Modern synthesis (20th century)

The modern synthesis^[a] was the early 20th-century synthesis of Charles Darwin's theory of evolution and Gregor Mendel's ideas on heredity into a joint mathematical framework. Julian Huxley coined the term in his 1942 book, Evolution: The Modern Synthesis. The synthesis combined the ideas of natural selection, Mendelian genetics, and population genetics. It also related the broad-scale macroevolution seen by palaeontologists to the small-scale microevolution of local populations.

The synthesis was defined differently by its founders, with

Darwin's evolution by natural selection, 1859

The eclipse of Darwinism, 1880s onwards

From the 1880s onwards, biologists grew skeptical of Darwinian evolution. This

Weismann's germ plasm, 1892

August Weismann's idea, set out in his 1892 book Das Keimplasma: eine Theorie der Vererbung ("The Germ Plasm: a Theory of Inheritance"),^[13] was that the hereditary material, which he called the germ plasm, and the rest of the body (the soma) had a one-way relationship: the germ-plasm formed the body, but the body did not influence the germ-plasm, except indirectly in its participation in a population subject to natural selection. If correct, this made Darwin's pangenesis wrong, and Lamarckian inheritance impossible. His experiment on mice, cutting off their tails and showing that their offspring had normal tails, demonstrated that inheritance was 'hard'.^[b] He argued strongly and dogmatically^[15] for Darwinism and against Lamarckism, polarising opinions among other scientists. This increased anti-Darwinian feeling, contributing to its eclipse.^[16][17]

Disputed beginnings

Genetics, mutationism and biometrics, 1900–1918

While carrying out breeding experiments to clarify the mechanism of inheritance in 1900, Hugo de Vries and Carl Correns independently rediscovered Gregor Mendel's work. News of this reached William Bateson in England, who reported on the paper during a presentation to the Royal Horticultural Society in May 1900.^[18] In Mendelian inheritance, the contributions of each parent retain their integrity, rather than blending with the contribution of the other parent. In the case of a cross between two true-breeding varieties such as Mendel's round and wrinkled peas, the first-generation offspring are all alike, in this case, all round. Allowing these to cross, the original characteristics reappear (segregation): about 3/4 of their offspring are round, 1/4 wrinkled. There is a discontinuity between the appearance of the offspring; de Vries coined the term allele for a variant form of an inherited characteristic.^[19] This reinforced a major division of thought, already present in the 1890s, between gradualists who followed Darwin, and saltationists such as Bateson.^[20]

The two schools were the Mendelians, such as Bateson and de Vries, who favoured mutationism, evolution driven by mutation, based on genes whose alleles

A traditional view is that the biometricians and the Mendelians rejected natural selection and argued for their separate theories for 20 years, the debate only resolved by the development of population genetics.^[23][25] A more recent view is that Bateson, de Vries,

Thomas Hunt Morgan began his career in genetics as a saltationist and started out trying to demonstrate that mutations could produce new species in fruit flies. However, the experimental work at his lab with the fruit fly, Drosophila melanogaster^[c] showed that rather than creating new species in a single step, mutations increased the supply of genetic variation in the population.^[34] By 1912, after years of work on the genetics of fruit flies, Morgan showed that these insects had many small Mendelian factors (discovered as mutant flies) on which Darwinian evolution could work as if the variation was fully continuous. The way was open for geneticists to conclude that Mendelism supported Darwinism.^[35]

An obstruction: Woodger's positivism, 1929

The

In 1918, R. A. Fisher wrote "The Correlation between Relatives on the Supposition of Mendelian Inheritance,"^[39] which showed how continuous variation could come from a number of discrete genetic loci. In this and other papers, culminating in his 1930 book The Genetical Theory of Natural Selection,^[40] Fisher showed how Mendelian genetics was consistent with the idea of evolution by natural selection.^[41][d]

In the 1920s, a series of papers by J. B. S. Haldane analyzed real-world examples of natural selection, such as the evolution of industrial melanism in peppered moths.^[41] and showed that natural selection could work even faster than Fisher had assumed.^[43] Both of these scholars, and others, such as Dobzhansky and Wright, wanted to raise biology to the standards of the physical sciences by basing it on mathematical modeling and empirical testing. Natural selection, once considered unverifiable, was becoming predictable, measurable, and testable.^[44]

De Beer's embryology, 1930

The traditional view is that

Wright's adaptive landscape, 1932

The population geneticist Sewall Wright focused on combinations of genes that interacted as complexes, and the effects of inbreeding on small relatively isolated populations, which could be subject to genetic drift. In a 1932 paper, he introduced the concept of an adaptive landscape in which phenomena such as cross breeding and genetic drift in small populations could push them away from adaptive peaks, which would in turn allow natural selection to push them towards new adaptive peaks.^[41][53] Wright's model would appeal to field naturalists such as Theodosius Dobzhansky and Ernst Mayr who were becoming aware of the importance of geographical isolation in real world populations.^[43] The work of Fisher, Haldane and Wright helped to found the discipline of theoretical population genetics.^[54][55][56]

Dobzhansky's evolutionary genetics, 1937

Theodosius Dobzhansky, an immigrant from the Soviet Union to the United States, who had been a postdoctoral worker in Morgan's fruit fly lab, was one of the first to apply genetics to natural populations. He worked mostly with Drosophila pseudoobscura. He says pointedly: "Russia has a variety of climates from the Arctic to sub-tropical... Exclusively laboratory workers who neither possess nor wish to have any knowledge of living beings in nature were and are in a minority."^[57] Not surprisingly, there were other Russian geneticists with similar ideas, though for some time their work was known to only a few in the West. His 1937 work Genetics and the Origin of Species^[58] was a key step in bridging the gap between population geneticists and field naturalists. It presented the conclusions reached by Fisher, Haldane, and especially Wright in their highly mathematical papers in a form that was easily accessible to others.^[41][43] Further, Dobzhansky asserted the physicality, and hence the biological reality, of the mechanisms of inheritance: that evolution was based on material genes, arranged in a string on physical hereditary structures, the chromosomes, and linked more or less strongly to each other according to their actual physical distances on the chromosomes. As with Haldane and Fisher, Dobzhansky's "evolutionary genetics"^[59] was a genuine science, now unifying cell biology, genetics, and both micro and macroevolution.^[44] His work emphasized that real-world populations had far more genetic variability than the early population geneticists had assumed in their models and that genetically distinct sub-populations were important. Dobzhansky argued that natural selection worked to maintain genetic diversity as well as by driving change. He was influenced by his exposure in the 1920s to the work of Sergei Chetverikov, who had looked at the role of recessive genes in maintaining a reservoir of genetic variability in a population, before his work was shut down by the rise of Lysenkoism in the Soviet Union.^[41][43] By 1937, Dobzhansky was able to argue that mutations were the main source of evolutionary changes and variability, along with chromosome rearrangements, effects of genes on their neighbours during development, and polyploidy. Next, genetic drift (he used the term in 1941), selection, migration, and geographical isolation could change gene frequencies. Thirdly, mechanisms like ecological or sexual isolation and hybrid sterility could fix the results of the earlier processes.^[60]

Ford's ecological genetics, 1940

E. B. Ford was an experimental naturalist who wanted to test natural selection in nature, virtually inventing the field of ecological genetics.^[61] His work on natural selection in wild populations of butterflies and moths was the first to show that predictions made by R. A. Fisher were correct. In 1940, he was the first to describe and define genetic polymorphism, and to predict that human blood group polymorphisms might be maintained in the population by providing some protection against disease.^[61][62] His 1949 book Mendelism and Evolution^[63] helped to persuade Dobzhansky to change the emphasis in the third edition of his famous textbook Genetics and the Origin of Species from drift to selection.^[64]

Schmalhausen's stabilizing selection, 1941

Ivan Schmalhausen developed the theory of stabilizing selection, the idea that selection can preserve a trait at some value, publishing a paper in Russian titled "Stabilizing selection and its place among factors of evolution" in 1941 and a monograph Factors of Evolution: The Theory of Stabilizing Selection^[65] in 1945. He developed it from J. M. Baldwin's 1902 concept that changes induced by the environment will ultimately be replaced by hereditary changes (including the Baldwin effect on behaviour), following that theory's implications to their Darwinian conclusion, and bringing him into conflict with Lysenkoism. Schmalhausen observed that stabilizing selection would remove most variations from the norm, most mutations being harmful.^[66][67][68] Dobzhansky called the work "an important missing link in the modern view of evolution".^[69]

Huxley's popularising synthesis, 1942

In 1942, Julian Huxley's serious but popularising^[70][71] Evolution: The Modern Synthesis^[2] introduced a name for the synthesis and intentionally set out to promote a "synthetic point of view" on the evolutionary process. He imagined a wide synthesis of many sciences: genetics, developmental physiology, ecology, systematics, palaeontology, cytology, and mathematical analysis of biology, and assumed that evolution would proceed differently in different groups of organisms according to how their genetic material was organised and their strategies for reproduction, leading to progressive but varying evolutionary trends.^[71] His vision was of an "evolutionary humanism",^[72] with a system of ethics and a meaningful place for "Man" in the world grounded in a unified theory of evolution which would demonstrate progress leading to humanity at its summit. Natural selection was in his view a "fact of nature capable of verification by observation and experiment", while the "period of synthesis" of the 1920s and 1930s had formed a "more unified science",^[72] rivalling physics and enabling the "rebirth of Darwinism".^[72]

However, the book was not the research text that it appeared to be. In the view of the philosopher of science

Simpson's palaeontology, 1944

George Gaylord Simpson was responsible for showing that the modern synthesis was compatible with palaeontology in his 1944 book Tempo and Mode in Evolution. Simpson's work was crucial because so many palaeontologists had disagreed, in some cases vigorously, with the idea that natural selection was the main mechanism of evolution. It showed that the trends of linear progression (in for example the evolution of the horse) that earlier palaeontologists had used as support for neo-Lamarckism and orthogenesis did not hold up under careful examination. Instead, the fossil record was consistent with the irregular, branching, and non-directional pattern predicted by the modern synthesis.^[41][43]

Society for the Study of Evolution, 1946

During World War II, Mayr edited a series of bulletins of the Committee on Common Problems of Genetics, Paleontology, and Systematics, formed in 1943, reporting on discussions of a "synthetic attack" on the interdisciplinary problems of evolution. In 1946, the committee became the Society for the Study of Evolution, with Mayr, Dobzhansky and Sewall Wright the first of the signatories. Mayr became the editor of its journal, Evolution. From Mayr and Dobzhansky's point of view, suggests the historian of science Betty Smocovitis, Darwinism was reborn, evolutionary biology was legitimised, and genetics and evolution were synthesised into a newly unified science. Everything fitted into the new framework, except "heretics" like Richard Goldschmidt who annoyed Mayr and Dobzhansky by insisting on the possibility of speciation by macromutation, creating "hopeful monsters". The result was "bitter controversy".^[52]

After the synthesis, evolutionary biology continued to develop with major contributions from workers including W. D. Hamilton,[85] George C. Williams,^[86] E. O. Wilson,^[87] Edward B. Lewis^[88] and others.

Hamilton's inclusive fitness, 1964

In 1964, W. D. Hamilton published two papers on "The Genetical Evolution of Social Behaviour". These defined inclusive fitness as the number of offspring equivalents an individual rears, rescues or otherwise supports through its behaviour. This was contrasted with personal reproductive fitness, the number of offspring that the individual directly begets. Hamilton, and others such as John Maynard Smith, argued that a gene's success consisted in maximising the number of copies of itself, either by begetting them or by indirectly encouraging begetting by related individuals who shared the gene, the theory of kin selection.^[85][89]

Williams's gene-centred evolution, 1966

In 1966,

In 1975, E. O. Wilson published his controversial^[93] book Sociobiology: The New Synthesis, the subtitle alluding to the modern synthesis^[87] as he attempted to bring the study of animal society into the evolutionary fold. This appeared radically new, although Wilson was following Darwin, Fisher, Dawkins and others.^[87] Critics such as Gerhard Lenski noted that he was following Huxley, Simpson and Dobzhansky's approach, which Lenski considered needlessly reductive as far as human society was concerned.^[94] By 2000, the proposed discipline of sociobiology had morphed into the relatively well-accepted discipline of evolutionary psychology.^[87]

Lewis's homeotic genes, 1978

In 1977,

By the late 20th century, however, the modern synthesis was showing its age, and fresh syntheses to remedy its defects and fill in its gaps were proposed from different directions. These have included such diverse fields as the study of society,^[87] developmental biology,^[50] epigenetics,^[96] molecular biology, microbiology, genomics,^[3] symbiogenesis, and horizontal gene transfer.^[97] The physiologist Denis Noble argues that these additions render neo-Darwinism in the sense of the early 20th century's modern synthesis "at the least, incomplete as a theory of evolution",^[97] and one that has been falsified by later biological research.^[97]

Michael Rose and Todd Oakley note that evolutionary biology, formerly divided and "Balkanized", has been brought together by genomics. It has in their view discarded at least five common assumptions from the modern synthesis, namely that the genome is always a well-organised set of genes; that each gene has a single function; that species are well adapted biochemically to their ecological niches; that species are the durable units of evolution, and all levels from organism to organ, cell and molecule within the species are characteristic of it; and that the design of every organism and cell is efficient. They argue that the "new biology" integrates genomics, bioinformatics, and evolutionary genetics into a general-purpose toolkit for a "Postmodern Synthesis".^[54]

Pigliucci's extended evolutionary synthesis, 2007

In 2007, more than half a century after the modern synthesis, Massimo Pigliucci called for an extended evolutionary synthesis to incorporate aspects of biology that had not been included or had not existed in the mid-20th century.^[98][99] It revisits the relative importance of different factors, challenges assumptions made in the modern synthesis, and adds new factors^[99][100] such as multilevel selection, transgenerational epigenetic inheritance, niche construction, and evolvability.^{[101][96][102]}

Koonin's 'post-modern' evolutionary synthesis, 2009

Biologists, alongside scholars of the history and philosophy of biology, have continued to debate the need for, and possible nature of, a replacement synthesis. For example, in 2017 Philippe Huneman and Denis M. Walsh stated in their book Challenging the Modern Synthesis that numerous theorists had pointed out that the disciplines of embryological developmental theory, morphology, and ecology had been omitted. They noted that all such arguments amounted to a continuing desire to replace the modern synthesis with one that united "all biological fields of research related to evolution, adaptation, and diversity in a single theoretical framework."

Looking back at the conflicting accounts of the modern synthesis, the historian Betty Smocovitis notes in her 1996 book Unifying Biology: The Evolutionary Synthesis and Evolutionary Biology that both historians and philosophers of biology have attempted to grasp its scientific meaning, but have found it "a moving target";[107] the only thing they agreed on was that it was a historical event.^[107] In her words

"by the late 1980s the notoriety of the evolutionary synthesis was recognized ... So notorious did 'the synthesis' become, that few serious historically minded analysts would touch the subject, let alone know where to begin to sort through the interpretive mess left behind by the numerous critics and commentators".^[108]

See also

• Developmental systems theory
• Gene-centered view of evolution
• History of evolutionary thought
• Neo-Darwinism
• Objections to evolution

Notes