Modern synthesis (20th century)

The modern evolutionary synthesis refers to the set of ideas from biology that form the unified theory of evolution and is accepted by the great majority of working biologists. This synthesis was produced over a period of about a decade (1936-1947) and was closely connected with the development from 1918 to 1932 of the biological subdiscipline of population genetics, which integrated the theory of natural selection with Mendalian genetics. Major figures in the development of the modern synthesis include R. A. Fisher, Theodosius Dobzhansky, J.B.S. Haldane, Sewall Wright, Julian Huxley, Ernst Mayr, Bernhard Rensch, George Gaylord Simpson, and G. Ledyard Stebbins.

The modern synthesis solved difficulties and confusions caused by the specialisation and poor communication between biologists in the early years of the twentieth century. Discoveries of early geneticists were difficult or impossible to reconcile with gradual evolution and the mechanism of natural selection. The synthesis reconciled the two schools of thought, while providing evidence that studies of populations in the field were crucial to evolutionary theory. It unifies which draws from several branches of biology that had become separated, particularly genetics, cytology, systematics, botany, morphology, ecology and paleontology.

The key individuals that lead this synthesis were Dobzhansky

Developments leading up to the modern synthesis

1859-1899

Two of Darwin's theories were widely accepted: evolution itself and the idea of common descent. However, two other ideas met with 'determined resistance': gradual change and natural selection (see T.H. Huxley). Variations of Lamarckism, orthogenesis ("progressive" evolution), and saltationism (evolution by "jumps" or mutations) were discussed as alternatives. Also, Darwin did not quite offer a real theory of how new species arise; strange, given the title of his book. For example, he did not explicitly suggest that geographical separation might be the start of speciation. As part of the disagreement about whether natural selection was sufficient by itself to explain speciation, George John Romanes coined the term "neo-Darwinism" to refer to the version of evolution advocated by Alfred Russel Wallace and August Weismann with its heavy dependence on natural selection. Weismann and Wallace rejected the Lamarckian idea of inheritance of acquired characteristics, something that Darwin didn't rule out.

Weismann's key idea was that the relationship between the hereditary material, which he called the germ plasm, and the rest of the body (the soma) was 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. Weismann was translated into English, and though he was influential, it took many years for the full significance of his work to be appreciated. Later, after the completion of the modern synthesis, the term neo-Darwinism would come to be associated with its core concept of evolution being driven by natural selection acting on variation produced by genetic mutation and crossing-over.

1900-1915

Gregor Mendel's work was re-discovered by Hugo de Vries and Carl Correns. It showed that the contributions of parents retained their integrity and blending inheritance was out. However, the early Mendelians viewed hard inheritance as incompatible with natural selection and favored saltationism (large mutations or jumps) instead. The biometric school, led by Karl Pearson and Walter Weldon, argued vigorously against it, saying that empirical evidence indicated that variation was continuous in most organisms not discrete as Mendelalism predicted. The relevance of Mendelism to evolution was unclear and hotly debated.

T. H. 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 Drosophila melanogaster, which helped establish the link between Mendelian genetics and the chromosomal theory of inheritance, demonstrated that rather than creating new species in a single step, mutations increased the genetic variation in the population.

This period also contains the first important result of population genetics, the Hardy-Weinberg equilibrium. This simple calculation proves that (with no migration or selection, and disregarding mutation) a random-mating population has the proportion of alleles at all loci in equilibrium. The implication is that without natural selection (or some other agency) natural populations would not change their genetic structure.

Knowledge of cytology was growing: the famous textbook of E.B. Wilson, The cell, in each of its three editions managed to summarise cytological research for a generation (Wilson 1896, 1909, 1924).

1916-1935

The key precursor to the synthesis was the development of population genetics. J.B.S. Haldane and Ronald Fisher, and Sewall Wright provided critical contributions to this field. A first step was taken by Fisher, who in 1918 produced the paper The Correlation Between Relatives on the Supposition of Mendelian Inheritance, which showed how the continuous variation measured by the biometricians could be the result of the action of many discrete genetic loci. In this and subsequent papers culminating in his 1930 book Genetical Theory of Natural Selection Fisher was able to show how Mendelian genetics was consistent with the main elements of neo-Darwinism.

During the 1920s a series of papers by J.B.S. Haldane applied mathematical analysis to real world examples of natural selection such as the evolution of industrial melanism in peppered moths. Haldane established that natural selection could work in the real world at a faster rate than even Fisher had assumed.

Sewall Wright focused on combinations of genes that interacted as complexes, and the effects of inbreeding on small relatively isolated populations, which could exhibit 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. 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.

The work of Fisher, Haldane and Wright founded the discipline of population genetics. This is the precursor of the modern synthesis, which is an even broader coalition of ideas. The work of these population geneticists did not achieve a comprehensive synthesis because events at the level of the gene do not explain phenomena studied by systematists, ecologists, ethologists or palaeontologists (Mayr & Provine 1998).

Also of note was Julian Huxley's 600+ pages on evolution (in Wells, Huxley & Wells 1930). "Huxley's discussion of evolution was the single most encompassing presentation of a neo-Darwinian viewpoint available in 1930".

1936-1947

The publication of Dobzhansky's Genetics and the Origin of Species in 1937 was a critical event at the start, and the conference at Princeton in 1947 marks the completion of the new synthesis, though some books from this period were published later, such as Stebbins (1950) and Ford (1964!). At the Princeton conference all branches of biology accepted the synthesis except developmental biology; integration of this field took a further thirty or so years.

Microbiology played no part in the synthesis: almost all that we know about the molecular biology, genetics, structure and evolution of bacteria, archaea and viruses dates to the post-DNA period. Even fungi were so little understood as to be excluded. So the synthesis is concerned entirely with eukaryotes with (mostly) sexual forms of reproduction, mostly the traditional objects of natural history: animals, plants and their fossil remains.

The modern synthesis

Theodosius Dobzhansky was one of the first to apply genetics to natural populations of, mostly, Drosophila pseudoobscura. He says pointedly: "Russia has a variety of climates from the arctic to sub-tropical... rivers, lakes and seas. Laboratory workers who neither possess nor wish to have any knowledge of living beings in nature were and are in a minority". 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.

Sergei Chetverikov (1926, transl. 1961) was interested in micro-evolutionary processes in nature such as natural variability, speciation, and selection. He was a mentor of Dobzhansky; Haldane arranged for his publications to be translated, but they were not published in English at the time. Another geneticist, Timoféeff-Ressovsky, eventually left Russia for Berlin, and was published in time for the synthesis (in Huxley 1940). The title of his paper, Mutations and geographical variation, was close to Dobzhansky's interests. Dobzhansky acknowledged these and other Russian workers in his 1980 paper.

Dobzhansky's early work in the U.S. was done mostly in collaboration with Alfred Sturtevant. Dobzhansky organised the collection of D. pseudo-obscura flies. The larval salivary glands could be dissected, stained and prepared, and their polytene chromosomes mapped under the microscope. Then chromosomal re-arrangements such as inversions could be identified rapidly (Painter 1933). Sturtevant did all the genetic crossing. Soon, four different kinds of Y chromosome were discovered: they found a great deal of variability in this undomesticated wild species, which was not yet stripped of its natural genetic diversity. D. pseudo-obscura was variable and polytypic: significant differences were found between sub-populations in the wild, and they were not always geographically separated. Dobzhansky began to construct phylogenies of the regional strains. It was this work which took centre stage in the Jesup Lectures at Columbia University, which became Genetics and the origin of species (Kohler 1994).^{[No page numbers provided]}

After Dobzhansky split from Sturtevant and the group at Cal Tech, he formed an occasional partnership with Sewall Wright, in which Wright supplied some of the design and analysis for Dobzhansky's investigations. These investigations used ideas from population genetics in a way that was accessible to others, but perhaps even more important was to reveal to the mathematical theorists what was really going on in wild populations. In the 1950s and 60s Dobzhansky's main collaborator was Boris Spassky, a Russian originally trained as a forester, who escaped the Soviet Union via Harbin in China (Adams 1994 p27). After he had published a series of papers under the title Genetics of natural populations (reprinted in Dobzhansky 1981), Dobzhansky wrote his last book, the massive Genetics of the evolutionary process (1970); by then there were many other biologists drawn to the theme which Chetverikov had initiated and which Dobzhansky developed so successfully.

Julian Huxley coined the terms evolutionary synthesis and modern synthesis in Evolution: the modern synthesis in 1942. This was "more comprehensive in subject matter and documentation than the other major works of the evolutionary synthesis period" (Provine in Mayr & Provine 1998 p332). This survey was made possible by the extraordinarily complete set of offprints arranged in over a hundred boxfiles in Huxley's office. Huxley edited two important collections of papers and reviews towards the synthesis; the first is The new systematics (1940) and the second is Evolution as a process (1954, edited with AC Hardy and E.B. Ford). Both volumes included essays by leading synthesisers; the overall quality of the 1954 volume is exceptionally high. Huxley's earlier synthesis (in Wells et al 1930) never saw the light of day as a separate volume; apparently H.G. Wells paid Huxley in cash, and kept the copyright himself (Huxley 1970 Chapter 12 says Huxley earned nearly £10,000 from this book). Evolution in action (Huxley 1953) is a readable later account.

Ernst Mayr: his key contribution to the synthesis was Systematics and the Origin of Species, 1942. "That mutation, recombination, selection and isolation are the four cornerstones of evolution is now generally acknowledged" is an absolutely characteristic statement from Mayr (in Huxley, Hardy and Ford 1954 p157). The isolation he usually had in mind was geographical isolation: allopatric speciation. He was sceptical of the reality of sympatric speciation: for much of his long career he did not believe sympatric speciation possible.

The role of geographical isolation as a causative agent in speciation goes back to Moritz Wagner (1813-1887), who explored Algeria from 1836-38. He made a study of flightless beetles (Pimelia and Melasoma) on the north coast. The land between the Atlas mountains and the Mediterranean is sectioned by a series of rivers running from the mountains to the coast. Wagner found that each species was restricted to a stretch of the coast between two rivers. What is more, he found a similar situation in the Caucasus, and also for montane species where the valleys between peaks acted as the isolating mechanism. This illustrates the way naturalists who travelled picked up evidence, and adds Wagner to the list of traveller/naturalists who became convinced of evolution as a result of their observations on natural populations. A long correspondence ensued between Charles Darwin and Wagner; the latter's reputation was later diminished because of his adherence to Lamarkism; however, his observations are a permanent contribution to biology (Wagner 1868).

One thing Mayr pointed out was that earlier workers had not clearly appreciated was the distinction between geographical and reproductive isolation; this was one of the major theses in Mayr's Systematics and the origin of species: geographical isolation as a prerequisite for building up intrinsic (reproductive) isolating mechanisms. (Mayr 1982 p556 et seq)

George Simpson was responsible for showing that the genetical theory of evolution was compatible with palaeontology (Tempo and Mode in Evolution 1944). Simpson's work was crucial for the synthesis because so many palaeontologists had disagreed—in some cases vigorously—with the earlier neo-darwinian theory. E.D. Cope, the ablest palaeontologist of the nineteenth century in the USA was a Lamarkist. Semi-vitalistic ideas such as orthogenesis and a general disavowal of gradualism were widespread. Often a palaeontologist would believe that if he, personally, could not see a function for some structure, then it actually did not have a function! Of course these difficulties are basically caused by the nature of fossil evidence being so difficult to apply to the kind of questions which interest systematists or naturalists. Consequently palaeontologists are usually most interested in macroevolution, and Simpson is a good example of this, as his second (and even better) book on evolution illustrates (Simpson 1953).

The synthesis between genetics and palaeontology took place in stages (Mayr 1982 p606 et seq). The first step was to decide whether there are any macroevolutionary phenomena which clearly are not consistent with the genetical account of evolution (genetic variation and natural selection). After a lengthy examination, Simpson decided the answer was no. Secondly, can all the laws and principles of palaeontology be developed simply by studying gene frequencies in populations? Again, the answer was no, which amounts to saying that the data of palaeontology cannot simply be reduced (see reductionism) to studies of the Dobzhansky type.

Bernhard Rensch was called up for war duty in 1939, so his work, which included a wide survey of all the main issues faced by the synthesis, was published before and after the synthetic period (Rensch 1939, 1959). The story of his master-work Evolution above the species level (first edition in English: Rensch 1959) is quite interesting. In the Preface Rensch says "The greater part of this book was written in the last years of the war". Not until the second German edition in 1954 could the material in the books by Huxley, Mayr and Simpson be incorporated. In it Rensch emphasised how common were polytypic species (Rassenkreise) and how widespread are character gradients (called clines by Huxley). These were key observations supporting the idea of geographic speciation.

Almost unknown outside Germany, not surprisingly in view of the date, was the collection of papers edited by Gustav Heberer, published in 1943 (the book ran to three editions and ended up as a massive multi-volume work). Yet this also illustrates how many workers had independently come to similar conclusions. The synthesis was an idea whose time had arrived.

The botanist G. Ledyard Stebbins was a major contributor to the synthesis. He worked for many years with E. B. Babcock on the genetics and cytology of Crespis, a plant which showed both polyploidy and apomixis (Stebbins 1940). They discovered that polyploidy was important in developing large, complex and widespread genera. They found polyploidy was only common in herbaceous perennials and infrequent in other plants. It seemed that polyploidy was a conservative force in the longer term.

After being raised to a full chair at UC Berkeley Stebbins taught a course on evolution which resulted in his active co-operation in the new synthesis. His major work, Variation and Evolution in Plants was published after the war (Stebbins 1950).

Cyril Darlington was a cytologist who emphasised the evolutionary significance of crossing-over (chiasmata). As the process of meiosis operates, potential variation gets displayed in the phenotype, and so the variation is potentially open to natural selection. Instead of mutation and natural selection, in sexual reproduction we have mutation, crossing-over (or recombination) and selection as the basic mechanism of evolution. Many biologists have underestimated what a difference this makes. At the restart of genetics crossing-over was not known, and when it was discovered neither William Bateson nor T.H. Morgan would believe it! In both cases their co-workers had a considerable struggle to persuade their Directors. Darlington was lead to consider the evolution of genetic systems, perhaps the first time this issue had been faced squarely (Darlington 1939).

E.B. Ford worked for many years on genetic polymorphism (Ford 1940, 1964). Polymorphism in natural populations is frequent; the key feature here is the occurrence together of two or more discontinuous forms of a species in some kind of balance. So long as the proportions of each form is above mutation rate, then selection must be the cause. As early as 1930 Fisher had discussed a situation where, with alleles at a single locus, the heterozygote is more viable than either homozygote. That is a typical genetic mechanism for causing this type of polymorphism. The work involves a synthesis of field observations, taxonomy, and laboratory genetics (Huxley 1955).

It can be seen from this summary that the synthesis was achieved by collaboration between biologists from four countries in particular: USA, England, Russia and Germany, despite all the disruptions and difficulties cause by a world war involving all four parties. In retrospect, that does seem to be a remarkable occurrence. Much of the work done in Russia and Germany was not widely recognised until later.

Tenets of the modern synthesis

The modern evolutionary synthesis bridged the gap between experimental geneticists and naturalists and palaeontologists. The following conclusions are generally agreed (Mayr 1982 p567 et seq):

• All evolutionary phenomena can be explained in a way consistent with known genetic mechanisms and the observational evidence of naturalists.

• Evolution is gradual: small genetic changes, recombination ordered by natural selection.

• On selection:

• selection is overwhelmingly the main mechanism of change.
• even slight advantages are important when continued.
• the object of selection is the phenotype in its surrounding environment.
• discontinuities amongst species (or other taxa) can be explained as originating gradually through geograpical separation and extinction (not saltation).

• Population thinking:

• the strength of natural selection in the wild.
• the genetic structure of natural populations is a key factor in evolution: the amount of genetic diversity carried by populations.
• species as reproductively isolated aggregates of populations;
• the effect of ecological factors (such as niche occupation, competition, adaptive radiation) on diversity.
• the significance of barriers to gene flow.

• Palaeontology:

• ability to explain historical observations by extrapolation from micro-evolution at the population level.
• historical contingency: explanations at different levels may co-exist.
• gradualism does not mean constant rate: small steps, but varying rates of change.
• the vagaries of deposition (etc) do result in unequal and broken record of descent.

Modern research

The biological sciences changed materially after the second world war. The fields of cell biology and molecular biology grew almost from scratch; the discovery of the structure of DNA lead to the complete genetic analysis of many different types of organism. The realisation that RNA is heavily involved, not just in protein synthesis, but in gene regulation and in embryonic development is opening new worlds. Microbiology has grown from an off-shoot of medicine into a fundamental part of our modern view of life; we are now aware that the genomes of prokaryotes, both bacterial and archaea, have contributed to the eukaryotic cell, of which all higher forms of life are built. The field of palaeontology opened up after many years of stagnation, with a vast array of important discoveries, especially in China. Discovery of significant pre-human fossils (hominid or hominin) in Africa has also grown hugely. Much of this has to do with the invention of new techniques and technologies, at least in the area of cell and molecular biology. Also, the huge increase in funding for the biological sciences should not be underestimated.

Almost unnoticed amidst these great advances, the evolutionary synthesis has survived quite well. Another of Mayr's pronouncements went "The evolutionary synthesis... was clearly the most decisive event in the history of evolutionary biology since the publication of the Origin of Species in 1859 (Mayr 1982 p569). New problems, such as altruism, kin selection, the evolution of sex, have arisen (Cronin 1991; Hamilton 1996-2005; Maynard Smith 1978, 1988); old problems have returned, such as sexual selection (Cronin 1991), mimicry (Naisbit et al 2003), speciation (Mallet 2006).

One or two apparent challenges to the synthesis faded somewhat after a time, for example the idea of punctuated equilibrium (Eldredge and Gould 1972). Eldredge and Gould claimed that the gradualism espoused by Charles Darwin was virtually nonexistent in the fossil record, and that stasis dominates the history of most fossil species, punctuated by rare bursts of adaptive radiation. Now, gradualism in evolution is an ambiguous idea: it might mean evolution is small steps (as opposed to larger jumps), or it might mean constant slow development. The first version is absolutely consistent with the evolutionary synthesis. The second version is inconsistent with the fossil record.

In an article Can we complete Darwin's revolution? Stephen Gould said "Evolutionary events are concentrated in episodes of branching speciation within small, isolated populations" (Gould 1996), which is completely orthodox so far as the evolutionary synthesis goes; indeed much the same idea of speciation had been put forward by Mayr long before. Another example, from W.D. Hamilton, perhaps the greatest innovator in evolutionary theory in recent times: "I was and still am a Darwinian gradualist for most of the issues of evolutionary change" (Hamilton 1996, vol 1 p27). The synthesis is in good health, though that certainly does not mean that all evolutionary problems are solved.

See also

• History of evolutionary thought
• Gene-centered view of evolution
• Population genetics
• Symbiogenesis

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