Alternatives to Darwinian evolution have been proposed by scholars investigating biology to explain signs ofevolution and therelatedness 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 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.

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 ofcreationism, 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 place; notably, proponents oftheistic evolution, such as the biologistAsa 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 byCharles Darwin,natural selection, was denied, explanations of evolution such asLamarckism,catastrophism,orthogenesis,vitalism,structuralism andmutationism (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 forteleology or the concept of progress (orthogenesis) 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 butnatural selection was in eclipse.^[2] 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. Themodern 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.

Aristotle did not embrace either divine creation or evolution, instead arguingin his biology that each species (eidos) was immutable, breeding true to itsideal eternal form (not the same as Plato'stheory of forms).^[3][4] Aristotle's suggestion inDe Generatione Animalium of a fixed hierarchy in nature - ascala naturae ("ladder of nature") provided an early explanation of the continuity of living things.^[5][6][7] Aristotle saw that animals wereteleological (functionally end-directed), and had parts that werehomologous with those of other animals, but he did not connect these ideas into a concept of evolutionary progress.^[8]

In the Middle Ages,Scholasticism developed Aristotle's view into the idea of agreat chain of being.^[1] The image of a ladder inherently suggests the possibility of climbing, but both the ancient Greeks and mediaeval scholastics such asRamon Llull^[1] maintained that each species remained fixed from the moment of its creation.^[9][8]

By 1818, however,Étienne Geoffroy Saint-Hilaire argued in hisPhilosophie anatomique that the chain was "a progressive series", where animals like molluscs low on the chain could "rise, by addition of parts, from the simplicity of the first formations to the complication of the creatures at the head of the scale", given sufficient time. Accordingly, Geoffroy and later biologists looked for explanations of such evolutionary change.^[10]

Georges Cuvier's 1812Recherches sur les Ossements Fossiles set out his doctrine of the correlation of parts, namely that since an organism was a whole system, all its parts mutually corresponded, contributing to the function of the whole. So, from a single bone the zoologist could often tell what class or even genus the animal belonged to. And if an animal had teeth adapted for cutting meat, the zoologist could be sure without even looking that its sense organs would be those of a predator and its intestines those of a carnivore. A species had an irreducible functional complexity, and "none of its parts can change without the others changing too".^[11] Evolutionists expected one part to change at a time, one change to follow another. In Cuvier's view, evolution was impossible, as any one change would unbalance the whole delicate system.^[11]

Louis Agassiz's 1856 "Essay on Classification" exemplified German philosophical idealism. This held that each species was complex within itself, had complex relationships to other organisms, and fitted precisely into its environment, as a pine tree in a forest, and could not survive outside those circles. The argument from such ideal forms opposed evolution without offering an actual alternative mechanism.Richard Owen held a similar view in Britain.^[12]

The Lamarckian social philosopher and evolutionistHerbert Spencer, ironically the author of the phrase "survival of the fittest" adopted byDarwin,^[13] used an argument like Cuvier's to oppose natural selection. In 1893, he stated that a change in any one structure of the body would require all the other parts to adapt to fit in with the new arrangement. From this, he argued that it was unlikely that all the changes could appear at the right moment if each one depended on random variation; whereas in a Lamarckian world, all the parts would naturally adapt at once, through a changed pattern of use and disuse.^[14]

Where the fact of evolutionary change was accepted by biologists butnatural selection was denied, including but not limited to the late 19th centuryeclipse of Darwinism, alternative scientific explanations such asLamarckism,orthogenesis,structuralism,catastrophism,vitalism and theistic evolution^[a] were entertained, not necessarily separately. (Purely religious points of view such as young or old earthcreationism orintelligent design are not considered here.) Different factors motivated people to propose non-Darwinian evolutionary mechanisms. Natural selection, with its emphasis on death and competition, did not appeal to some naturalists because they felt it immoral, leaving little room forteleology or the concept of progress in the development of life.^[15][16] Some of these scientists and philosophers, likeSt. George Jackson Mivart andCharles Lyell, who came to accept evolution but disliked natural selection, raised religious objections.^[17] Others, such as the biologist and philosopherHerbert Spencer, the botanistGeorge Henslow (son of Darwin's mentorJohn Stevens Henslow, also a botanist), and the authorSamuel Butler, felt that evolution was an inherently progressive process that natural selection alone was insufficient to explain. Still others, including the American paleontologistsEdward Drinker Cope andAlpheus Hyatt, had an idealist perspective and felt that nature, including the development of life, followed orderly patterns that natural selection could not explain.^[18]

Some felt that natural selection would be too slow, given the estimates of theage of the earth and sun (10–100 million years) being made at the time by physicists such asLord Kelvin, and some felt that natural selection could not work because at the time the models for inheritance involved blending of inherited characteristics, an objection raised by the engineerFleeming Jenkin in a review ofOrigin written shortly after its publication.^[18][19] Another factor at the end of the 19th century was the rise of a new faction of biologists, typified by geneticists likeHugo de Vries andThomas Hunt Morgan, who wanted to recast biology as an experimental laboratory science. They distrusted the work of naturalists like Darwin andAlfred Russel Wallace, dependent on field observations of variation, adaptation, andbiogeography, as being overly anecdotal. Instead they focused on topics likephysiology andgenetics that could be investigated with controlled experiments in the laboratory, and discounted less accessible phenomena like natural selection andadaptation to the environment.^[20]

Vitalism holds that living organisms differ from other things in containing something non-physical, such as a fluid or vital spirit, that makes them live.^[33] The theory dates to ancient Egypt.^[34][21]SinceEarly Modern times, vitalism stood in contrast to the mechanistic explanation of biological systems started byDescartes. Nineteenth century chemists set out to disprove the claim that forming organic compounds required vitalist influence.^[33] In 1828,Friedrich Wöhler showed thaturea could be made entirely from inorganic chemicals.^[35]Louis Pasteur believed thatfermentation required whole organisms, which he supposed carried out chemical reactions found only in living things. The embryologistHans Driesch, experimenting onsea urchin eggs, showed that separating the first two cells led to two complete but smallblastulas, seemingly showing that cell division did not divide the egg into sub-mechanisms, but created more cells each with the vital capability to form a new organism. Vitalism faded out with the demonstration of more satisfactory mechanistic explanations of each of the functions of a living cell or organism.^[33][36] By 1931, biologists had "almost unanimously abandoned vitalism as an acknowledged belief."^[37]

The American botanistAsa Gray used the name "theistic evolution"^[b] for his point of view, presented in his 1876 bookEssays and Reviews Pertaining to Darwinism.^[38] He argued that the deity supplies beneficial mutations to guide evolution.St George Jackson Mivart argued instead in his 1871On the Genesis of Species that the deity, equipped with foreknowledge, sets the direction of evolution by specifying the (orthogenetic) laws that govern it, and leaves species to evolve according to the conditions they experience as time goes by.The Duke of Argyll set out similar views in his 1867 bookThe Reign of Law.^[23][39] According to the historian Edward Larson, the theory failed as an explanation in the minds of late 19th century biologists as it broke the rules ofmethodological naturalism which they had grown to expect.^[22] Accordingly, by around 1900, biologists no longer saw theistic evolution as a valid theory. In Larson's view, by then it "did not even merit a nod among scientists."^[23] In the 20th century, theistic evolution could take other forms, such as theorthogenesis of Teilhard de Chardin.^[40]

Orthogenesis or Progressionism is the hypothesis that life has an innate tendency to change, developing in a unilinear fashion in a particular direction, or simply making some kind of definite progress. Many different versions have been proposed, some such as that ofTeilhard de Chardin openly spiritual, others such asTheodor Eimer's apparently simply biological. These theories often combined orthogenesis with other supposed mechanisms. For example, Eimer believed in Lamarckian evolution, but felt that internal laws of growth determined which characteristics would be acquired and would guide the long-term direction of evolution.^[41][42]

Orthogenesis was popular among paleontologists such asHenry Fairfield Osborn. They believed that the fossil record showed unidirectional change, but did not necessarily accept that the mechanism driving orthogenesis wasteleological (goal-directed). Osborn argued in his 1918 bookOrigin and Evolution of Life that trends inTitanothere horns were both orthogenetic and non-adaptive, and could be detrimental to the organism. For instance, they supposed that the large antlers of theIrish elk had caused its extinction.^[41][42]

Support for orthogenesis fell during themodern synthesis in the 1940s when it became apparent that it could not explain the complex branching patterns of evolution revealed by statistical analysis of thefossil record.^[18][19] Work in the 21st century has supported the mechanism and existence of mutation-biased adaptation (a form of mutationism), meaning that constrained orthogenesis is now seen as possible.^[43][44][45] Moreover, theself-organizing processes involved in certain aspects ofembryonic development often exhibit stereotypical morphological outcomes, suggesting that evolution will proceed in preferred directions once key molecular components are in place.^[46]

Jean-Baptiste Lamarck's 1809 evolutionary theory,transmutation of species, was based on a progressive (orthogenetic) drive toward greater complexity. Lamarck also shared the belief, common at the time, thatcharacteristics acquired during an organism's life could be inherited by the next generation, producing adaptation to the environment. Such characteristics were caused by the use or disuse of the affected part of the body. This minor component of Lamarck's theory became known, much later, asLamarckism.^[26] Darwin includedEffects of the increased Use and Disuse of Parts, as controlled by Natural Selection inOn the Origin of Species, giving examples such as large ground feeding birds getting stronger legs through exercise, and weaker wings from not flying until, like theostrich, they could not fly at all.^[47] In the late 19th century,neo-Lamarckism was supported by the German biologistErnst Haeckel, the AmericanpaleontologistsEdward Drinker Cope andAlpheus Hyatt, and the AmericanentomologistAlpheus Packard. Butler and Cope believed that this allowed organisms to effectively drive their own evolution.^[48] Packard argued that the loss of vision in the blind cave insects he studied was best explained through a Lamarckian process of atrophy through disuse combined with inheritance of acquired characteristics.^[48][49][50] Meanwhile, the English botanistGeorge Henslow studied how environmental stress affected the development of plants, and he wrote that the variations induced by such environmental factors could largely explain evolution; he did not see the need to demonstrate that such variations could actually be inherited.^[51] Critics pointed out that there was no solid evidence for the inheritance of acquired characteristics. Instead, the experimental work of the German biologistAugust Weismann resulted in the germ plasm theory of inheritance, which Weismann said made the inheritance of acquired characteristics impossible, since theWeismann barrier would prevent any changes that occurred to the body after birth from being inherited by the next generation.^[49][52]

In modernepigenetics, biologists observe thatphenotypes depend on heritable changes togene expression that do not involve changes to theDNA sequence.These changes can cross generations in plants, animals, andprokaryotes. This is not identical to traditional Lamarckism, as the changes do not last indefinitely and do not affect the germ line and hence the evolution of genes.^[53]

Catastrophism is thehypothesis, argued by the Frenchanatomist andpaleontologistGeorges Cuvier in his 1812Recherches sur les ossements fossiles de quadrupèdes, that the variousextinctions and the patterns offaunal succession seen in thefossil record were caused by large-scale natural catastrophes such as volcanic eruptions and, for the most recent extinctions in Eurasia, the inundation of low-lying areas by thesea. This was explained purely by natural events: he did not mentionNoah's flood,^[54] nor did he ever refer to divine creation as the mechanism for repopulation after an extinction event, though he did not support evolutionary theories such as those of his contemporaries Lamarck and Geoffroy Saint-Hilaire either.^[55][56] Cuvier believed that thestratigraphic record indicated that there had been several such catastrophes, recurring natural events, separated by long periods of stability during the history of life on earth. This led him to believe the Earth was several million years old.^[57]

Catastrophism has found a place in modern biology with theCretaceous–Paleogene extinction event at the end of theCretaceous period, as proposed in a paper byWalter andLuis Alvarez in 1980. It argued that a 10 kilometres (6.2 mi)asteroidstruck Earth 66 million years ago at the end of theCretaceous period. The event, whatever it was, made about 70% of all species extinct, including thedinosaurs, leaving behind theCretaceous–Paleogene boundary.^[58] In 1990, a 180 kilometres (110 mi) candidate crater marking the impact was identified atChicxulub in theYucatán Peninsula ofMexico.^[59]

Biological structuralism objects to an exclusively Darwinian explanation of natural selection, arguing that other mechanisms also guide evolution, and sometimes implying that these supersede selection altogether.^[28] Structuralists have proposed different mechanisms that might have guided the formation ofbody plans. Before Darwin,Étienne Geoffroy Saint-Hilaire argued that animals sharedhomologous parts, and that if one was enlarged, the others would be reduced in compensation.^[60] After Darwin,D'Arcy Thompson hinted atvitalism and offered geometric explanations in his classic 1917 bookOn Growth and Form.^[28]Adolf Seilacher suggested mechanical inflation for "pneu" structures inEdiacaran biota fossils such asDickinsonia.^[61][62]Günter P. Wagner argued for developmental bias, structural constraints onembryonic development.^[63][64]Stuart Kauffman favouredself-organisation, the idea that complex structure emergesholistically and spontaneously from the dynamic interaction of all parts of anorganism.^[65]Michael Denton argued for laws of form by whichPlatonic universals or "Types" are self-organised.^[66] In 1979Stephen J. Gould andRichard Lewontin proposedbiological "spandrels", features created as a byproduct of the adaptation of nearby structures.^[63]Gerd Müller andStuart Newman argued that the appearance in thefossil record of most of the currentphyla in theCambrian explosion was "pre-Mendelian" evolution caused by plastic responses of morphogenetic systems that were partly organized by physical mechanisms.^[67][68]Brian Goodwin, described by Wagner as part of "afringe movement in evolutionary biology",^[63] denied that biological complexity can bereduced to natural selection, and argued thatpattern formation is driven bymorphogenetic fields.^[69] Darwinian biologists have criticised structuralism, emphasising that there is plentiful evidence fromdeep homology thatgenes have been involved in shaping organisms throughoutevolutionary history. They accept that some structures such as thecell membrane self-assemble, but question the ability of self-organisation to drive large-scale evolution.^[70][71]

Saltationism^[72][73] held that new species arise as a result of largemutations. It was seen as a much faster alternative to the Darwinian concept of a gradual process of small random variations being acted on by natural selection. It was popular with early geneticists such asHugo de Vries, who along withCarl Correns helped rediscoverGregor Mendel's laws of inheritance in 1900,William Bateson, a British zoologist who switched to genetics, and early in his career,Thomas Hunt Morgan. These ideas developed intomutationism, the mutation theory of evolution.^[29][30] This held that species went through periods of rapid mutation, possibly as a result of environmental stress, that could produce multiple mutations, and in some cases completely new species, in a single generation, based on de Vries's experiments with the evening primrose,Oenothera, from 1886. The primroses seemed to be constantly producing new varieties with striking variations in form and color, some of which appeared to be new species because plants of the new generation could only be crossed with one another, not with their parents.^[74] However,Hermann Joseph Muller showed in 1918 that the new varieties de Vries had observed were the result ofpolyploid hybrids rather than rapid genetic mutation.^[75]

Initially, de Vries and Morgan believed that mutations were so large as to create new forms such as subspecies or even species instantly. Morgan's 1910 fruit fly experiments, in which he isolated mutations for characteristics such as white eyes, changed his mind. He saw that mutations represented smallMendelian characteristics that would only spread through a population when they were beneficial, helped by natural selection. This represented the germ of themodern synthesis, and the beginning of the end for mutationism as an evolutionary force.^[76]

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.^[77]Masatoshi Nei argues instead that the production of more efficient genotypes by mutation is fundamental for evolution, and that evolution is often mutation-limited.^[78] Theendosymbiotic theory implies rare but major events of saltational evolution bysymbiogenesis.^[79]Carl Woese and colleagues suggested that the absence of RNA signature continuum betweendomains ofbacteria,archaea, andeukarya shows that these major lineages materialized via large saltations in cellular organization.^[80] Saltation at a variety of scales is agreed to be possible by mechanisms includingpolyploidy, which certainly can create new species of plant,^[81][82]gene duplication,lateral gene transfer,^[83] andtransposable elements (jumping genes).^[84]

Theneutral theory of molecular evolution, proposed byMotoo Kimura in 1968, holds that at themolecular level mostevolutionary changes and most of the variation within and between species is not caused bynatural selection but bygenetic drift ofmutantalleles that are neutral. Aneutral mutation is one that does not affect an organism's ability to survive and reproduce. The neutral theory allows for the possibility that most mutations are deleterious, but holds that because these are rapidly purged by natural selection, they do not make significant contributions to variation within and between species at the molecular level. Mutations that are not deleterious are assumed to be mostly neutral rather than beneficial.^[31]

The theory was controversial as it sounded like a challenge to Darwinian evolution; controversy was intensified by a 1969 paper byJack Lester King andThomas H. Jukes, provocatively but misleadingly titled "Non-Darwinian Evolution". It provided a wide variety of evidence includingprotein sequence comparisons, studies of the Treffers mutator gene inE. coli, analysis of the genetic code, and comparativeimmunology, to argue that most protein evolution is due to neutral mutations and genetic drift.^[85][86]

According to Kimura, the theory applies only for evolution at the molecular level, whilephenotypic evolution is controlled by natural selection, so the neutral theory does not constitute a true alternative.^[31][87]

The various alternatives to Darwinian evolution by natural selection were not necessarily mutually exclusive. The evolutionary philosophy of the American palaeontologistEdward Drinker Cope is a case in point. Cope, a religious man, began his career denying the possibility of evolution. In the 1860s, he accepted that evolution could occur, but, influenced by Agassiz, rejected natural selection. Cope accepted instead the theory of recapitulation of evolutionary history during the growth of the embryo - thatontogeny recapitulates phylogeny, which Agassiz believed showed a divine plan leading straight up to man, in a pattern revealed both inembryology andpalaeontology. Cope did not go so far, seeing that evolution created a branching tree of forms, as Darwin had suggested. Each evolutionary step was however non-random: the direction was determined in advance and had a regular pattern (orthogenesis), and steps were not adaptive but part of a divine plan (theistic evolution). This left unanswered the question of why each step should occur, and Cope switched his theory to accommodate functional adaptation for each change. Still rejecting natural selection as the cause of adaptation, Cope turned to Lamarckism to provide the force guiding evolution. Finally, Cope supposed that Lamarckian use and disuse operated by causing a vitalist growth-force substance, "bathmism", to be concentrated in the areas of the body being most intensively used; in turn, it made these areas develop at the expense of the rest. Cope's complex set of beliefs thus assembled five evolutionary philosophies: recapitulationism, orthogenesis, theistic evolution, Lamarckism, and vitalism.^[88] Other palaeontologists and field naturalists continued to hold beliefs combining orthogenesis and Lamarckism until the modern synthesis in the 1930s.^[89]

By the start of the 20th century, during theeclipse of Darwinism, biologists were doubtful of natural selection, but equally 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.^[90] Themodern synthesis a generation later, roughly between 1918 and 1932, broadly swept away all the alternatives to Darwinism, though some including forms of orthogenesis,^[45]epigenetic mechanisms that resemble Lamarckianinheritance of acquired characteristics,^[53] catastrophism,^[58] structuralism,^[69] and mutationism^[78] have been revived, such as through the discovery of molecular mechanisms.^[91]

Biology has become Darwinian, but belief in some form of progress (orthogenesis) remains both in the public mind and among biologists. Ruse argues that evolutionary biologists will probably continue to believe in progress for three reasons. Firstly, theanthropic principle demands people able to ask about the process that led to their own existence, as if they were the pinnacle of such progress. Secondly, scientists in general and evolutionists in particular believe that their work is leading them progressively closer toa true grasp of reality, as knowledge increases, and hence (runs the argument) there is progress in nature also. Ruse notes in this regard thatRichard Dawkins explicitly compares cultural progress withmemes to biological progress with genes. Thirdly, evolutionists are self-selected; they are people, such as the entomologist and sociobiologistE. O. Wilson, who are interested in progress to supply a meaning for life.^[92]

• Coloration evidence for natural selection
• History of evolutionary thought
• Objections to evolution
• Extended evolutionary synthesis
• Lysenkoism

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• Evolutionary biology
• History of evolutionary biology
• Non-Darwinian evolution

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