Mutationism

Mutationism is one of several alternatives to evolution by natural selection that have existed both before and after the publication of Charles Darwin's 1859 book, On the Origin of Species. In the theory, mutation was the source of novelty, creating new forms and new species, potentially instantaneously,[1] in sudden jumps.[2] This was envisaged as driving evolution, which was thought to be limited by the supply of mutations.

Before Darwin, biologists commonly believed in saltationism, the possibility of large evolutionary jumps, including immediate speciation. For example, in 1822 Étienne Geoffroy Saint-Hilaire argued that species could be formed by sudden transformations, or what would later be called macromutation. Darwin opposed saltation, insisting on gradualism in evolution as in geology (uniformitarianism). In 1864, Albert von Kölliker revived Geoffroy's theory. In 1901 the geneticist Hugo de Vries gave the name "mutation" to seemingly new forms that suddenly arose in his experiments on the evening primrose Oenothera lamarckiana, and in the first decade of the 20th century, mutationism, or as de Vries named it mutationstheorie, became a rival to Darwinism supported for a while by geneticists including William Bateson, Thomas Hunt Morgan, and Reginald Punnett.

Understanding of mutationism is clouded by the mid-20th century portrayal of the early mutationists by supporters of the modern synthesis as opponents of Darwinian evolution and rivals of the biometrics school who argued that selection operated on continuous variation. In this portrayal, mutationism was defeated by a synthesis of genetics and natural selection that supposedly started later, around 1918, with work by the mathematician Ronald Fisher. However, the alignment of Mendelian genetics and natural selection began as early as 1902 with a paper by Udny Yule, and built up with theoretical and experimental work in Europe and America. Despite the controversy, the early mutationists had by 1918 already accepted natural selection and explained continuous variation as the result of multiple genes acting on the same characteristic, such as height.

Mutationism, along with other alternatives to Darwinism like Lamarckism and orthogenesis, was discarded by most biologists as they came to see that Mendelian genetics and natural selection could readily work together; mutation took its place as a source of the genetic variation essential for natural selection to work on. However, mutationism did not entirely vanish. In 1940, Richard Goldschmidt again argued for single-step speciation by macromutation, describing the organisms thus produced as "hopeful monsters", earning widespread ridicule. In 1987, Masatoshi Nei argued controversially that evolution was often mutation-limited. Modern biologists such as Douglas J. Futuyma conclude that essentially all claims of evolution driven by large mutations can be explained by Darwinian evolution.

Hugo de Vries (1848-1935), by Thérèse Schwartze (1851-1918)
Painting of Hugo de Vries, making a painting of an evening primrose, the plant which had apparently produced new forms by large mutations in his experiments, by Thérèse Schwartze, 1918

Developments leading up to mutationism

Geoffroy72
Étienne Geoffroy Saint-Hilaire believed that "monstrosities" could immediately found new species in a single large jump or saltation.

Geoffroy's monstrosities, 1822

Prior to Charles Darwin, most naturalists were saltationists,[a] believing that species evolved and that speciation took place in sudden jumps.[4] Jean-Baptiste Lamarck was a gradualist but similar to other scientists of the period had written that saltational evolution was possible.[5]

In 1822, in the second volume of his Philosophie anatomique, Étienne Geoffroy Saint-Hilaire endorsed a theory of saltational evolution that "monstrosities could become the founding fathers (or mothers) of new species by instantaneous transition from one form to the next."[6] Geoffroy wrote that environmental pressures could produce sudden transformations to establish new species instantaneously.[7]

Darwin's anti-saltationist gradualism, 1859

In his 1859 book On the Origin of Species, Charles Darwin denied saltational evolution. He argued that evolutionary transformation always proceeds gradually, never in jumps: "natural selection acts solely by accumulating slight successive favourable variations, it can produce no great or sudden modification; it can act only by very short steps". Darwin continued in this belief throughout his life.[8]

Kölliker Rudolph Albert von 1818-1902
Rudolph Albert von Kölliker revived Geoffroy's saltationist ideas, calling his theory heterogenesis. It depended on a nonmaterial directive force (orthogenesis).

Thomas Henry Huxley warned Darwin that he had taken on "an unnecessary difficulty in adopting Natura non facit saltum ["Nature does not take leaps"] so unreservedly."[9] Huxley feared this assumption could discourage naturalists (catastrophists) who believed that major leaps and cataclysms played a significant role in the history of life.[10]

von Kölliker's heterogenesis, 1864

In 1864 Albert von Kölliker revived Geoffroy's theory that evolution proceeds by large steps, under the name of heterogenesis, but this time assuming the influence of a nonmaterial force[b] to direct the course of evolution.[11][12]

Galton's "sports", 1892

Darwin's cousin, Francis Galton, considered Darwin's evidence for evolution, and came to an opposite conclusion about the type of variation on which natural selection must act. He carried out his own experiments and published a series of papers and books setting out his views. Already by 1869 when he published Hereditary Genius, he believed in evolution by saltation. In his 1889 book Natural Inheritance he argued that natural selection would benefit from accepting that the steps need not, as Darwin had stated, be minute. In his 1892 book Finger Prints, he stated directly that "The progress of evolution is not a smooth and uniform progression, but one that proceeds by jerks, through successive 'sports' (as they are called), some of them implying considerable organic changes; and each in its turn being favoured by Natural Selection".[13]

From 1860 to 1880 saltation had been a minority viewpoint, to the extent that Galton felt his writings were being universally ignored. By 1890 it became a widely held theory, and his views helped to launch a major controversy.[14][15]

Bateson@72 (detail)
Drawing of William Bateson, 1909, by the biologist Dennis G. Lillie

Bateson's discontinuous variation, 1894

William Bateson's 1894 book Materials for the Study of Variation, Treated with Especial Regard to Discontinuity in the Origin of Species marked the arrival of mutationist thinking, before the rediscovery of Mendel's laws.[16] He examined discontinuous variation (implying a form of saltation[17]) where it occurred naturally, following William Keith Brooks, Galton, Thomas Henry Huxley and St. George Jackson Mivart.[17]

Early 20th century mutationism

De Vries and Mendelian mutationstheorie, 1901

The main principle of the mutation theory is that species and varieties have originated by mutation, but are, at present, not known to have originated in any other way. — Hugo de Vries[18]

Hugo de Vries's careful 1901 studies of wild variants of the evening primrose Oenothera lamarckiana showed that distinct new forms could arise suddenly in nature, apparently at random, and could be propagated for many generations without dissipation or blending. He gave such changes the name "mutation".[c][20][21] By this, de Vries meant that a new form of the plant was created in a single step (not the same as a mutation in the modern sense); no long period of natural selection was required for speciation, and nor was reproductive isolation.[22] In the view of the historian of science Peter J. Bowler, De Vries used the term to mean[1]

large-scale genetic changes capable of producing a new subspecies, or even species, instantaneously.[1]

The historian of science Betty Smocovitis described mutationism as[2]

the case of purported saltatory evolution that Hugo de Vries had mistakenly interpreted for the evening primrose, Oenothera.[2]

De Vries set out his position, known as Mutationstheorie (mutation theory) on the creative nature of mutation in his 1905 book Species and Varieties: their Origin by Mutation.[23] In the view of the historian of science Edward Larson, de Vries was the person largely responsible for transforming Victorian era saltationism into early 20th century mutation theory, "and in doing so pushed Darwinism near the verge of extinction as a viable scientific theory".[24]

Johannsen's "pure line" experiments, 1903

Wilhelm Johannsen 1857-1927
Wilhelm Johannsen's "pure line" experiments seemed to show that evolution could not work on continuous variation.

In the early 1900s, Darwin's mechanism of natural selection was understood by believers in continuous variation, principally the biometricians Walter Weldon and Karl Pearson, to be able to work on a continuously varying characteristic, whereas de Vries argued that selection on such characteristics would be ineffective. Wilhelm Johannsen's "pure line" experiments on Phaseolus vulgaris beans appeared to refute this mechanism. Using the true-breeding Princess variety of bean, carefully inbred within weight classes, Johannsen's work appeared to support de Vries. The offspring had a smooth random distribution. Johanssen believed that his results showed that continuous variability was not inherited, so evolution must rely on discontinuous mutations, as de Vries had argued.[25][26][27][28] Johanssen published his work in Danish in a 1903 paper Om arvelighed i samfund og i rene linier (On inheritance in populations and in pure lines),[29] and in his 1905 book Arvelighedslærens Elementer (The Elements of Heredity).[30]

Punnett's mimicry, 1915

Common Mormon Papilio polytes Female Form Romulus by kadavoor
Papilio polytes has 3 forms with differing wing patterns, here the "Romulus" morph. Reginald Punnett argued that this polymorphism demonstrated discontinuous evolution. However, Ronald Fisher showed that this could have arisen by small changes in additional modifier genes.

In 1915, Reginald Punnett argued in his book Mimicry in Butterflies that the 3 morphs (forms) of the butterfly Papilio polytes, which mimic different host species of butterfly, demonstrated discontinuous evolution in action. The different forms existed in a stable polymorphism controlled by 2 Mendelian factors (genes). The alleles of these genes were certainly discontinuous, so Punnett supposed that they must have evolved in discontinuous leaps.[31]

The undermining of mutationism

Yule's analysis of Mendelism and continuous variation, 1902

The undermining of mutationism began almost at once, in 1902, as the statistician Udny Yule analysed Mendel's theory and showed that given full dominance of one allele over another, a 3:1 ratio of alleles would be sustained indefinitely. This meant that the recessive allele could remain in the population with no need to invoke mutation. He also showed that given multiple factors, Mendel's theory enabled continuous variation, as indeed Mendel had suggested, removing the central plank of the mutationist theory, and criticised Bateson's confrontational approach.[32] However, the "excellent"[33] paper did not prevent the Mendelians and the biometricians from falling out.[33]

Nilsson-Ehle's experiments on Mendelian inheritance and continuous variation, 1908

The Swedish geneticist H. Nilsson-Ehle demonstrated in 1908, in a paper published in German in a Swedish journal, Einige Ergebnisse von Kreuzungen bei Hafer und Weizen (Observations on Crosses in Oats and Wheat),[34] that continuous variation could readily be produced by multiple Mendelian genes. He found numerous Mendelian 3:1 ratios, implying a dominant and a recessive allele, in oats and wheat; a 15:1 ratio for a cross of oat varieties with black and white glumes respectively, implying two pairs of alleles (two Mendelian factors); and that crossing a red-grained Swedish velvet wheat with a white one gave in the third (F3) generation the complex signature of ratios expected of three factors at once, with 37 grains giving only red offspring, 8 giving 63:1 in their offspring, 12 giving 15:1, and 6 giving 3:1. There weren't any grains giving all white, but as he had only expected 1 of those in his sample, 0 was not an unlikely outcome. Genes could clearly combine in almost infinite combinations: ten of his factors allowed for almost 60,000 different forms, with no need to suppose that any new mutations were involved. The results implied that natural selection would work on Mendelian genes, helping to bring about the unification of Darwinian evolution and genetics.[35]

Similar work in America by Edward East on maize in 1910[36] showed the same thing for biologists without access to Nilsson-Ehle's work.[37] On the same theme, the mathematician Ronald Fisher published "The Correlation Between Relatives on the Supposition of Mendelian Inheritance" in 1918,[38] again showing that continuous variation could readily be produced by multiple Mendelian genes. It showed, too, that Mendelian inheritance had no essential link with mutationism: Fisher stressed that small variations (per gene) would be sufficient for natural selection to drive evolution.[39]

Castle's selection experiments on hooded rats, 1911

Starting in 1906, William Castle carried out a long study of the effect of selection on coat colour in rats. The piebald or hooded pattern was recessive to the grey wild type. He crossed hooded rats with the black-backed Irish type, and then back-crossed the offspring with pure hooded rats. The dark stripe on the back was bigger. He then tried selecting different groups for bigger or smaller stripes for 5 generations, and found that it was possible to change the characteristics way beyond the initial range of variation. This effectively refuted de Vries's claim that continuous variation could not be inherited permanently, requiring new mutations. By 1911 Castle noted that the results could be explained by Darwinian selection on heritable variation of Mendelian genes.[40]

Morgan's small Mendelian genes in Drosophila, 1912

Drosophila melanogaster - side (aka)
Thomas Hunt Morgan's work on Drosophila melanogaster found many small Mendelian factors for natural selection to work on.

By 1912, after years of work on the genetics of Drosophila fruit flies, Thomas Hunt Morgan showed that these animals had many small Mendelian factors on which Darwinian evolution could work as if variation was fully continuous. The way was open for geneticists to conclude that Mendelism supported Darwinism.[41]

Muller's balanced lethal explanation of Oenothera "mutations", 1918

De Vries's mutationism was dealt a serious if not fatal blow in 1918 by the American geneticist Hermann Joseph Muller. He compared the behaviour of balanced lethals in Drosophila with De Vries's supposed mutations in Oenothera, showing that they could work the same way.[42] No actual mutations were involved, but infrequent chromosome crossovers accounted for the sudden appearance of traits which had been present in the genes all along.[43]

Fisher's explanation of polymorphism, 1927

In 1927, Fisher explicitly attacked Punnett's 1915 theory of discontinuous evolution of mimicry. Fisher argued that selection acting on genes making small modifications to the butterfly's phenotype (its appearance) would allow the multiple forms of a polymorphism to be established.[39]

Later mutationist theories

The understanding that Mendelian genetics could both preserve discrete variations indefinitely, and support continuous variation for natural selection to work on gradually, meant that most biologists from around 1918 onwards accepted natural selection as the driving force of evolution.[44] Mutationism and other alternatives to evolution by natural selection did not however vanish entirely.[45][46][47]

Berg's nomogenesis, 1922

Lev Berg proposed a combination of mutationism and directed (orthogenetic) evolution in his 1922 book Nomogenesis; or, Evolution Determined by Law. He used evidence from paleontology, zoology, and botany to argue that natural selection had limitations which set a direction for evolution. He claimed that speciation was caused by "mass transformation of a great number of individuals" by directed mass mutations.[48][45]

The course of evolution by differentiation or divergent mutation rather than by selection (1940) (20709042401)
John Christopher Willis's The Course of Evolution by Differentiation Or Divergent Mutation Rather Than by Selection, 1940

Willis's macromutations, 1923

In 1923, the botanist John Christopher Willis proposed that species were formed by large mutations, not gradual evolution by natural selection,[49][50] and that evolution was driven by orthogenesis, which he called "differentiation", rather than by natural selection.[46]

Goldschmidt's hopeful monsters, 1940

Masatoshi Nei - 2013
Masatoshi Nei argues that evolution is often mutation-limited.[51]

In his 1940 book The Material Basis of Evolution, the German geneticist Richard Goldschmidt argued for single-step speciation by macromutation, describing the organisms thus produced as "hopeful monsters". Goldschmidt's thesis was universally rejected and widely ridiculed by biologists, who favoured the neo-Darwinian explanations of Fisher, J. B. S. Haldane and Sewall Wright.[47][52] However, interest in Goldschmidt's ideas has reawakened in the field of evolutionary developmental biology.[53][54][55][56][57]

Nei's mutation-driven evolution, 1987

Contemporary biologists accept that mutation and selection both play roles in evolution; the mainstream view is that while mutation supplies material for selection in the form of variation, all non-random outcomes are caused by natural selection.[58] Masatoshi Nei argues instead that the production of more efficient genotypes by mutation is fundamental for evolution, and that evolution is often mutation-limited.[51][59][60][61][62][63][64][65][66] Nei's book received thoughtful reviews; while Wright,[67] in the conservative journal Evolution, rejected Nei's thinking as mistaken, Galtier,[68] Weiss,[69] Stoltzfus,[51] and Wagner,[58] although not necessarily agreeing with Nei's position, treated it as a relevant alternative view.

Contemporary approaches

Reviewing the history of macroevolutionary theories, the American evolutionary biologist Douglas J. Futuyma notes that since 1970, two very different alternatives to Darwinian gradualism have been proposed, both by Stephen Jay Gould: mutationism, and punctuated equilibria.[70][71] Gould's macromutation theory gave a nod to his predecessor with an envisaged "Goldschmidt break" between evolution within a species and speciation. His advocacy of Goldschmidt was attacked with "highly unflattering comments"[70] by Brian Charlesworth[72] and Alan Templeton.[73] Futuyma concludes, following other biologists reviewing the field such as K.Sterelny[74] and A. Minelli,[75] that essentially all the claims of evolution driven by large mutations could be explained within the Darwinian evolutionary synthesis.[70] James A. Shapiro's claim that molecular genetics undermines Darwinism has been described as mutationism and an extreme view by the zoologist Andy Gardner.[76]

Alternatives to Darwinism
Multiple explanations have been offered since the 19th century for how evolution took place, given that many scientists initially had objections to natural selection.[77] Many of these, including mutationism, led to some form of orthogenesis (solid blue arrows), with or without theistic intervention (dotted blue arrows). Better understanding of mutation led instead (dashed orange arrows) to the modern synthesis of Mendelian genetics with natural selection, establishing Darwinian evolution throughout biology.[78]

Historiography

Biologists at the start of the 20th century broadly agreed that evolution occurred, but felt that the mechanisms suggested by Darwin, including natural selection, would be ineffective. Large mutations looked likely to drive evolution quickly, and avoided the difficulty which had rightly worried Darwin, namely that blending inheritance would average out any small favourable changes.[d][80] Further, large saltatory mutation, able to create species in a single step, offered a ready explanation of why the fossil record should contain large discontinuities and times of rapid change.[81]

These discoveries were often framed by supporters of the mid-20th century modern synthesis, such as Julian Huxley and Ernst Mayr, as a controversy between the early geneticists—the "Mendelians"—including Bateson, Johannsen, de Vries, Morgan, and Punnett, who advocated Mendelism and mutation, and were understood as opponents of Darwin's original gradualist view, and the biometricians such as Pearson and Weldon, who opposed Mendelism and were more faithful to Darwin. In this version, little progress was made during the eclipse of Darwinism, and the debate between mutationist geneticists such as de Vries and biometricians such as Pearson ended with the victory of the modern synthesis between about 1918 and 1950.[82][64] According to this account, the new population genetics of the 1940s demonstrated the explanatory power of natural selection, while mutationism, alongside other non-Darwinian approaches such as orthogenesis and structuralism, was essentially abandoned.[83] This view became dominant in the second half of the 20th century, and was accepted by both biologists and historians.[84]

A more recent view, advocated by the historians Arlin Stoltzfus and Kele Cable, is that Bateson, de Vries, Morgan and Punnett had by 1918 formed a synthesis of Mendelism and mutationism. The understanding achieved by these geneticists spanned the action of natural selection on alleles (alternative forms of a gene), the Hardy-Weinberg equilibrium, the evolution of continuously-varying traits (like height), and the probability that a new mutation will become fixed. In this view, the early geneticists accepted natural selection alongside mutation, but rejected Darwin's non-Mendelian ideas about variation and heredity, and the synthesis began soon after 1900.[64][85] The traditional claim that Mendelians rejected the idea of continuous variation outright is simply false; as early as 1902, Bateson and Edith Saunders wrote that "If there were even so few as, say, four or five pairs of possible allelomorphs, the various homo- and hetero-zygous combinations might, on seriation, give so near an approach to a continuous curve, that the purity of the elements would be unsuspected".[86]

Historians have interpreted the history of mutationism in different ways.[82][87][26][88]The classical view is that mutationism, opposed to Darwin's gradualism, was an obvious error; the decades-long delay in synthesizing genetics and Darwinism is an "inexplicable embarrassment";[89] genetics led logically to the modern synthesis and mutationism was one of several anti-Darwinian "blind alleys" separate from the main line leading from Darwin to the present.[90] A revisionist view is that mutationists accepted both mutation and selection, with broadly the same roles they have today, and early on accepted and indeed offered a correct explanation for continuous variation based on multiple genes, paving the way for gradual evolution. At the time of the Darwin centennial in Cambridge in 1909, mutationism and Lamarckism were contrasted with natural selection as competing ideas; 50 years later, at the 1959 University of Chicago centennial of the publication of On the Origin of Species, mutationism was no longer seriously considered.[91][84]

See also

Notes

  1. ^ The term mutation was not used in biology until the 20th century, but macromutation and saltation are essentially equivalent descriptions.[3]
  2. ^ Orthogenesis, possibly vitalism.
  3. ^ The changes in the evening primrose were later shown to be caused by chromosome duplications (polyploidy) rather than gene mutation.[19]
  4. ^ Mendelian inheritance, with discrete alleles, solves Darwin's problem, as blending does not occur.[79]

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Alternatives to evolution by natural selection

Alternatives to evolution by natural selection, also described as non-Darwinian mechanisms of evolution, have been proposed by scholars investigating biology since classical times to explain signs of evolution and the relatedness of different groups of living things.

The alternatives in question do not deny that evolutionary changes over time are the origin of the diversity of life, nor deny that the organisms alive today share a common ancestor from the distant past (or ancestors, in some proposals); rather, they propose alternative mechanisms of evolutionary change over time, arguing against mutations acted on by natural selection as the most important driver of evolutionary change. (In most cases, they do not deny that mutations or natural selection occur, or that they play a role in evolutionary change, but instead deny that they are fully sufficient primary causes for the evidence of evolutionary change that is observed in the natural world.)

This distinguishes them from certain other kinds of arguments that deny that large scale evolution of any sort has taken place, as in some forms of creationism, which do not propose alternative mechanisms of evolutionary change but instead deny that evolutionary change has taken place at all. Not all forms of creationism deny that evolutionary change takes places; notably, proponents of theistic evolution, such as the biologist Asa Gray, assert that evolutionary change does occur and is responsible for the history of life on Earth, with the proviso that this process has been influenced by a god or gods in some meaningful sense.

Where the fact of evolutionary change was accepted but the mechanism proposed by Charles Darwin, natural selection, was denied, explanations of evolution such as Lamarckism, catastrophism, orthogenesis, vitalism, structuralism and mutationism (called saltationism before 1900) were entertained. Different factors motivated people to propose non-Darwinian mechanisms of evolution. Natural selection, with its emphasis on death and competition, did not appeal to some naturalists because they felt it immoral, leaving little room for teleology or the concept of progress in the development of life. Some who came to accept evolution, but disliked natural selection, raised religious objections. Others felt that evolution was an inherently progressive process that natural selection alone was insufficient to explain. Still others felt that nature, including the development of life, followed orderly patterns that natural selection could not explain.

By the start of the 20th century, evolution was generally accepted by biologists but natural selection was in eclipse. Many alternative theories were proposed, but biologists were quick to discount theories such as orthogenesis, vitalism and Lamarckism which offered no mechanism for evolution. Mutationism did propose a mechanism, but it was not generally accepted. The modern synthesis a generation later claimed to sweep away all the alternatives to Darwinian evolution, though some have been revived as molecular mechanisms for them have been discovered.

History of evolutionary thought

Evolutionary thought, the conception that species change over time, has roots in antiquity – in the ideas of the ancient Greeks, Romans, and Chinese as well as in medieval Islamic science. With the beginnings of modern biological taxonomy in the late 17th century, two opposed ideas influenced Western biological thinking: essentialism, the belief that every species has essential characteristics that are unalterable, a concept which had developed from medieval Aristotelian metaphysics, and that fit well with natural theology; and the development of the new anti-Aristotelian approach to modern science: as the Enlightenment progressed, evolutionary cosmology and the mechanical philosophy spread from the physical sciences to natural history. Naturalists began to focus on the variability of species; the emergence of paleontology with the concept of extinction further undermined static views of nature. In the early 19th century Jean-Baptiste Lamarck (1744 – 1829) proposed his theory of the transmutation of species, the first fully formed theory of evolution.

In 1858 Charles Darwin and Alfred Russel Wallace published a new evolutionary theory, explained in detail in Darwin's On the Origin of Species (1859). Unlike Lamarck, Darwin proposed common descent and a branching tree of life, meaning that two very different species could share a common ancestor. Darwin based his theory on the idea of natural selection: it synthesized a broad range of evidence from animal husbandry, biogeography, geology, morphology, and embryology. Debate over Darwin's work led to the rapid acceptance of the general concept of evolution, but the specific mechanism he proposed, natural selection, was not widely accepted until it was revived by developments in biology that occurred during the 1920s through the 1940s. Before that time most biologists regarded other factors as responsible for evolution. Alternatives to natural selection suggested during "the eclipse of Darwinism" (c. 1880 to 1920) included inheritance of acquired characteristics (neo-Lamarckism), an innate drive for change (orthogenesis), and sudden large mutations (saltationism). Mendelian genetics, a series of 19th-century experiments with pea plant variations rediscovered in 1900, was integrated with natural selection by Ronald Fisher, J. B. S. Haldane, and Sewall Wright during the 1910s to 1930s, and resulted in the founding of the new discipline of population genetics. During the 1930s and 1940s population genetics became integrated with other biological fields, resulting in a widely applicable theory of evolution that encompassed much of biology—the modern synthesis.

Following the establishment of evolutionary biology, studies of mutation and genetic diversity in natural populations, combined with biogeography and systematics, led to sophisticated mathematical and causal models of evolution. Paleontology and comparative anatomy allowed more detailed reconstructions of the evolutionary history of life. After the rise of molecular genetics in the 1950s, the field of molecular evolution developed, based on protein sequences and immunological tests, and later incorporating RNA and DNA studies. The gene-centered view of evolution rose to prominence in the 1960s, followed by the neutral theory of molecular evolution, sparking debates over adaptationism, the unit of selection, and the relative importance of genetic drift versus natural selection as causes of evolution. In the late 20th-century, DNA sequencing led to molecular phylogenetics and the reorganization of the tree of life into the three-domain system by Carl Woese. In addition, the newly recognized factors of symbiogenesis and horizontal gene transfer introduced yet more complexity into evolutionary theory. Discoveries in evolutionary biology have made a significant impact not just within the traditional branches of biology, but also in other academic disciplines (for example: anthropology and psychology) and on society at large.

Hugo de Vries

Hugo Marie de Vries (Dutch pronunciation: [ˈɦyɣoː də ˈvris]) (16 February 1848 – 21 May 1935) was a Dutch botanist and one of the first geneticists. He is known chiefly for suggesting the concept of genes, rediscovering the laws of heredity in the 1890s while apparently unaware of Gregor Mendel's work, for introducing the term "mutation", and for developing a mutation theory of evolution.

John Christopher Willis

John Christopher Willis FRS (20 February 1868 – 21 March 1958) was an English botanist known for his Age and Area hypothesis and criticism of natural selection.

Lev Berg

Lev Semyonovich Berg (also known as Leo S. Berg) (Russian: Лев Семёнович Берг; 14 March 1876, Bender – 24 December 1950, Leningrad) was a leading Russian geographer, biologist and ichthyologist who served as President of the Soviet Geographical Society between 1940 and 1950.

He is known for his own evolutionary theory, nomogenesis (a form of orthogenesis incorporating mutationism) as opposed to the theories of Darwin and Lamarck.

Louis Blaringhem

Louis Florimond Joseph Blaringhem (1 February 1878 in Locon – 1 January 1958 in Paris) was a French agronomist and botanist.

Luís Wittnich Carrisso

Luís Wittnich Carrisso (14 February 1886 – 14 June 1937) was a Portuguese botanist, professor at the University of Coimbra.

Carrisso was born in Figueira da Foz. He attended the Faculty of Philosophy of University of Coimbra (1904-1910). After graduating he became a student of botanist Julio Augusto Henriques. Carrisso took interest in evolution and heredity and presented his PhD thesis Hereditariedade in 1911. He published scientific work on ecology and plant systematics.In 1918, he became Professor Botany at University of Coimbra's Botanical Garden. He was a supporter of Charles Darwin's theory of evolution but was skeptical of the role of natural selection. He embraced mutationism.

Masatoshi Nei

Masatoshi Nei (根井正利, Nei Masatoshi) is a population geneticist currently affiliated with the Department of Biology at Temple University as a Carnell Professor. He was, until recently, Evan Pugh Professor of Biology at Pennsylvania State University and Director of the Institute of Molecular Evolutionary Genetics and had been there from 1990 to 2015.

He was born in 1931 in Miyazaki Prefecture, on Kyūshū Island, Japan. He was associate professor and professor of biology at Brown University from 1969 to 1972 and professor of population genetics at the Center for Demographic and Population Genetics, University of Texas at Houston, from 1972 to 1990. He is a theoretical population geneticist and evolutionary biologist. Acting alone or working with his students, he has continuously developed statistical theories of molecular evolution taking into account discoveries in molecular biology. He has also developed concepts in evolutionary theory and advanced the theory of mutation-driven evolution.

Modern synthesis (20th century)

The modern synthesis was the early 20th-century synthesis reconciling Charles Darwin's theory of evolution and Gregor Mendel's ideas on heredity in a joint mathematical framework. Julian Huxley coined the term in his 1942 book, Evolution: The Modern Synthesis.

The 19th century ideas of natural selection and Mendelian genetics were put together with population genetics, early in the twentieth century. The modern synthesis also addressed the relationship between the broad-scale changes of macroevolution seen by palaeontologists and the small-scale microevolution of local populations of living organisms. The synthesis was defined differently by its founders, with Ernst Mayr in 1959, G. Ledyard Stebbins in 1966 and Theodosius Dobzhansky in 1974 offering differing numbers of basic postulates, though they all included natural selection, working on heritable variation supplied by mutation. Other major figures in the synthesis included E. B. Ford, Bernhard Rensch, Ivan Schmalhausen, and George Gaylord Simpson. An early event in the modern synthesis was R. A. Fisher's 1918 paper on mathematical population genetics, but William Bateson, and separately Udny Yule, were already starting to show how Mendelian genetics could work in evolution in 1902.

Different syntheses followed, accompanying the gradual breakup of the early 20th century synthesis, including with social behaviour in E. O. Wilson's sociobiology in 1975, evolutionary developmental biology's integration of embryology with genetics and evolution, starting in 1977, and Massimo Pigliucci's proposed extended evolutionary synthesis of 2007. In the view of the evolutionary biologist Eugene Koonin in 2009, the modern synthesis will be replaced by a 'post-modern' synthesis that will include revolutionary changes in molecular biology, the study of prokaryotes and the resulting tree of life, and genomics.

Mutation

In biology, a mutation is the alteration of the nucleotide sequence of the genome of an organism, virus, or extrachromosomal DNA.Mutations result from errors during DNA replication (especially during meiosis) or other types of damage to DNA (such as may be caused by exposure to radiation or carcinogens), which then may undergo error-prone repair (especially microhomology-mediated end joining), or cause an error during other forms of repair, or else may cause an error during replication (translesion synthesis). Mutations may also result from insertion or deletion of segments of DNA due to mobile genetic elements. Mutations may or may not produce discernible changes in the observable characteristics (phenotype) of an organism. Mutations play a part in both normal and abnormal biological processes including: evolution, cancer, and the development of the immune system, including junctional diversity.

The genomes of RNA viruses are based on RNA rather than DNA. The RNA viral genome can be double-stranded (as in DNA) or single-stranded. In some of these viruses (such as the single-stranded human immunodeficiency virus) replication occurs quickly and there are no mechanisms to check the genome for accuracy. This error-prone process often results in mutations.

Mutation can result in many different types of change in sequences. Mutations in genes can either have no effect, alter the product of a gene, or prevent the gene from functioning properly or completely. Mutations can also occur in nongenic regions. One study on genetic variations between different species of Drosophila suggests that, if a mutation changes a protein produced by a gene, the result is likely to be harmful, with an estimated 70 percent of amino acid polymorphisms that have damaging effects, and the remainder being either neutral or marginally beneficial. Due to the damaging effects that mutations can have on genes, organisms have mechanisms such as DNA repair to prevent or correct mutations by reverting the mutated sequence back to its original state.

Oenothera lamarckiana

Oenothera lamarckiana (evening primrose or evening-primrose) is a plant species in family Onagraceae, indigenous to North American Plains (all regions of North America). The species was important to the study of genetics, the debate about the cause of evolution, whether that was natural selection or one of the alternatives such as mutationism, and particularly to the discovery of polyploidy.

Otto Schindewolf

Otto Heinrich Schindewolf (7 June 1896 – 10 June 1971) was a German paleontologist who studied the evolution of corals and cephalopods.

Reginald Punnett

Reginald Crundall Punnett FRS (; 20 June 1875 – 3 January 1967) was a British geneticist who co-founded, with William Bateson, the Journal of Genetics in 1910. Punnett is probably best remembered today as the creator of the Punnett square, a tool still used by biologists to predict the probability of possible genotypes of offspring. His Mendelism (1905) is sometimes said to have been the first textbook on genetics; it was probably the first popular science book to introduce genetics to the public.

Richard Goldschmidt

Richard Benedict Goldschmidt (April 12, 1878 – April 24, 1958) was a German-born American geneticist. He is considered the first to attempt to integrate genetics, development, and evolution. He pioneered understanding of reaction norms, genetic assimilation, dynamical genetics, sex determination, and heterochrony. Controversially, Goldschmidt advanced a model of macroevolution through macromutations popularly known as the "Hopeful Monster" hypothesis.Goldschmidt also described the nervous system of the nematode, a piece of work that influenced Sydney Brenner to study the wiring diagram of Caenorhabditis elegans, winning Brenner and his colleagues the Nobel Prize in 2002.

Robert Heath Lock

Robert Heath Lock (January 19, 1879 – June 26, 1915) was an English botanist and geneticist who wrote the first English textbook on genetics.

Saltation (biology)

In biology, saltation (from Latin, saltus, "leap") is a sudden and large mutational change from one generation to the next, potentially causing single-step speciation. This was historically offered as an alternative to Darwinism. Some forms of mutationism were effectively saltationist, implying large discontinuous jumps.

Speciation, such as by polyploidy in plants, can sometimes be achieved in a single and in evolutionary terms sudden step. Evidence exists for various forms of saltation in a variety of organisms.

Søren Løvtrup

Soren Løvtrup (1922–2002) was a Danish embryologist and historian of science in the Department of Animal Physiology at the Umeå University, Sweden. Løvtrup was known for his macromutation theory of evolution, which was in opposition to traditional neo-Darwinism. In 1987 Løvtrup published his controversial book Darwinism: The Refutation of a Myth which challenged Charles Darwin's role as the intellectual founder of evolutionary theory and accused Darwin of plagiarism.

The eclipse of Darwinism

Julian Huxley used the phrase “the eclipse of Darwinism” to describe the state of affairs prior to what he called the modern synthesis, when evolution was widely accepted in scientific circles but relatively few biologists believed that natural selection was its primary mechanism. Historians of science such as Peter J. Bowler have used the same phrase as a label for the period within the history of evolutionary thought from the 1880s to around 1920, when alternatives to natural selection were developed and explored—as many biologists considered natural selection to have been a wrong guess on Charles Darwin's part, or at least as of relatively minor importance. An alternative term, the interphase of Darwinism, has been proposed to avoid the largely incorrect implication that the putative eclipse was preceded by a period of vigorous Darwinian research.While there had been multiple explanations of evolution including vitalism, catastrophism, and structuralism through the 19th century, four major alternatives to natural selection were in play at the turn of the 20th century:

Theistic evolution was the belief that God directly guided evolution.

Neo-Lamarckism was the idea that evolution was driven by the inheritance of characteristics acquired during the life of the organism.

Orthogenesis was the belief that organisms were affected by internal forces or laws of development that drove evolution in particular directions

Mutationism was the idea that evolution was largely the product of mutations that created new forms or species in a single step.Theistic evolution largely disappeared from the scientific literature by the end of the 19th century as direct appeals to supernatural causes came to be seen as unscientific. The other alternatives had significant followings well into the 20th century; mainstream biology largely abandoned them only when developments in genetics made them seem increasingly untenable, and when the development of population genetics and the modern synthesis demonstrated the explanatory power of natural selection. Ernst Mayr wrote that as late as 1930 most textbooks still emphasized such non-Darwinian mechanisms.

William Bateson

William Bateson (8 August 1861 – 8 February 1926) was an English biologist who was the first person to use the term genetics to describe the study of heredity, and the chief populariser of the ideas of Gregor Mendel following their rediscovery in 1900 by Hugo de Vries and Carl Correns. His 1894 book Materials for the Study of Variation was one of the earliest formulations of the new approach to genetics.

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