This article is a revised version of Evolution in evolution (2019), written for the Magdalene College Magazine (2024–25).
The conception of Evolution as proceeding through the gradual transformation of masses of individuals by the accumulation of impalpable changes is one that the study of genetics shows immediately to be false. Once for all, that burden so gratuitously undertaken in ignorance of genetic physiology by the evolutionists of the last century may be cast into oblivion.
William Bateson (1909), p. 289
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The first edition of Charles Darwin’s On the Origin of Species by Means of Natural Selection (1859). (Credit: Scott Thomas Images.) |
THERE IS A WIDESPREAD popular construction of scientific revolutions as singular events which unfold, bolt-like and final, in the blink of an eye. Names like Galileo, Newton or Einstein are typically invoked as those of mythical figures with a miraculous capacity single-handedly to transform the way we see the world. It appears, however, that the kind of sweeping, dramatic breaks of paradigm which we have come to associate with scientific revolutions are rather hard to come across today. One may speculate — and be forgiven for it — that this may be the result of certain changes in the nature of academic work, by which the progress of research has been throttled to make space for an ever-swelling volume of inescapable paperwork. But the truth is that, rather than stalling, scientific progress is now considerably faster than it has ever been. The actual reason why sharp and sudden scientific revolutions of the kind encountered in popular science books are nowhere to be found today, is that such events are not revolutions in the usual sense of the word. Rather than cataclysmic changes, these are painfully protracted processes which require decades of cumulative scientific work to mature and develop. While both science popularisers and scientists themselves — not to mention the film industry — are very often guilty of misrepresenting scientific discoveries by filtering them through an almost Wagnerian dramatic lens, the reality is that, academics being sceptical and proud creatures by nature, every great conceptual shift must be slowly percolated, rather than poured, into the pool of accepted knowledge. To name but one example, the double-helix structure of the DNA molecule, now hailed as the central biological breakthrough of the second half of the twentieth century, was regarded by many as little more than a theoretical possibility years after it was first proposed. Even Sir Isaac Newton, that weary archetype of supernatural scientific genius, had to endure a decades-long intellectual war of attrition with his Continental competitors before his law of universal gravitation became widely accepted outside Britain.
Among the documented cases of gradually unfolding scientific revolutions, one stands out for being both particularly interesting and surprisingly obscure; this is the story of how Charles Darwin’s theory of evolution by natural selection came to be the main unifying idea of biology. Contrary to popular belief, this was no swift revolution, but rather a drawn-out process of fierce scholarly debate which began with the publication of Darwin’s ideas in 1859, and which would not relent until the late 1940s. During this period, the differentiation of biology into several new disciplines created the conditions for a chasm to grow between classically trained naturalists and a new breed of experimental biologists. As a result, evolutionary thought split into two mutually opposed currents which would only be reconciled with the eventual development of a unified theory of evolution.
During his lifetime, Darwin witnessed his theory of natural selection gain acceptance and esteem among a small circle of naturalists and evolutionary biologists. This cadre of early Darwinians included Alfred Newton, the first Professor of Zoology at Cambridge, who wrote: ‘I never doubted for one moment, then nor since, that we had one of the grandest discoveries of the age — a discovery all the more grand because it was so simple’ (Newton, 1888, p. 244). This limited success notwithstanding, Darwin never experienced the ultimate development of his theory into the undisputed cornerstone of biology which it is today — a status best encapsulated by Theodosius Dobzhansky’s famous aphorism that ‘Nothing in biology makes sense except in the light of evolution’. In fact, it might be difficult for present-day biologists even to conceive the extremes of opposition which so-called ‘Darwinism’ faced throughout the late nineteenth century, and until as late as the 1930s.
At the time, Darwinism was only one among a number of discordant theories attempting to explain the processes whereby biological species develop. Some of these, now referred to as ‘essentialist’ theories, were built on a notion of species as uniform ‘lines’ of virtually identical individuals, each made in the image of an unchanging ‘essence’ (a concept plainly borrowed from Platonism). Essentialist thinking therefore rejected the existence of significant natural variation within a species. On the other hand, ‘populationist’ theories viewed species as populations composed of distinct, unique individuals, and thus inevitably carrying a substantial degree of natural biological variation; examples of such variation could be differences in adult size, coat colour or leaf shape. Furthermore, some theories presumed the existence of ‘soft inheritance’, characterised by the notion that the hereditary material (what we now call ‘genes’) can be altered to some extent through the interaction of the organism with its environment. Lamarck’s theory of evolution by inheritance of acquired characters stands out as a notorious example of this current, positing that any physiological changes acquired by an individual during its life will be inherited by its descendants. Other theories, in contrast, admitted only ‘hard inheritance’, by which the hereditary material cannot be modified through interaction with the environment, meaning that the characters acquired by an individual during its own life are not passed to its offspring. Modern biology has supplied overwhelming evidence against the notion of soft inheritance; we know that, at least in animals, the germ cells which transmit an individual’s genes to the next generation are sequestered away from other tissues, such that environmental modification of the genetic material in these cells is prevented. (This does not include systemic exposure to certain aggressive agents not normally found in nature, such as X-rays and chemical carcinogens, which are known to induce changes to the germ cells’ DNA; furthermore, while recent discoveries of heritable epigenetic changes in some species have been argued to challenge the notion of strict hard inheritance, the validity of such arguments is still under debate.) It might therefore come as a surprise that nearly all the early theories of evolution, including Darwin’s, allowed some degree of soft inheritance. In particular, Darwinism originally assumed a certain plasticity of the genetic material, such that it could be modified to an extent through the use or disuse of certain organs during life; Darwin believed that such a process would assist natural selection in allowing species to adapt effectively to their environment. Some of Darwin’s supporters, notably the biologists August Weismann and Alfred Russell Wallace, would later develop an elaboration of Darwin’s theory known as ‘neo-Darwinism’, which definitely rejected the possibility of any kind of soft inheritance. Through his own extensive studies of natural populations in Southeast Asia, Wallace had independently arrived at a theory of evolution which was fundamentally similar, though less developed, than Darwin’s; it was knowledge of this fact which finally spurred Darwin to publish the theory on which he had been quietly working for two decades. Before the publication of Darwin’s book, Darwin and Wallace (1858) decided to present a summary of their conclusions in a joint communication to the Linnean Society.
Based on principles such as soft and hard inheritance, essentialism and populationism, a diverse array of evolutionary theories was put forward between the 1860s and the 1940s, of which Darwinism was seldom among the favourites. The chief factor compelling authors to support one theory over another was their particular field of expertise, and the number and variety of such fields within biology was expanding as never before, with emergent disciplines including embryology, cytology and ecology. Yet, from the standpoint of evolutionary thought, one of these new sciences was undoubtedly more impactful than any other: the science of genetics, born out of the unexpected rediscovery of Gregor Mendel’s laws of biological inheritance in the year 1900. The early geneticists built on the knowledge recovered from Mendel’s writings and began developing a detailed understanding of the principles of genetic mutation and inheritance. The spark of this new understanding, however, far from kindling any concerted progress in evolutionary biology, would serve to ignite a long and vicious conflict among the different biological disciplines.
From the outset, the founding fathers of genetics stood in opposition to Darwin’s idea of natural selection as the main driving force in evolution. Both the first geneticists and the earlier palaeontologists interpreted their own observations as being plainly in accordance with the hypothesis that new biological forms emerge by means of discontinuous change, or ‘mutation’. A mutation was defined as a discrete modification of the genetic material causing a visible and often disruptive physiological change in the organism. Such events, the geneticists argued, would sometimes result in the instantaneous transformation of an existing species into a new one, without the production of intermediate forms. This theory, which drew implicitly on essentialist principles, was known as ‘saltationism’ because of its belief in speciation by ‘saltation’ — a large evolutionary leap leading from one form to another. It provided a counterpoint to Darwin’s theory, which relied on a ‘gradualist’ conception of evolution derived from populationist thinking, whereby species gave rise to new species in a gradual manner, through a continuous succession of intermediate forms. Outlandish as it may sound today, saltationism fitted the experimental observations of geneticists, as well as prior palaeontological evidence, outstandingly well. The extreme sparsity of the fossil record meant that palaeontologists could never witness a continuous progression of forms linking two related species, whereas geneticists were accustomed to working with uniform stocks of nearly identical individuals — typically plants or mice — as a means of minimising experimental interference. The mutants produced in these genetic experiments presented dramatic physical modifications which were inherited by their offspring in accordance to Mendel’s laws. It seemed logical, then, to suppose that mutations such as these, infrequent but highly disruptive events, were the force behind the origin of new species. In the geneticists’ defence, it must be pointed out that we now know of cases where new species have indeed emerged through a singular genetic alteration, such as the duplication of the entire genome in some plants. The idea of speciation by saltation is therefore not impossible, but saltationism as a theory lacks the generality required to explain the evolution of most known species.
Furthermore, there was an additional problem plaguing Darwinism. The physiological basis of inheritance was entirely unknown in the nineteenth century, and Darwin had implicitly made recourse to a theory known as ‘blending inheritance’, according to which an organism’s constitution is a smooth average of its parents’ constitutions. The rediscovery and confirmation of Mendel’s work quickly proved that inheritance does not operate in this way, but rather through the segregation of discrete, individual genes from parent to offspring. Indeed, it could be shown mathematically that blending inheritance would lead to a situation where every individual in a species would display the exact same form of every trait, rendering evolution impossible. Geneticists thus argued that Darwin’s entire notion of gradual evolution, based on continuous variation, blending inheritance and natural selection, was simply untenable in the light of their experimental results. Some of the most distinguished early geneticists, including T. H. Morgan and William Bateson — the latter of whom translated Mendel’s work into English and coined the very term ‘genetics’ — went so far as to declare that genetics had finally put an end to Darwinism (see Bateson’s words at the beginning of this article). It should be borne in mind, however, that genetics was itself a controversial discipline at the time, composed of multiple competing strands; and the early geneticists, or ‘Mendelians’, were just as anxious to establish the validity of their own views on heredity as were the Darwinians to see their evolutionary ideas vindicated. Moreover, in spite of their opposition to Darwinism, the contributions of this first generation of geneticists — most notably the elucidation of the laws of heredity, the discovery of genes and chromosomes, and the refutation of the notion of soft inheritance — would ultimately prove essential to the refinement of evolutionary theory.
In contrast to the geneticists, those biologists who had been trained as naturalists, including zoologists and botanists, were used to deriving their conclusions from the direct study of natural populations, and they insisted that their observations of natural diversity were in perfect agreement with Darwin’s theory of gradual evolution through natural selection. The true root of the disagreement probably lay in the utter lack of communication between the two camps: naturalists and geneticists not only held competing theories, but also followed very distinct approaches to scientific enquiry, pursued divergent biological questions, attended different meetings, read and published in different journals, and even employed distinct vocabulary, including incompatible definitions for such fundamental terms as ‘species’ and ‘mutation’. In addition, geneticists appeared to view naturalists as speculation-lovers who were incapable of subjecting their ideas to proper testing, while naturalists had a tendency to dismissing geneticists as narrow-minded experimentalists who lacked experience of real natural populations. Misunderstanding and resentment compounded easily under such an atmosphere, gradually carving an ever deeper chasm between both disciplines. Astonishing proof of this circumstance comes from the fact that, when a younger generation of theoretical and experimental geneticists — including Sir Ronald Fisher, J. B. S. Haldane, Sewall Wright and H. J. Muller — began to obtain, from the late 1910s, fresh results demonstrating how the accumulation of effects from many discretely inherited genes can give rise to the continuous diversity described by naturalists (see figure below), and therefore how Mendelism and neo-Darwinism were in fact compatible, this did little to bridge the huge divide between geneticists and naturalists. Instead, because of the alienation brought about by constant hostility, scholarly communication was impaired to such an extent that the naturalists would spend decades persevering in their efforts to refute the already obsolete ideas of the earlier geneticists.
Among the documented cases of gradually unfolding scientific revolutions, one stands out for being both particularly interesting and surprisingly obscure; this is the story of how Charles Darwin’s theory of evolution by natural selection came to be the main unifying idea of biology. Contrary to popular belief, this was no swift revolution, but rather a drawn-out process of fierce scholarly debate which began with the publication of Darwin’s ideas in 1859, and which would not relent until the late 1940s. During this period, the differentiation of biology into several new disciplines created the conditions for a chasm to grow between classically trained naturalists and a new breed of experimental biologists. As a result, evolutionary thought split into two mutually opposed currents which would only be reconciled with the eventual development of a unified theory of evolution.
During his lifetime, Darwin witnessed his theory of natural selection gain acceptance and esteem among a small circle of naturalists and evolutionary biologists. This cadre of early Darwinians included Alfred Newton, the first Professor of Zoology at Cambridge, who wrote: ‘I never doubted for one moment, then nor since, that we had one of the grandest discoveries of the age — a discovery all the more grand because it was so simple’ (Newton, 1888, p. 244). This limited success notwithstanding, Darwin never experienced the ultimate development of his theory into the undisputed cornerstone of biology which it is today — a status best encapsulated by Theodosius Dobzhansky’s famous aphorism that ‘Nothing in biology makes sense except in the light of evolution’. In fact, it might be difficult for present-day biologists even to conceive the extremes of opposition which so-called ‘Darwinism’ faced throughout the late nineteenth century, and until as late as the 1930s.
At the time, Darwinism was only one among a number of discordant theories attempting to explain the processes whereby biological species develop. Some of these, now referred to as ‘essentialist’ theories, were built on a notion of species as uniform ‘lines’ of virtually identical individuals, each made in the image of an unchanging ‘essence’ (a concept plainly borrowed from Platonism). Essentialist thinking therefore rejected the existence of significant natural variation within a species. On the other hand, ‘populationist’ theories viewed species as populations composed of distinct, unique individuals, and thus inevitably carrying a substantial degree of natural biological variation; examples of such variation could be differences in adult size, coat colour or leaf shape. Furthermore, some theories presumed the existence of ‘soft inheritance’, characterised by the notion that the hereditary material (what we now call ‘genes’) can be altered to some extent through the interaction of the organism with its environment. Lamarck’s theory of evolution by inheritance of acquired characters stands out as a notorious example of this current, positing that any physiological changes acquired by an individual during its life will be inherited by its descendants. Other theories, in contrast, admitted only ‘hard inheritance’, by which the hereditary material cannot be modified through interaction with the environment, meaning that the characters acquired by an individual during its own life are not passed to its offspring. Modern biology has supplied overwhelming evidence against the notion of soft inheritance; we know that, at least in animals, the germ cells which transmit an individual’s genes to the next generation are sequestered away from other tissues, such that environmental modification of the genetic material in these cells is prevented. (This does not include systemic exposure to certain aggressive agents not normally found in nature, such as X-rays and chemical carcinogens, which are known to induce changes to the germ cells’ DNA; furthermore, while recent discoveries of heritable epigenetic changes in some species have been argued to challenge the notion of strict hard inheritance, the validity of such arguments is still under debate.) It might therefore come as a surprise that nearly all the early theories of evolution, including Darwin’s, allowed some degree of soft inheritance. In particular, Darwinism originally assumed a certain plasticity of the genetic material, such that it could be modified to an extent through the use or disuse of certain organs during life; Darwin believed that such a process would assist natural selection in allowing species to adapt effectively to their environment. Some of Darwin’s supporters, notably the biologists August Weismann and Alfred Russell Wallace, would later develop an elaboration of Darwin’s theory known as ‘neo-Darwinism’, which definitely rejected the possibility of any kind of soft inheritance. Through his own extensive studies of natural populations in Southeast Asia, Wallace had independently arrived at a theory of evolution which was fundamentally similar, though less developed, than Darwin’s; it was knowledge of this fact which finally spurred Darwin to publish the theory on which he had been quietly working for two decades. Before the publication of Darwin’s book, Darwin and Wallace (1858) decided to present a summary of their conclusions in a joint communication to the Linnean Society.
Based on principles such as soft and hard inheritance, essentialism and populationism, a diverse array of evolutionary theories was put forward between the 1860s and the 1940s, of which Darwinism was seldom among the favourites. The chief factor compelling authors to support one theory over another was their particular field of expertise, and the number and variety of such fields within biology was expanding as never before, with emergent disciplines including embryology, cytology and ecology. Yet, from the standpoint of evolutionary thought, one of these new sciences was undoubtedly more impactful than any other: the science of genetics, born out of the unexpected rediscovery of Gregor Mendel’s laws of biological inheritance in the year 1900. The early geneticists built on the knowledge recovered from Mendel’s writings and began developing a detailed understanding of the principles of genetic mutation and inheritance. The spark of this new understanding, however, far from kindling any concerted progress in evolutionary biology, would serve to ignite a long and vicious conflict among the different biological disciplines.
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| Illustration of the inheritance of seed characters in pea (from Fig. 3 in Bateson 1909). A plant from a variety with green round seeds, when fertilised with pollen from a variety with yellow wrinkled seeds, produces yellow round (YR) seeds (F1). In genetic terms, this indicates that the characters of ‘yellowness’ and ‘roundness’ are both dominant. When crossed among themselves, however, the seeds borne by these new plants (F2) present a distribution of characters which is closely predicted by Mendel’s laws of inheritance. |
From the outset, the founding fathers of genetics stood in opposition to Darwin’s idea of natural selection as the main driving force in evolution. Both the first geneticists and the earlier palaeontologists interpreted their own observations as being plainly in accordance with the hypothesis that new biological forms emerge by means of discontinuous change, or ‘mutation’. A mutation was defined as a discrete modification of the genetic material causing a visible and often disruptive physiological change in the organism. Such events, the geneticists argued, would sometimes result in the instantaneous transformation of an existing species into a new one, without the production of intermediate forms. This theory, which drew implicitly on essentialist principles, was known as ‘saltationism’ because of its belief in speciation by ‘saltation’ — a large evolutionary leap leading from one form to another. It provided a counterpoint to Darwin’s theory, which relied on a ‘gradualist’ conception of evolution derived from populationist thinking, whereby species gave rise to new species in a gradual manner, through a continuous succession of intermediate forms. Outlandish as it may sound today, saltationism fitted the experimental observations of geneticists, as well as prior palaeontological evidence, outstandingly well. The extreme sparsity of the fossil record meant that palaeontologists could never witness a continuous progression of forms linking two related species, whereas geneticists were accustomed to working with uniform stocks of nearly identical individuals — typically plants or mice — as a means of minimising experimental interference. The mutants produced in these genetic experiments presented dramatic physical modifications which were inherited by their offspring in accordance to Mendel’s laws. It seemed logical, then, to suppose that mutations such as these, infrequent but highly disruptive events, were the force behind the origin of new species. In the geneticists’ defence, it must be pointed out that we now know of cases where new species have indeed emerged through a singular genetic alteration, such as the duplication of the entire genome in some plants. The idea of speciation by saltation is therefore not impossible, but saltationism as a theory lacks the generality required to explain the evolution of most known species.
Furthermore, there was an additional problem plaguing Darwinism. The physiological basis of inheritance was entirely unknown in the nineteenth century, and Darwin had implicitly made recourse to a theory known as ‘blending inheritance’, according to which an organism’s constitution is a smooth average of its parents’ constitutions. The rediscovery and confirmation of Mendel’s work quickly proved that inheritance does not operate in this way, but rather through the segregation of discrete, individual genes from parent to offspring. Indeed, it could be shown mathematically that blending inheritance would lead to a situation where every individual in a species would display the exact same form of every trait, rendering evolution impossible. Geneticists thus argued that Darwin’s entire notion of gradual evolution, based on continuous variation, blending inheritance and natural selection, was simply untenable in the light of their experimental results. Some of the most distinguished early geneticists, including T. H. Morgan and William Bateson — the latter of whom translated Mendel’s work into English and coined the very term ‘genetics’ — went so far as to declare that genetics had finally put an end to Darwinism (see Bateson’s words at the beginning of this article). It should be borne in mind, however, that genetics was itself a controversial discipline at the time, composed of multiple competing strands; and the early geneticists, or ‘Mendelians’, were just as anxious to establish the validity of their own views on heredity as were the Darwinians to see their evolutionary ideas vindicated. Moreover, in spite of their opposition to Darwinism, the contributions of this first generation of geneticists — most notably the elucidation of the laws of heredity, the discovery of genes and chromosomes, and the refutation of the notion of soft inheritance — would ultimately prove essential to the refinement of evolutionary theory.
In contrast to the geneticists, those biologists who had been trained as naturalists, including zoologists and botanists, were used to deriving their conclusions from the direct study of natural populations, and they insisted that their observations of natural diversity were in perfect agreement with Darwin’s theory of gradual evolution through natural selection. The true root of the disagreement probably lay in the utter lack of communication between the two camps: naturalists and geneticists not only held competing theories, but also followed very distinct approaches to scientific enquiry, pursued divergent biological questions, attended different meetings, read and published in different journals, and even employed distinct vocabulary, including incompatible definitions for such fundamental terms as ‘species’ and ‘mutation’. In addition, geneticists appeared to view naturalists as speculation-lovers who were incapable of subjecting their ideas to proper testing, while naturalists had a tendency to dismissing geneticists as narrow-minded experimentalists who lacked experience of real natural populations. Misunderstanding and resentment compounded easily under such an atmosphere, gradually carving an ever deeper chasm between both disciplines. Astonishing proof of this circumstance comes from the fact that, when a younger generation of theoretical and experimental geneticists — including Sir Ronald Fisher, J. B. S. Haldane, Sewall Wright and H. J. Muller — began to obtain, from the late 1910s, fresh results demonstrating how the accumulation of effects from many discretely inherited genes can give rise to the continuous diversity described by naturalists (see figure below), and therefore how Mendelism and neo-Darwinism were in fact compatible, this did little to bridge the huge divide between geneticists and naturalists. Instead, because of the alienation brought about by constant hostility, scholarly communication was impaired to such an extent that the naturalists would spend decades persevering in their efforts to refute the already obsolete ideas of the earlier geneticists.
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Illustration of Fisher’s (1918) ‘infinitesimal model’, explaining the emergence of continuous biological variation from the combined contribution of a large number of discrete Mendelian genes, or loci. Each row in the diagram presents a simulated distribution of population values for a trait determined by an increasing number of individual genes. The bars on the left-hand side indicate the individual effect of each gene contributing to the trait (ranging from only two genes in the top case to 500 in the bottom case). The right-hand side provides the corresponding distributions of trait values in the simulated population, showing how the values for a trait become more normally distributed as the number of genes increases. This explains why many physiological characters in humans and other species follow a normal (or Gaussian) distribution. (Credit: Chamaemelum/Wikimedia Commons.) |
In this way, naturalists and geneticists would go on treading along separate paths for the first three decades of the twentieth century, each dragging their own conceptual burdens: the naturalists held obsolete views about the nature of genetic mutation and inheritance; the geneticists were hampered by saltationist views and by the belief that the evolution of species could be understood by extrapolation from the evolution of single mutations in experimental settings. Even as late as the 1930s, when crucial experiments in artificial selection, together with the work of the first mathematical geneticists, were demonstrating beyond any doubt the reality of evolution by natural selection, specialist textbooks still listed up to six potentially correct theories of evolution.
This stagnant atmosphere would finally be cleared in the 1940s, mainly through the insight of one palaeontologist, George Simpson, and two zoologists, Julian Huxley and Bernhard Rensch. Perhaps the only scientists of their generation who had amassed a profound knowledge of the latest advances in each of the relevant disciplines, they published three independent books (Huxley, 1942; Simpson, 1944; Rensch, 1947) describing how the findings of zoologists, botanists, geneticists, palaeontologists and others could be integrated into one self-consistent theoretical framework which could explain the entire evolutionary process. In his book (which happened to be published first due to circumstances arising from the Second World War), Huxley christened this new theory with the name by which it is known today: the ‘modern evolutionary synthesis’. The modern synthesis states that the gradual evolution of species can be explained in terms of the accumulation of myriad genetic mutations with generally small effects, in conjunction with recombination (the shuffling of genetic material as it is passed from parent to offspring), and the action of both natural selection and stochastic processes on the genetic diversity produced by mutation and recombination. One key feature of the theory is that it explains how these low-level genetic and selective mechanisms give rise to high-level evolutionary processes, including the origin of species, genera and higher taxonomic levels.
The forging of the modern evolutionary synthesis was not in itself a scientific revolution, but rather the conclusion of a protracted paradigm shift initiated by Darwin and Wallace nearly a century earlier. Such a conclusion did not entail the victory of one scientific tradition over another, but the fusion of two radically different conceptual frameworks — naturalism and experimentalism — into one whole. For such a milestone to arrive, a number of obstacles, grown through the persistent isolation between the opposing camps, first had to be cleared up. In the end, this was achieved by those who, rather than focusing on their own narrow specialism, were sufficiently curious to learn about the advances made in other fields, and sufficiently open-minded to notice the commonalities latent underneath the conflict. The legacy of the modern synthesis was the unification of evolutionary biology into a single field; after its arrival, the discord and hostility which had reigned for half a century gave way to widespread agreement. And the bridges erected then would remain solidly in place until the present day: although there is still debate regarding particular aspects of the theory — such as the conceptual implications of epigenetic memory and horizontal exchange of genes between organisms — the basic framework of the synthesis has remained essentially intact since the 1940s.
The history of the modern evolutionary synthesis, our current framework for understanding evolution, is of value to scientists and historians alike. The long series of discoveries and conceptual advances linking Darwin’s original theory to our unified interpretation of the evolutionary process provides an illuminating example of the consequences of such phenomena as have manifested themselves time and again in the history of science: resistance to new ideas, deficient communication compounded by semantic differences, and excessive specialisation leading to tribalistic sentiments of superiority towards foreign disciplines. Hopefully, this story also offers a lesson in how exploring the history of scientific ideas allows much deeper understanding than the mere study of their definitions; for while definitions carry a pretension to simplicity and finality, the history of science conveys the truth that science is a living process, the progress of which is fundamentally arduous, incremental, and positively fraught with quarrel.
This stagnant atmosphere would finally be cleared in the 1940s, mainly through the insight of one palaeontologist, George Simpson, and two zoologists, Julian Huxley and Bernhard Rensch. Perhaps the only scientists of their generation who had amassed a profound knowledge of the latest advances in each of the relevant disciplines, they published three independent books (Huxley, 1942; Simpson, 1944; Rensch, 1947) describing how the findings of zoologists, botanists, geneticists, palaeontologists and others could be integrated into one self-consistent theoretical framework which could explain the entire evolutionary process. In his book (which happened to be published first due to circumstances arising from the Second World War), Huxley christened this new theory with the name by which it is known today: the ‘modern evolutionary synthesis’. The modern synthesis states that the gradual evolution of species can be explained in terms of the accumulation of myriad genetic mutations with generally small effects, in conjunction with recombination (the shuffling of genetic material as it is passed from parent to offspring), and the action of both natural selection and stochastic processes on the genetic diversity produced by mutation and recombination. One key feature of the theory is that it explains how these low-level genetic and selective mechanisms give rise to high-level evolutionary processes, including the origin of species, genera and higher taxonomic levels.
The forging of the modern evolutionary synthesis was not in itself a scientific revolution, but rather the conclusion of a protracted paradigm shift initiated by Darwin and Wallace nearly a century earlier. Such a conclusion did not entail the victory of one scientific tradition over another, but the fusion of two radically different conceptual frameworks — naturalism and experimentalism — into one whole. For such a milestone to arrive, a number of obstacles, grown through the persistent isolation between the opposing camps, first had to be cleared up. In the end, this was achieved by those who, rather than focusing on their own narrow specialism, were sufficiently curious to learn about the advances made in other fields, and sufficiently open-minded to notice the commonalities latent underneath the conflict. The legacy of the modern synthesis was the unification of evolutionary biology into a single field; after its arrival, the discord and hostility which had reigned for half a century gave way to widespread agreement. And the bridges erected then would remain solidly in place until the present day: although there is still debate regarding particular aspects of the theory — such as the conceptual implications of epigenetic memory and horizontal exchange of genes between organisms — the basic framework of the synthesis has remained essentially intact since the 1940s.
The history of the modern evolutionary synthesis, our current framework for understanding evolution, is of value to scientists and historians alike. The long series of discoveries and conceptual advances linking Darwin’s original theory to our unified interpretation of the evolutionary process provides an illuminating example of the consequences of such phenomena as have manifested themselves time and again in the history of science: resistance to new ideas, deficient communication compounded by semantic differences, and excessive specialisation leading to tribalistic sentiments of superiority towards foreign disciplines. Hopefully, this story also offers a lesson in how exploring the history of scientific ideas allows much deeper understanding than the mere study of their definitions; for while definitions carry a pretension to simplicity and finality, the history of science conveys the truth that science is a living process, the progress of which is fundamentally arduous, incremental, and positively fraught with quarrel.
References
Bateson, W. (1909). Mendel’s Principles of Heredity (Cambridge University Press).
Darwin, C., Wallace, A.R. (1858). On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection. Journal of the Proceedings of the Linnean Society of London. Zoology, 3 (9): 45–62.
Darwin, C. (1859). On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life (John Murray).
Fisher, R.A. (1918). The Correlation between Relatives on the Supposition of Mendelian Inheritance. Transactions of the Royal Society of Edinburgh, 52 (2): 399–433.
Huxley, J. (1942). Evolution: The Modern Synthesis (Allen and Unwin).
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).
Newton, A. (1888). Early days of Darwinism. Macmillan’s Magazine, 57: 241–249.
Rensch, B. (1947). Neuere Probleme der Abstammungslehre (Enke).
Simpson, G.G. (1944). Tempo and Mode in Evolution (Columbia University Press).
Bateson, W. (1909). Mendel’s Principles of Heredity (Cambridge University Press).
Darwin, C., Wallace, A.R. (1858). On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection. Journal of the Proceedings of the Linnean Society of London. Zoology, 3 (9): 45–62.
Darwin, C. (1859). On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life (John Murray).
Fisher, R.A. (1918). The Correlation between Relatives on the Supposition of Mendelian Inheritance. Transactions of the Royal Society of Edinburgh, 52 (2): 399–433.
Huxley, J. (1942). Evolution: The Modern Synthesis (Allen and Unwin).
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).
Newton, A. (1888). Early days of Darwinism. Macmillan’s Magazine, 57: 241–249.
Rensch, B. (1947). Neuere Probleme der Abstammungslehre (Enke).
Simpson, G.G. (1944). Tempo and Mode in Evolution (Columbia University Press).
