Alternatives to Darwinian evolution

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Ramon Lull
's Ladder of Ascent and Descent of the Mind, 1305

Alternatives to Darwinian evolution have been proposed by scholars investigating biology 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 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 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 place; 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 (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 but

natural 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. 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.

Unchanging forms

Main articles: Hylomorphism and Great chain of being

De Generatione Animalium of a fixed hierarchy in nature - a scala naturae ("ladder of nature") provided an early explanation of the continuity of living things.[5][6][7] Aristotle saw that animals were teleological (functionally end-directed), and had parts that were homologous with those of other animals, but he did not connect these ideas into a concept of evolutionary progress.[8]

In the Middle Ages,

Ramon Lull[1] maintained that each species remained fixed from the moment of its creation.[9][8]

By 1818, however, Étienne Geoffroy Saint-Hilaire argued in his Philosophie 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 1812 Recherches 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 evolutionist Herbert Spencer, ironically the author of the phrase "survival of the fittest" adopted by Darwin,[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]

Alternative explanations of change

Further information: The eclipse of Darwinism

Where the fact of evolutionary change was accepted by biologists but

eclipse of Darwinism, alternative scientific explanations such as Lamarckism, orthogenesis, structuralism, catastrophism, vitalism and theistic evolution[a] were entertained, not necessarily separately. (Purely religious points of view such as young or old earth creationism or intelligent 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 for teleology or the concept of progress in the development of life.[15][16] Some of these scientists and philosophers, like St. George Jackson Mivart and Charles Lyell, who came to accept evolution but disliked natural selection, raised religious objections.[17] Others, such as the biologist and philosopher Herbert Spencer, the botanist George Henslow (son of Darwin's mentor John Stevens Henslow, also a botanist), and the author Samuel Butler, felt that evolution was an inherently progressive process that natural selection alone was insufficient to explain. Still others, including the American paleontologists Edward Drinker Cope and Alpheus 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 the

age of the earth and sun (10–100 million years) being made at the time by physicists such as Lord 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 engineer Fleeming Jenkin in a review of Origin 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 like Hugo de Vries and Thomas Hunt Morgan, who wanted to recast biology as an experimental laboratory science. They distrusted the work of naturalists like Darwin and Alfred Russel Wallace, dependent on field observations of variation, adaptation, and biogeography, as being overly anecdotal. Instead they focused on topics like physiology and genetics that could be investigated with controlled experiments in the laboratory, and discounted less accessible phenomena like natural selection and adaptation to the environment.[20]

Theory Date Notable
proponent 		Species
can change? 		Mechanism
of change 		Mechanism
is physical? 		Extinction
possible? 		Notes
Scala naturae[6] c. 350 BC Aristotle No None N/A No Characteristics of groups do not fit on linear scale, as Aristotle observed.[6] Teleology and homology recognised but not connected as evolution with adaptation; not spiritual
Great chain of being[1] 1305 Llull, Ramon;
scholastics 		No None N/A No Aristotelian, fitted into Christian theology
Vitalism[21] 1759 Wolff, Caspar Friedrich Yes A life force in embryo No No? Varieties of theory from Ancient Egypt onwards, often spiritual. Dropped from biology with chemical synthesis of organic molecules e.g. of urea in 1828
Theistic evolution 1871–6
Mivart, St George J.
 Yes Deity supplies beneficial mutations (Gray 1876), or sets (orthogenetic) direction (Mivart 1871) No Yes "Failed the test of
methodological naturalism that had come to define science".[22] Discounted by biologists by 1900[23]
Orthogenesis[24] 1859
Baer, Karl von
 Yes "Purposeful creation" No Yes? Many variants in 19th and 20th centuries
Orthogenesis[25]
inc. emergent evolution 		1959 Teilhard de Chardin, Pierre Yes "Inherent progressive tendency" (teleological, vitalist) No Yes Spiritual theory, emergence of mind, Omega Point
Lamarckism[26] 1809 Lamarck, Jean-Baptiste Yes Use and disuse; inheritance of acquired characteristics So it was thought, but none was found No Part of his view of orthogenesis. Dropped from biology as Weismann barrier prevents changes in somatic cells from affecting germ line in gonads
Catastrophism[27] 1812 Cuvier, Georges No Extinctions caused by natural events such as volcanism, floods Yes, for reducing number of species Yes To explain extinctions and
fossil record
; repopulation by new species after such events noted but left unexplained
Structuralism[28] 1917
Thompson, D'Arcy
 Yes Self-organization, physical forces Yes Yes? Many variants, some influenced by vitalism
Saltationism[29][30]
or Mutationism 		1831 Geoffroy Saint-Hilaire, Étienne Yes Large mutations Yes Yes? Sudden production of new species under environmental pressure
Neutral theory of molecular evolution[31] 1968 Kimura, Motoo Yes Genetic drift Yes Yes Only at molecular level; fits in with natural selection at higher levels. Observed 'molecular clock' supports neutral drift; not a rival to natural selection, as does not cause evolution of phenotype
Darwinian evolution[32]
 1859 Darwin, Charles Yes Natural selection Yes Yes Lacked mechanisms of mutation and heredity until birth of genetics, 1900; Darwin instead proposed pangenesis and some degree of inheritance of acquired characteristics

Vitalism

Louis Pasteur believed that only living things could carry out fermentation. Painting by Albert Edelfelt, 1885
Main article: Vitalism

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] Since

blastulas, 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]

Theistic evolution

Main article: Theistic evolution

The American botanist

methodological 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 the orthogenesis of Teilhard de Chardin.[40]

Orthogenesis

Titanothere horns showed a non-adaptive orthogenetic
trend.
Main article: Orthogenesis

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 of

Teilhard de Chardin openly spiritual, others such as Theodor 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 as

Titanothere horns were both orthogenetic and non-adaptive, and could be detrimental to the organism. For instance, they supposed that the large antlers of the Irish elk had caused its extinction.[41][42]

Support for orthogenesis fell during the

embryonic development often exhibit stereotypical morphological outcomes, suggesting that evolution will proceed in preferred directions once key molecular components are in place.[46]

Lamarckism

Main article: Lamarckism
Jean-Baptiste Lamarck, drawn by Jules Pizzetta, 1893

paleontologists Edward Drinker Cope and Alpheus Hyatt, and the American entomologist Alpheus 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 botanist George 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 biologist August Weismann resulted in the germ plasm theory of inheritance, which Weismann said made the inheritance of acquired characteristics impossible, since the Weismann barrier would prevent any changes that occurred to the body after birth from being inherited by the next generation.[49][52]

In modern

DNA sequence. These changes can cross generations in plants, animals, and prokaryotes. 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]

fossil record
.

Catastrophism

Main article: Catastrophism

Catastrophism is the

stratigraphic 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 the Cretaceous–Paleogene extinction event at the end of the Cretaceous period, as proposed in a paper by

Chicxulub in the Yucatán Peninsula of Mexico.[59]

Structuralism

Main article: Structuralism (biology)
geometric transformation of one fish's body form into another with a 20° shear mapping. He did not discuss the evolutionary causes of such a change, raising suspicions of vitalism.[28]

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.

evolutionary history. They accept that some structures such as the cell membrane self-assemble, but question the ability of self-organisation to drive large-scale evolution.[70][71]

Saltationism, mutationism

saltation, by Thérèse Schwartze
, 1918
Main articles:
Saltationism and Mutationism

polyploid 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 small

Mendelian characteristics that would only spread through a population when they were beneficial, helped by natural selection. This represented the germ of the modern 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.

polyploidy, which certainly can create new species of plant,[81][82] gene duplication, lateral gene transfer,[83] and transposable elements (jumping genes).[84]

Genetic drift

Many mutations are neutral or silent, having no effect on the amino acid sequence that is produced when the gene involved is translated to protein, and accumulate over time, forming a molecular clock. However this does not cause phenotypic evolution.
Main article: Neutral theory of molecular evolution

The

molecular level most evolutionary changes and most of the variation within and between species is not caused by natural selection but by genetic drift of mutant alleles that are neutral. A neutral 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 by

E. coli, analysis of the genetic code, and comparative immunology, 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, while phenotypic evolution is controlled by natural selection, so the neutral theory does not constitute a true alternative.[31][87]

Combined theories

Multiple explanations have been offered since the 19th century for how evolution took place, given that many scientists initially had objections to natural selection (dashed orange arrows). Many of these theories led (blue arrows) to some form of directed evolution (orthogenesis), with or without invoking divine control (dotted blue arrows) directly or indirectly. For example, evolutionists like Edward Drinker Cope believed in a combination of theistic evolution, Lamarckism, vitalism, and orthogenesis,[88] represented by the sequence of arrows on the extreme left of the diagram.

The various alternatives to Darwinian evolution by natural selection were not necessarily mutually exclusive. The evolutionary philosophy of the American palaeontologist

palaeontology. 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]

Rebirth of natural selection, with continuing alternatives

Main article: Modern synthesis (20th century)

By the start of the 20th century, during the

inheritance 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, the anthropic 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 to a true grasp of reality, as knowledge increases, and hence (runs the argument) there is progress in nature also. Ruse notes in this regard that Richard Dawkins explicitly compares cultural progress with memes to biological progress with genes. Thirdly, evolutionists are self-selected; they are people, such as the entomologist and sociobiologist E. O. Wilson, who are interested in progress to supply a meaning for life.[92]

See also

Notes

  1. ^ Not to be confused with the more recent use of the term theistic evolution, which refers to a theological belief in the compatibility of science and religion.
  2. ^ Gray, and later historians of science, did not refer to a 20th century usage of theistic evolution (described in that article), that one can accept Darwinian evolution without being an atheist.

References

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  3. ^ Larson 2004, pp. 12–13.
  4. ^ Leroi 2015, p. 89.
  5. ^ Lloyd 1968, pp. 166–169.
  6. ^ a b c Leroi 2015, pp. 276–278.
  7. ^ Mayr 1985, pp. 201–202.
  8. ^ a b Ruse 1996, p. 43.
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  13. ^ Bowler 1989, pp. 239, 342.
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Sources

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