as it passes from parents to offspring), and the action of natural selection on this genetic diversity. The key feature of the modern synthesis is that it explains how these low-level mechanisms give rise to higher-level evolutionary processes, such as speciation and macroevolution.

In his lifetime, Darwin saw his theory gain acceptance and regard among some naturalists, but he never witnessed its ultimate development into the unquestionable pillar of biology which it is today. In fact, it might be difficult for us to conceive the extreme opposition which the Darwinian theory of evolution faced throughout the late nineteenth century, and until as late as the 1930s. At the time, Darwinism was only one of a number of different theories which attempted to explain the processes whereby biological species originate. Some of these, known as essentialist theories, were based on the notion that species were uniform ‘pure lines’ composed of virtually identical individuals, and thus claimed that no significant natural variation existed within each species. In contrast, populationist theories interpreted species as populations composed of distinct, unique individuals, and therefore harbouring a considerable amount of biological variation. Furthermore, some theories accepted the existence of soft inheritance, characterised by the notion that the genetic material can be modified to some extent by the interaction of the organism with its environment; a particularly notorious example of this current was Lamarck’s theory of the inheritance of acquired characters, which posited that the physiological changes acquired by an individual during its lifetime are inherited by its descendants. Other theories recognised only hard inheritance, meaning that the hereditary material cannot be altered by the action of the environment. Today, we consider soft inheritance to be false, and the genetic material to be immutable by means of interaction with the environment (although recent discoveries of heritable epigenetic variation in some species may potentially pose a challenge to this idea). It might therefore appear surprising that nearly all of the early theories of evolution, including Darwinism, recognised some degree of soft inheritance. In particular, Darwin’s theory assumed a certain ‘plasticity’ of the genetic material, such that it could be modified to some extent through the use or disuse of biological traits. Neo-Darwinism, an elaboration of the theory developed by some of Darwin’s supporters, among them the naturalist Alfred Russel Wallace, later rejected this possibility for an inheritance of acquired characters, adopting a hard-inheritance standpoint.

Based on these and other principles, a wide array of evolutionary theories were developed between the 1860s and the 1940s, of which Darwinism was seldom among the favourites. The main factor which compelled authors to support one theory or another was their field of expertise, and the number of these was growing as never before. Over the second half of the nineteenth century, the broad science of biology, theretofore split into the disciplines of zoology and botany, rapidly differentiated into several new fields, including embryology, cytology and ecology. From the viewpoint of evolutionary theory, however, the most influential of the new disciplines was arguably genetics, the study of genes and heredity, which was born from the rediscovery of Gregor Mendel’s laws of genetic inheritance in 1900. From this year onwards, geneticists would develop an increasing understanding of the principles of mutation and inheritance; this fresh knowledge, however, rather than sparking advances in the study of evolution, would ignite a long and vicious conflict between the different biological disciplines.

From the outset, the founding fathers of genetics were firm opposers of Darwinism and natural selection. The very first geneticists, together with the palaeontologists, thought that the emergence of new species happened by means of discontinuous changes; in their view, an isolated and disruptive modification of the genetic material (which they termed a mutation) produced a radical physiological change in the organism, resulting in the instantaneous transformation of one species into another, without any intermediates. This theory, based on essentialist principles, was known as saltationism, because of its belief in speciation by ‘saltation’, or huge evolutionary leaps leading from one species to the next. Bizarre as it may sound today, this explanation fitted the initial observations of geneticists, as well as previous palaeontological evidence, remarkably well. In their experiments, geneticists relied on uniform ‘stocks’ of nearly identical individuals (normally fruit flies, which were relatively easy to breed and study), as a means of avoiding experimental interference. In their stocks of flies, the early geneticists observed that isolated genetic mutations led to drastic, heritable modifications of traits such as eye colour or wing shape. It seemed reasonable, then, to suppose that evolution proceeded in the same manner: mutations were infrequent and highly disruptive events, causing the instantaneous transformation of one species into another. Gradual evolution by means of natural selection acting on existing natural variation within a species appeared to be in complete contradiction with these early results, and some geneticists went as far as to declare that Darwinian evolution had been positively disproved by genetics. Notwithstanding these misconceptions, genetics made some significant contributions to evolutionary theory during this period, most notably the refutation of the existence of soft inheritance.

On the other hand, those biologists who had been trained as naturalists, such as zoologists and botanists, were used to deriving their conclusions directly from the study of natural populations, and insisted that all their observations supported Darwin’s theory of gradual evolution, rather than saltationism. The root of the disagreement, however, was certainly the lack of communication between both fields: naturalists and geneticists did not only defend different theories, but also had distinct approaches to science, pursued divergent biological interests, attended different meetings, published in different scientific journals, and even used distinct vocabulary (including incompatible definitions for essential concepts such as ‘species’ and ‘mutation’). In addition, geneticists tended to regard naturalists as speculative scholars who could never test their ideas in the laboratory, and therefore possessed no objectivity; naturalists, in turn, viewed geneticists as myopic experimentalists who had no real knowledge about natural populations, and were insensitive to the crucial difference between heredity and evolution. All this inevitably led to mounting misunderstanding and resentment, and perhaps more importantly, to an immense communication lag between both disciplines. An astonishing proof of this circumstance is given by the fact that, when a younger generation of geneticists — including names such as Hermann Muller, J.B.S. Haldane and Ronald Fisher — began to obtain, from the late 1910s onwards, new evidence against saltationism and in favour of neo-Darwinism and natural selection, this did not help to bridge the gap between geneticists and naturalists. Instead, because of the utter alienation brought about by the endless disagreements between both fields, communication was damaged to such an extent that naturalists would spend decades persevering to refute the already obsolete ideas of the earlier geneticists. It was mainly because of this abysmal academic segregation that the arrival of the modern evolutionary synthesis was deferred until the 1940s.

In this way, naturalists and geneticists progressed along isolated paths for the first three decades of the twentieth century, each dragging their own conceptual burdens: the former held wrong and obsolete views about the nature of genetic mutation and inheritance, while the latter were dominated by the belief that the evolution of species and higher taxonomic levels could be understood by simple extrapolation from knowledge about how single genes evolve in isolated, ideal populations. Up until the 1920s, when crucial experiments on artificial selection, together with the work of the first mathematical geneticists, contributed to establish a firm belief in natural selection, specialist textbooks still presented up to six theories of evolution as being potentially valid.

This bleak scientific panorama was completely transformed in the 1940s, thanks to the insight of one palaeontologist, George Gaylord Simpson, and two zoologists, Julian Huxley and Bernhard Rensch. Perhaps the only scientists of their generation who had amassed a detailed knowledge of all the latest advances in each of the relevant disciplines, they published three independent books in which they demonstrated how the findings of zoologists, palaeontologists, geneticists and others could be integrated in order to explain all of evolution, from the emergence of changes in individual genes to the origin of species, genera and higher levels, within a single consistent framework. In his book, Huxley christened this new theory with the name by which it is known today — the modern synthesis.

The forging of the modern evolutionary synthesis was not in itself a scientific revolution, but rather the completion of a shift of paradigm initiated by Darwin nearly a century earlier. Moreover, the synthesis did not imply the victory of one scientific tradition over another, but rather the fusion of two radically different conceptual frameworks — naturalism and experimentalism — into a new harmonious whole. For this fusion to arrive, it was first necessary to remove conceptual misunderstanding and communication barriers between the opposing camps, something that could only be achieved by those who, rather than focusing on narrow specialisation, were curious enough to learn about the advances made outside their own respective fields, and open-minded enough to appreciate commonality rather than disagreement. The real impact of the modern synthesis was the unification of evolutionary biology into a single field; after its arrival, the complete discord and hostility which had reigned over the three previous decades was replaced by widespread agreement. Bridges had been built which would remain solidly in place until the present day; although discussion is still ongoing regarding some aspects of the theory (such as the role of epigenetic inheritance and horizontal gene transfer), the basic framework of the synthesis has remained essentially untouched since it was first outlined in the 1940s.

The history of the modern synthesis, our current framework for studying evolution, is of value to scientists and historians alike. The long series of discoveries and conceptual advances that led from Darwin’s original theory to the arrival of a unified interpretation of evolution are a particularly informative illustration of phenomena which have manifested time and again throughout the history of science: resistance to new ideas, exceeding specialisation, terminological barriers, communication lags, sentiments of superiority and hostility between disciplines, and the critical importance of collaboration and mutual understanding for scientific advance. The story of the modern synthesis thus reflects the true method of scientific progress, which is of course harder, messier and more gradual than we like to imagine. It also constitutes a telling example of how exploring the history of scientific ideas provides us with a much deeper understanding than the mere study of their definitions; for while the latter pretend to be static and set in stone, the former conveys the truth that science is alive and restless, and that the search for knowledge is fundamentally arduous, incremental, collaborative, and eternal.

Mayr, E. (1980). ‘Some Thoughts on the History of the Evolutionary Synthesis’, in The Evolutionary Synthesis: Perspectives on the Unification of Biology (Harvard University Press).
Huxley, J. (1942). Evolution: The Modern Synthesis (Allen and Unwin).
Simpson, G. G. (1944). Tempo and Mode in Evolution (Columbia University Press).
Rensch, B. (1947). Neuere Probleme der Abstammungslehre (Enke).