Evolutionary Biology

Evolutionary Ideas: The Modern Synthesis

by Peter J Bowler

Introduction

In the early years of the twentieth century the Darwinian selection theory was still under threat. Many field naturalists and palaeontologists accepted non-Darwinian mechanisms such as Lamarckism, while the new Mendelian genetics was linked to the theory that new species were produced by sudden saltations. By the 1920s the hostility between the geneticists and the Darwinians was beginning to break down as the new discipline of population genetics emerged. Three biologists, R. A. Fisher, J. B. S. Haldane and Sewall Wright, played a major role in using population genetics to show that a theory of natural selection based on the genetical model of inheritance offered the most plausible explanation of how evolution worked. The geneticists had meanwhile discredited Lamarckism, forcing evolutionists more generally to reconsider the role of natural selection. At the same time, field studies had focused attention on the role of isolation and local adaptation, effects that could easily be explained in terms of the genetical selection theory. By the 1940s a number of evolutionists had begun to proclaim the emergence of a new evolutionary synthesis that would unify biology.

The synthesis has enjoyed the broad support of biologists ever since and has been the source of many important developments, including its application to the study of animal social behaviour via sociobiology. There have been numerous attempts to modify its underlying foundations, such as the theory of punctuated equilibrium, but also many efforts to insist that Darwinism really does provide a complete explanation of the development of all living things, including the human species. Yet the origin and true nature of the synthesis remain controversial. Some of the biologists actually involved in the synthesis began to interact with historians and philosophers interested in how this major development occurred, and many different perspectives have been offered. Did population genetics lead the way, or did the work of the field naturalists also play a major formative role? Was there a synthesis of two originally distinct theories (Darwinism and genetics), or is it more fruitful to see the new evolutionism as a successful unification of separate disciplines (genetics, field studies, palaeontology)? Was the degree of unity achieved really all that significant, or was the rhetoric of unity largely a device used to create a discipline of evolutionary biology that could stave off the challenge of the professionally more successful molecular biologists? Was it not so much a synthesis as a ‘constriction’ – an elimination of non-Darwinian ideas that still left a great deal of flexibility within a very loosely defined Darwinian framework? Has the synthesis ‘hardened’ its support for Darwinism since its inception? These debates about the origin of the synthesis are crucial for our understanding of its present significance in biology.

Population Genetics

In the last decade of the nineteenth century, the biometrical school under Karl Pearson and W. F. R. Weldon had developed sophisticated statistical tools for studying the variation of wild populations. They were convinced that natural selection acting on the range of natural variability in every population was the cause of evolution. Pearson was opposed by William Bateson, who argued that evolution was both nonadaptive and saltative – the position eventually identified with Hugo De Vries’ ‘mutation theory’. Bateson and other early supporters of Mendelian genetics retained their support for saltation and their opposition to Darwinism, and the hostility between Bateson and Pearson ruled out any hope of an immediate reconciliation between the two theories. In the 1920s a number of biologists began to explore the possibility of a synthesis, creating a science of population genetics that exploited the sophisticated mathematical techniques of biometry to study the effects of genetic variability in large populations. From this emerged the genetical theory of natural selection, effectively confirming that Darwin had been right to focus on gradual, adaptive evolution brought about by natural selection. Genetics now seemed to offer the perfect foundation for Darwinism, undermining the credibility of the old claim that the effects of a favourable variation would be diluted in a large population. Mendelism's particulate model of heredity guaranteed that genetic mutations breed true, and if their effects are favourable, the frequency of the mutated gene within the population's gene pool will increase. This was a crucial step in the foundation of the modern synthesis.

As early as 1902 G. Udney Yule had noted that Mendelism was not incompatible with the biometricians’ model of a continuous range of variation within the population. If a character was influenced by many different genes (not by one, as in Mendel's classic experiments), then the effects of the genes would blend together in a large population to give a continuous range of variation for that character. Yule was ignored in the heat of the debate between Bateson and Pearson, but over the next few decades a number of developments began to soften the geneticists’ support for De Vries’ and Bateson's claim that new species were founded instantaneously by large-scale mutations that bred true and thus formed a separate population. In America, Thomas Hunt Morgan's studies of true genetic mutations revealed that they did not form separate breeding populations, but merely added to the range of characters within the existing population. Many mutations had quite a small effect, while large macromutations were usually deleterious or even lethal. It seemed that the variability of the population was the result of mutations building up a fund of genetic differences. Originally a saltationist himself, Morgan now began to admit that selection must play a role in determining the ultimate fate of mutated genes. A gene that conferred some adaptive benefit would increase in frequency, while one that was disadvantageous would gradually be eliminated. Meanwhile the Swedish biologist H. Nilsson-Ehle and the American Edward East showed that many characters are affected by more than one Mendelian factor, paving the way for a recognition of Yule's point. In 1915 a study of mimicry in butterflies by R. C. Punnett was published with a table prepared by the mathematician H. T. J. Norton showing how rapidly a gene conferring adaptive advantage would spread into the population.

These insights became the foundation for the British contribution to population genetics. Ronald Aylmer Fisher was a Cambridge-trained mathematician who took a close interest in Pearson's biometrical techniques, but who also came to appreciate that the inheritance of variability in populations could be explained using Mendel's laws. His first paper on this was published in 1918, and over the next decade he developed a theoretical model for the action of natural selection on variation maintained in this way. His classic Genetical Theory of Natural Selection was published in 1930. Fisher assumed that selection acted uniformly and very slowly on large populations, gradually increasing the frequency of any gene conferring adaptive advantage to the organism. He believed that a large population favoured the action of selection by keeping up a high level of variability through mutation and recombination. At the same time J. B. S. Haldane developed a similar model of selection, although he used practical examples such as the development of industrial melanism to show that selection could act more quickly than Fisher supposed. The spread of a gene conferring a dark (melanic) colour in the peppered moth, Biston betularia, had occurred quite rapidly when the population was forced to adapt to a darker background in areas subject to heavy industrial pollution. The advantage conferred by better camouflage had a substantial effect on the gene's rate of reproduction.

In Russia, Sergei S. Chetverikov began to study the genetic variability of wild populations and concluded that, contrary to the ideas of the British school, variability would be expressed more freely in small populations. His school introduced the concept of the population's ‘gene pool’ – the reservoir of genetic variability maintained by mutation and capable of generating new phenotypic effects through recombination. Chetverikov's work influenced N. W. Timofeeff-Ressovsky, who moved to Germany in 1925 and did further work on the role of mutation in supplying the genetic variability of populations. Although not a member of the Chetverikov school, Theodosius Dobzhansky was aware of this work before he joined T. H. Morgan's team in America in 1927. (Russian genetics was coming under increasing threat from the activities of T. D. Lysenko, who exploited Lamarckian ideas now discredited in the West, but which could be linked more easily to Marxist philosophy. With Stalin's support, genetics and Darwinism were purged from Soviet biology until the 1950s.) In America, William E. Castle showed that inbreeding within small groups could elicit genetic variation concealed in large populations. His student Sewall Wright developed mathematical techniques different from those used in Britain to show how evolution could proceed in populations that were broken up into partially isolated groups. Initially, Wright believed that the random effect of ‘genetic drift’ within the subgroups could allow them to move between the ‘peaks’ of adaptive fitness for the population as a whole, thus promoting changes that would later be sifted by natural selection. Meanwhile, Dobzhansky worked with models of isolation and natural selection, and was particularly successful in translating the mathematics used by population genetics into terms that could be understood and exploited by field naturalists studying wild populations. Dobzhansky's Genetics and the Origin of Species of 1937 thus played a key role in bringing the experimentalists and the field workers together via the synthesis of Darwinism and genetics. In the following year, Dobzhansky and Wright began a long and fruitful collaboration to study the genetics of natural populations.

Systematics and Speciation

No one doubts the importance of the genetical theory of natural selection for the emergence of the evolutionary synthesis, but since 1959 the longest-surviving founder of the synthesis, Ernst Mayr, has conducted a campaign to argue that it is not the whole story. According to Mayr, field biologists working on geographical variation and adaptation were independently beginning to recognize the role of adaptive evolution and were instrumental in focusing attention onto local variation and the effects of evolution on isolated populations. Although many, including Mayr himself, began as Lamarckians in the 1920s, they increasingly realized that genetics had made the inheritance of acquired characters implausible and were thus ready to accept the models of evolution being offered by the population geneticists, especially once those models began to take localized isolation into account. The synthesis was thus a two-way affair, with major contributions from both the field workers and the mathematical population geneticists.

Mayr was trained in Germany when Darwinism was still ‘in eclipse’, and began his field work on the birds of New Guinea and the Solomon Islands under the influence of Bernhard Rensch, who was at the time a convinced Lamarckian. Mayr came to recognize the role of isolation in dividing species into subpopulations, which often had to be recognized as subspecies, or what had once been called ‘varieties’. He saw the link between these subspecies and the local climate, suggesting that adaptation to the prevailing conditions was the force that modified them to produce their distinctive characters. By the time he moved to America in 1930, Mayr had begun to appreciate that it was probably natural selection, not Lamarckism, that produced the changes. The influence of Dobzhansky showed him how the local variations could be studied in terms of the genetic differences built up by selection. Mayr's Systematics and the Origin of Species of 1942 showed the link between the patterns of relationship studied by systematics, the effects of geographical isolation, and the build-up of genetic differences between populations by natural selection. Geographical isolation was declared to be an essential first step in speciation (the division of one species into two or more), although ‘isolating mechanisms’ such as behavioural differences could maintain the separation between two populations even if they came back into contact with one another. Mayr explicitly rejected the renewed support for saltative evolution offered by Richard Goldschmidt – even if there was a ‘hopeful monster’ created by a macromutation, with whom would it be able to breed? Mayr and other supporters of the synthesis effectively marginalized Goldschmidt (another German refugee) within the American scientific community. Mayr has since led a campaign to insist that species must be viewed as populations of varying individuals – all traces of the old ‘typological’ viewpoint (in which the species is defined by an ideal type) must be eliminated from the new biology.

In Britain, E. B. Ford's Mendelism and Evolution of 1931 provided studies of the effects of selection on wild populations, confirming that the effect shown by industrial melanism was not an anomaly. Ford was in contact with Fisher, although his studies confirmed Haldane's point that selection could work more swiftly than Fisher expected. Fisher was also an important influence, along with T. H. Morgan, on Julian Huxley (grandson of Darwin's supporter, T. H. Huxley). Huxley had wide interests: he had studied embryology and growth rates, and had also made pioneering observations of animal behaviour showing the adaptive value of mating rituals in birds. He was aware of the prevailing anti-Darwinism of early twentieth century biology, and subsequently coined the phrase ‘eclipse of Darwinism’ to denote this episode. But Huxley turned to the selection theory early in his career, and in 1931 joined with the author H. G. Wells to write a popular survey of biology promoting Darwinism. Like Mayr, he became interested in the power of the new evolutionary theory to transform systematics by throwing light on the nature of species and the way speciation followed geographical isolation. The theories of the population geneticists could actually be checked against the evidence of how speciation worked. Huxley had deeper philosophical concerns – he wanted a theory of natural, yet progressive evolution that would both unify biology and provide the basis for an understanding of the human condition to replace what was once provided by religion. His 1942 book Evolution: the Modern Synthesis tried to provide an overview that would support these aims. Huxley continued to defend the claim that Darwinism gave progress in the long run, so that human values could be to some extent based on whether or not we contributed to nature's overall purpose. He even supported the theistic evolutionism of Pierre Teilhard De Chardin, which many biologists dismissed as mystical nonsense.

Palaeontology

Palaeontology had long been a hotbed of anti-Darwinian theorizing. In the early twentieth century many palaeontologists still thought that the fossil record for each group of animals displayed a multitude of lines evolving in parallel through the same pattern of development, as though driven by some common impulse. Lamarckism and orthogenesis were widely invoked to explain this parallelism, and many palaeontologists believed that whole groups were driven to extinction by internally programmed trends toward the production of nonadaptive factors. The incorporation of palaeontology into the synthesis was thus vital if the alleged support for these anti-Darwinian ideas was to be undermined. The most influential advocate of a new direction in the interpretation of the fossil record was the American, George Gaylord Simpson. He became convinced that palaeontologists had simply failed to appreciate the range of variation that could exist within a single population. Where they had numerous fossils from the same geological stratum, they had tended to divide them up into imaginary species – and had then visualized parallel lines linking the alleged species together. In many cases, the various ‘species’ from a single geological stratum were only variants within a single population, and the many parallel lines were simply artefacts created by dividing a single evolutionary lineage up along the time axis. Most of the cases in which evolution had been driven in an apparently nonadaptive direction could be accounted for in Darwinian terms if the lifestyle of the animals was studied more carefully. Thus the teeth of the sabretooth tiger did not become so long that the animals were unable to eat – they were an adaptation to piercing the thick skins of their prey. Simpson's Tempo and Mode in Evolution of 1944 took palaeontology firmly into the neo-Darwinian camp.

On the question of the apparently sudden appearance of new types in the fossil record, Simpson appealed to Darwin's argument on the record's imperfection. In his 1944 book, however, he did leave room for what he called ‘quantum evolution’ – relatively fast episodes of speciation that might pass through nonadaptive zones in the manner postulated by Wright's notion of genetic drift. His later writings abandoned this position, and Simpson became an advocate of a strongly materialistic Darwinian philosophy. Like Huxley, he was concerned about the broader implications of the synthesis and wanted to present it as a basis for a new vision of life. But Simpson was unwilling to concede any general progressive trend in evolution, and argued strongly against the idea that the human species was in any sense a preordained outcome of the development of life on Earth.

Later Developments

The modern synthesis was consolidated in the 1940s and soon dominated British and American evolutionary biology. Indeed, it was used to create a professional identity for the variety of scientific studies that could be illuminated by the new theory. In America, the Society for the Study of Evolution was founded in 1946 to promote the claim that the new evolutionism could unify biology in a hitherto unprecedented way. This strategy was a response to the threat posed by the continued rise of experimental biology, which tended to marginalize those areas once identified with ‘natural history’. To emphasize the scientific credentials of the movement, the synthesis was ‘hardened’ by the elimination of the residual non-Darwinian elements represented by Wright's genetic drift and Simpson's quantum evolution. To its critics, these developments suggest that the alleged unity brought about by the synthesis was more a product of rhetoric and professional strategy than a genuine conceptual and methodological reconciliation. Needless to say, the supporters of the synthesis dismiss these claims as nonsense. It should be noted that the success of the new synthesis was at first more limited outside Britain and America. In Russia the temporary triumph of Lysenkoism eliminated the study of genetics and of Darwinism. In Germany, Rensch produced an important pro-synthesis book in 1947, but palaeontology remained for some time locked into a non-Darwinian approach through the influence of Otto Schindewolf. It was only in the last decades of the twentieth century that French science and culture began to take Darwinism seriously.

The evolutionary synthesis retained its influence and extended its range of applications in the second half of the twentieth century. Perhaps the most striking extension was the application of Darwinian principles to the explanation of animal behaviour. John Maynard Smith applied game theory to understand the evolution of behavioural strategies. The science of sociobiology used natural selection to account for even apparently altruistic behaviour via the notion of ‘kin selection’. Richard Dawkin's notion of the ‘selfish gene’ seemed to sum up the essentially materialistic philosophy of the new Darwinism, and the same author has since led a campaign to replace religion with science as a source of human values. His position represents exactly the kind of threat seen in the new Darwinism by the Creationist movement. This has flourished, especially in America, since the evolutionary synthesis gave scientists the confidence to insist that evolution must be taught as an integral part of all school and college biology courses. The most extreme form of Creationism, however, rejects not just Darwinian evolutionism, but the whole picture of the Earth's past created by modern archaeology, palaeontology, geology and cosmology.

The success of the synthesis has attracted some critics even within science. In systematics, the German biologist Willi Hennig used the claim that classification should be based on genealogy to found the school of ‘cladism’, in which the identification of new characters in the founder members of a group is taken as the sole basis of affinity. This has been resisted by evolutionary taxonomists such as Mayr on the grounds that in some cases a phylum changes so much that it must be recognized as a distinct taxon even if its founder members cannot be identified by unique derived characters (strict application of cladist principles overturns, for instance, the traditional relationships between the birds, dinosaurs and other reptiles). Further development of the cladistic approach led to the group of ‘transformed cladists’, who claimed that it was impossible to recognize ancestor–descendant relationships between species and thus dismissed evolutionary ancestries as illusory. The resulting debates gave some support to the arguments of philosophers such as Karl Popper, who claimed that evolutionism is unscientific because its reconstructions of the past are untestable.

The palaeontologist Steven Jay Gould has become one of the late twentieth century's best-known opponents of a strictly Darwinian viewpoint. In 1977 he joined with Niles Eldredge to propose the theory of ‘punctuated equilibrium’, which challenged the prevailing faith in gradualism by arguing that the apparently sudden ‘jumps’ in the fossil record are not an artefact of poor preservation but are genuine evidence of speciation events that are very rapid by geological standards. This was not in itself a threat to the Darwinian position (which would expect small populations isolated in extreme environments to evolve comparatively rapidly), but Gould subsequently toyed with ideas about ‘hopeful monsters’. He also joined with the geneticist Richard Lewontin (who shares Gould's Marxist perspective) to argue that the synthesis has promoted an unthinking ‘adaptationism’ in which every character of every species must have an adaptive value. Gould and Lewontin proposed that many characters are merely by-products of the developmental processes by which the organism is constructed in ontogeny. Gould has also insisted that chance plays a more important role in evolution than Darwinists are willing to concede: if we could ‘rewind the tape’ of the evolution of life on Earth and let it play again, the results might be very different from what we now observe. Even orthodox Darwinists have been forced to accept the probability that the course of evolution has occasionally been disrupted by unpredictable mass extinctions caused by asteroid impacts. The neo-Darwinian synthesis has thus provided a framework, although by no means a rigid straightjacket, within which late twentieth century evolutionism has developed.

Further Reading

  • AdamsMB (ed.) (1994) The Evolution of Theodosius Dobzhansky. Princeton, NJ: Princeton University Press.
  • Allen GE (1978) Thomas Hunt Morgan: The Man and his Science. Princeton, NJ: Princeton University Press.
  • Bowler PJ (1989) Evolution: The History of an Idea. Berkeley, CA: University of California Press.
  • Depew DJ and Weber BH (1995) Darwinism Evolving: Systems Dynamics and the Genealogy of Natural Selection. Cambridge, MA: MIT Press.
  • Gould SJ (1983) The hardening of the modern synthesis. In: GreneM (ed.) Dimensions of Darwinism, pp. 71–93. Cambridge: Cambridge University Press.
  • Gould SJ (1989) Wonderful Life: The Burgess Shale and the Nature of History. London: Hutchinson.
  • Gould SJ and Eldredge N (1977) Punctuated equilibria: the tempo and mode of evolution reconsidered. Paleobiology 3: 115–151.
  • Gould SJ and Lewontin R (1979) The spandrels of San Marco and the panglossian paradigm: a critique of the adaptationist programme. Proceedings of the Royal Society B 205: 581–598.
  • GreneM (ed.) (1983) Dimensions of Darwinism: Themes and Counter Themes in Twentieth-century Evolutionary Thought. Cambridge: Cambridge University Press.
  • Hull DL (1988) Science as Process: An Evolutionary Account of the Growth and Conceptual Development of Science. Chicago, IL: University of Chicago Press.
  • Mayr E (1976) Evolution and the Diversity of Life. Cambridge, MA: Harvard University Press.
  • Mayr E (1982) The Growth of Biological Thought. Cambridge, MA: Harvard University Press.
  • MayrE and ProvineWB (eds) (1980) The Evolutionary Synthesis: Perspectives on the Unification of Biology. Cambridge: MA: Harvard University Press.
  • Provine WB (1971) The Origins of Theoretical Population Genetics. Chicago, IL: University of Chicago Press.
  • Provine WB (1986) Sewall Wright and Evolutionary Biology. Chicago, IL: University of Chicago Press.
  • Smocovitis VB (1996) Unifying Biology: The Evolutionary Synthesis and Evolutionary Biology. Princeton, NJ: Princeton University Press.
  • WatersKC and VanHeldenA (eds) (1992) Julian Huxley: Biologist and Statesman of Science. Houston, TX: Rice University Press.
  • Wilson EO (1975) Sociobiology: the New Synthesis. Cambridge, MA: Harvard University Press.
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