Modern evolutionary synthesis

The modern evolutionary synthesis refers to a set of ideas from several biological specialities that were brought together to form a unified theory of evolution 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 discipline of population genetics, which integrated the theory of natural selection with Mendelian genetics.

Julian Huxley invented the term, when he summarised the ideas in his book, Evolution: The Modern Synthesis in 1942. Though the 'Modern Synthesis' is the basis of current evolutionary thinking, it refers to a historical event that took place in the 1930s and 1940s. 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, Sergei Chetverikov, 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 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 drew together ideas from several branches of biology that had become separated, particularly genetics, cytology, systematics, botany, morphology, ecology and paleontology.

Modern evolutionary synthesis is also referred to as the new synthesis, the modern synthesis, and the evolutionary synthesis.

1859–1899
The Origin of Species was successful in convincing most of the scientific community of the fact that evolution had occurred, but was much less successful in convincing naturalists that natural selection was its primary mechanism. In the 19th and early 20th centuries variations of Lamarckism, orthogenesis ('progressive' evolution), and saltationism (evolution by jumps) were discussed as alternatives. Also, Darwin did not offer a precise explanation of how new species arise. As part of the disagreement about whether natural selection alone was sufficient to explain speciation, George 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 had not ruled out.

Weismann's 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 recombination (see crossing-over).

1900–1915
Gregor Mendel's work was re-discovered by Hugo de Vries and Carl Correns in 1900. News of this reached William Bateson in England, who reported on the paper during a presentation to the Royal Horticultural Society in May 1900. It showed that the contributions of each parent retained their integrity rather than blending with the contribution of the other parent. 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 Mendelism predicted. The relevance of Mendelism to evolution was unclear and hotly debated, especially by Bateson, who opposed the biometric ideas of his former teacher Weldon. This debate between the biometricians and the Mendelians continued for some twenty years.

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.

The foundation of population genetics
The first step towards the synthesis was the development of population genetics. R.A. Fisher, J.B.S. Haldane, and Sewall Wright provided critical contributions. In 1918 Fisher 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, contrary to the thinking of many early geneticists, completely consistent with the idea of evolution driven by natural selection. 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 modern synthesis
Theodosius Dobzhansky, a Russian emigre 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". 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 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. It also 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 driving change. Dobzhansky had been influenced by his exposure in the 1920s to the work of a Russian geneticist named 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.

Ernst Mayr's key contribution to the synthesis was Systematics and the Origin of Species, published in 1942. Mayr emphasized the importance of allopatric speciation, where geographically isolated sub-populations diverge so far that reproductive isolation occurs. He was sceptical of the reality of sympatric speciation believing that geographical isolation was a prerequisite for building up intrinsic (reproductive) isolating mechanisms. Mayr also introduced the biological species concept that defined a species as a group of interbreeding or potentially interbreeding populations that were reproductively isolated from all other populations. Before he left Germany for the United States in 1930, Mayr had been influenced by the work of German biologist Bernhard Rensch. In the 1920s Rensch, who like Mayr did field work in Indonesia, analyzed the geographic distribution of polytypic species and complexes of closely related species paying particular attention to how variations between different populations correlated with local environmental factors such as differences in climate. In 1947 Rensch would write a book, eventually translated into English under the title Evolution above the species level, that looked at how the same evolutionary mechanisms involved in speciation might be extended to explain the origins of the differences between the higher level taxa. His writings contributed to the rapid acceptance of the synthesis in Germany.

George Gaylord Simpson was responsible for showing that the modern synthesis was compatible with paleontology in his book Tempo and Mode in Evolution published in 1944. Simpson's work was crucial because so many paleontologists 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 paleontologists 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.

The botanist G. Ledyard Stebbins was another major contributor to the synthesis. His major work, Variation and Evolution in Plants, was published in 1950. It extended the synthesis to encompass botany including the important effects of hybridization and polyploidy in some kinds of plants.

Tenets of the modern synthesis
The modern synthesis bridged the gap between experimental geneticists and naturalists; and between both and palaeontologists, stating that:


 * 1) All evolutionary phenomena can be explained in a way consistent with known genetic mechanisms and the observational evidence of naturalists.
 * 2) Evolution is gradual: small genetic changes, recombination ordered by natural selection. Discontinuities amongst species (or other taxa) are explained as originating gradually through geograpical separation and extinction (not saltation).
 * 3) 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. The role of genetic drift is equivocal; though strongly supported initially by Dobzhansky, it was downgraded later as results from ecological genetics were obtained.
 * 4) The primacy of population thinking: the genetic diversity carried in natural populations is a key factor in evolution. The strength of natural selection in the wild was greater than expected; the effect of ecological factors such as niche occupation and the significance of barriers to gene flow are all important.
 * 5) In palaeontology, the ability to explain historical observations by extrapolation from micro to macro-evolution is proposed. Historical contingency means explanations at different levels may exist. Gradualism does not mean constant rate of change.

The idea that speciation occurs after populations are reproductively isolated has been much debated. In plants, polyploidy must be included in any view of speciation. Formulations such as 'evolution consists primarily of changes in the frequencies of alleles between one generation and another' were proposed rather later. Developmental biology ('evo-devo') played little part in the synthesis.

Almost all aspects of the synthesis have been challenged at times, with varying degrees of success. There is no doubt, however, that the synthesis was a great landmark in evolutionary biology. It cleared up many confusions, and was directly responsible for stimulating a great deal of research in the post-WWII era.

Further advances
The modern evolutionary synthesis continued to be developed and refined after the initial establishment in the 1930s and 1940s. The work of W. D. Hamilton, George C. Williams, John Maynard Smith and others led to the development of a gene-centric view of evolution in the 1960s. The synthesis as it exists now has extended the scope of the Darwinian idea of natural selection to include subsequent scientific discoveries and concepts unknown to Darwin, such as DNA and genetics, which allow rigorous, in many cases mathematical, analyses of phenomena such as kin selection, altruism, and speciation.

A particular interpretation most commonly associated with Richard Dawkins, author of The Selfish Gene, asserts that the gene is the only true unit of selection. Dawkins further extended the Darwinian idea to include non-biological systems exhibiting the same type of selective behavior of the 'fittest' such as memes in culture.