Alternatives to Darwinian evolution

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 ; 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 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.

Unchanging forms

did not embrace either divine creation or evolution, instead arguing in his biology that each species was immutable, breeding true to its ideal eternal form. Aristotle's suggestion in De Generatione Animalium of a fixed hierarchy in nature - a scala naturae provided an early explanation of the continuity of living things. Aristotle saw that animals were teleological, and had parts that were homologous with those of other animals, but he did not connect these ideas into a concept of evolutionary progress.
In the Middle Ages, Scholasticism developed Aristotle's view into the idea of a great chain of being. The image of a ladder inherently suggests the possibility of climbing, but both the ancient Greeks and mediaeval scholastics such as Ramon Llull maintained that each species remained fixed from the moment of its creation.
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.
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". 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.
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.
The Lamarckian social philosopher and evolutionist Herbert Spencer, ironically the author of the phrase "survival of the fittest" adopted by Darwin, 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.

Alternative explanations of change

Where the fact of evolutionary change was accepted by biologists but natural selection was denied, including but not limited to the late 19th century eclipse of Darwinism, alternative scientific explanations such as Lamarckism, orthogenesis, structuralism, catastrophism, vitalism and theistic evolution were entertained, not necessarily separately. 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. 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. Others, such as the biologist and philosopher Herbert Spencer, the botanist George Henslow, 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.
Some felt that natural selection would be too slow, given the estimates of the age of the earth and sun 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. 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.

Theory	Date	Notable proponent	Species can change?	Mechanism of change	Mechanism is physical?	Extinction possible?	Notes
Scala naturae	c. 350 BC	Aristotle	No	-	N/A	No	Characteristics of groups do not fit on linear scale, as Aristotle observed. Teleology and homology recognised but not connected as evolution with adaptation; not spiritual
Great chain of being	1305	Llull, Ramon; scholastics	No	-	N/A	No	Aristotelian, fitted into Christian theology
Vitalism	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	Gray, Asa Mivart, St George J.	Yes	Deity supplies beneficial mutations, or sets direction	No	Yes	"Failed the test of methodological naturalism that had come to define science". Discounted by biologists by 1900
Orthogenesis	1859	Baer, Karl von	Yes	"Purposeful creation"	No	Yes?	Many variants in 19th and 20th centuries
Orthogenesis inc. emergent evolution	1959	Teilhard de Chardin, Pierre	Yes	"Inherent progressive tendency"	No	Yes	Spiritual theory, emergence of mind, Omega Point
Lamarckism	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	1812	Cuvier, Georges	No	Extinctions caused by natural events such as volcanism, floods	Yes, for reducing number of species	Yes	To explain extinctions and faunal succession of tetrapods in fossil record; repopulation by new species after such events noted but left unexplained
Structuralism	1917	Thompson, D'Arcy	Yes	Self-organization, physical forces	Yes	Yes?	Many variants, some influenced by vitalism
Saltationism 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	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	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