Devolution, de-evolution, or backward evolution is the notion that species can revert to supposedly more primitive forms over time. The concept relates to the idea that evolution has a purpose (teleology) and is progressive (orthogenesis), for example that feet might be better than hooves or lungs than gills. However, evolutionary biology makes no such assumptions, and natural selection shapes adaptations with no foreknowledge of any kind. It is possible for small changes (such as in the frequency of a single gene) to be reversed by chance or selection, but this is no different from the normal course of evolution and as such de-evolution is not compatible with a proper understanding of evolution due to natural selection.
In the 19th century, when belief in orthogenesis was widespread, zoologists (such as Ray Lankester and Anton Dohrn) and the palaeontologists Alpheus Hyatt and Carl H. Eigenmann advocated the idea of devolution. The concept appears in Kurt Vonnegut's 1985 novel Galápagos, which portrays a society that has evolved backwards to have small brains.
Dollo's law of irreversibility, first stated in 1893 by the palaeontologist Louis Dollo, denies the possibility of devolution. The evolutionary biologist Richard Dawkins explains Dollo's law as being simply a statement about the improbability of evolution's following precisely the same path twice.
The term "devolution" and its associated concepts never were prominent in biology and now are at most of historical interest, except where they have been adopted by creationists.
Context
The idea of devolution is based on the presumption of orthogenesis, the view that evolution has a purposeful direction towards increasing complexity. Modern evolutionary theory, beginning with Darwin at least, poses no such presumption, and the concept of evolutionary change is independent of either any increase in complexity of organisms sharing a gene pool, or any decrease, such as in vestigiality or in loss of genes. Earlier views that species are subject to "cultural decay", "drives to perfection", or "devolution" are practically meaningless in terms of current (neo-)Darwinian theory. Early scientific theories of transmutation of species such as Lamarckism perceived species diversity as a result of a purposeful internal drive or tendency to form improved adaptations to the environment. In contrast, Darwinian evolution and its elaboration in the light of subsequent advances in biological research, have shown that adaptation through natural selection comes about when particular heritable attributes in a population happen to give a better chance of successful reproduction in the reigning environment than rival attributes do. By the same process less advantageous attributes are less "successful"; they decrease in frequency or are lost completely. Since Darwin's time it has been shown how these changes in the frequencies of attributes occur according to the mechanisms of genetics and the laws of inheritance originally investigated by Gregor Mendel. Combined with Darwin's original insights, genetic advances led to what has variously been called the modern evolutionary synthesis or the neo-Darwinism of the 20th century. In these terms evolutionary adaptation may occur most obviously through the natural selection of particular alleles. Such alleles may be long established, or they may be new mutations. Selection also might arise from more complex epigenetic or other chromosomal changes, but the fundamental requirement is that any adaptive effect must be heritable.
The concept of devolution on the other hand, requires that there be a preferred hierarchy of structure and function, and that evolution must mean "progress" to "more advanced" organisms. For example, it could be said that "feet are better than hooves" or "lungs are better than gills", so their development is "evolutionary" whereas change to an inferior or "less advanced" structure would be called "devolution". In reality an evolutionary biologist defines all heritable changes to relative frequencies of the genes or indeed to epigenetic states in the gene pool as evolution. All gene pool changes that lead to increased fitness in terms of appropriate aspects of reproduction are seen as (neo-)Darwinian adaptation because, for the organisms possessing the changed structures, each is a useful adaptation to their circumstances. For example, hooves have advantages for running quickly on plains, which benefits horses, and feet offer advantages in climbing trees, which some ancestors of humans did.
The concept of devolution as regress from progress relates to the ancient ideas that either life came into being through special creation or that humans are the ultimate product or goal of evolution. The latter belief is related to anthropocentrism, the idea that human existence is the point of all universal existence. Such thinking can lead on to the idea that species evolve because they "need to" in order to adapt to environmental changes. Biologists refer to this misconception as teleology, the idea of intrinsic finality that things are "supposed" to be and behave a certain way, and naturally tend to act that way to pursue their own good. From a biological viewpoint, in contrast, if species evolve it is not a reaction to necessity, but rather that the population contains variations with traits that favour their natural selection. This view is supported by the fossil record which demonstrates that roughly ninety-nine percent of all species that ever lived are now extinct.
People thinking in terms of devolution commonly assume that progress is shown by increasing complexity, but biologists studying the evolution of complexity find evidence of many examples of decreasing complexity in the record of evolution. The lower jaw in fish, reptiles and mammals has seen a decrease in complexity, if measured by the number of bones. Ancestors of modern horses had several toes on each foot; modern horses have a single hooved toe. Modern humans may be evolving towards never having wisdom teeth, and already have lost most of the tail found in many other mammals - not to mention other vestigial structures, such as the vermiform appendix or the nictitating membrane. In some cases, the level of organization of living creatures can also “shift” downwards (e.g., the loss of multicellularity in some groups of protists and fungi).
A more rational version of the concept of devolution, a version that does not involve concepts of "primitive" or "advanced" organisms, is based on the observation that if certain genetic changes in a particular combination (sometimes in a particular sequence as well) are precisely reversed, one should get precise reversal of the evolutionary process, yielding an atavism or "throwback", whether more or less complex than the ancestors where the process began. At a trivial level, where just one or a few mutations are involved, selection pressure in one direction can have one effect, which can be reversed by new patterns of selection when conditions change. That could be seen as reversed evolution, though the concept is not of much interest because it does not differ in any functional or effective way from any other adaptation to selection pressures. As the number of genetic changes rises however, one combinatorial effect is that it becomes vanishingly unlikely that the full course of adaptation can be reversed precisely. Also, if one of the original adaptations involved complete loss of a gene, one can neglect any probability of reversal. Accordingly, one might well expect reversal of peppered moth colour changes, but not reversal of the loss of limbs in snakes.
History
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The concept of degenerative evolution was used by scientists in the 19th century, at this time it was believed by most biologists that evolution had some kind of direction.
In 1857 the physician Bénédict Morel, influenced by Lamarckism, claimed that environmental factors such as taking drugs or alcohol would produce social degeneration in the offspring of those individuals, and would revert those offspring to a primitive state. Morel, a devout Catholic, had believed that mankind had started in perfection, contrasting modern humanity to the past. Morel claimed there had been "Morbid deviation from an original type". His theory of devolution was later advocated by some biologists.
According to Roger Luckhurst:
Darwin soothed readers that evolution was progressive, and directed towards human perfectibility. The next generation of biologists were less confident or consoling. Using Darwin's theory, and many rival biological accounts of development then in circulation, scientists suspected that it was just as possible to devolve, to slip back down the evolutionary scale to prior states of development.
One of the first biologists to suggest devolution was Ray Lankester, he explored the possibility that evolution by natural selection may in some cases lead to devolution, an example he studied was the regressions in the life cycle of sea squirts. Lankester discussed the idea of devolution in his book Degeneration: A Chapter in Darwinism (1880). He was a critic of progressive evolution, pointing out that higher forms existed in the past which have since degenerated into simpler forms. Lankester argued that "if it was possible to evolve, it was also possible to devolve, and that complex organisms could devolve into simpler forms or animals".
Anton Dohrn also developed a theory of degenerative evolution based on his studies of vertebrates. According to Dohrn many chordates are degenerated because of their environmental conditions. Dohrn claimed cyclostomes such as lampreys are degenerate fish as there is no evidence their jawless state is an ancestral feature but is the product of environmental adaptation due to parasitism. According to Dohrn if cyclostomes would devolve further then they would resemble something like an Amphioxus.
The historian of biology Peter J. Bowler has written that devolution was taken seriously by proponents of orthogenesis and others in the late 19th century who at this period of time firmly believed that there was a direction in evolution. Orthogenesis was the belief that evolution travels in internally directed trends and levels. The paleontologist Alpheus Hyatt discussed devolution in his work, using the concept of racial senility as the mechanism of devolution. Bowler defines racial senility as "an evolutionary retreat back to a state resembling that from which it began."
Hyatt who studied the fossils of invertebrates believed that up to a point ammonoids developed by regular stages up until a specific level but would later due to unfavourable conditions descend back to a previous level, this according to Hyatt was a form of lamarckism as the degeneration was a direct response to external factors. To Hyatt after the level of degeneration the species would then become extinct, according to Hyatt there was a "phase of youth, a phase of maturity, a phase of senility or degeneration foreshadowing the extinction of a type". To Hyatt the devolution was predetermined by internal factors which organisms can neither control or reverse. This idea of all evolutionary branches eventually running out of energy and degenerating into extinction was a pessimistic view of evolution and was unpopular amongst many scientists of the time.
Carl H. Eigenmann an ichthyologist wrote Cave vertebrates of America: a study in degenerative evolution (1909) in which he concluded that cave evolution was essentially degenerative. The entomologist William Morton Wheeler and the Lamarckian Ernest MacBride (1866–1940) also advocated degenerative evolution. According to Macbride invertebrates were actually degenerate vertebrates, his argument was based on the idea that "crawling on the seabed was inherently less stimulating than swimming in open waters."
Dollo's law
It has been observed that complex body parts evolve in a lineage over many generations; once lost, they are unlikely to re-evolve. This observation is sometimes generalized to a hypothesis known as Dollo's law, which states that evolution is not reversible. This does not imply that similar engineering solutions cannot be found by natural selection: the tails of the cetacea—whales, dolphins and porpoises, which are evolved from formerly land-dwelling mammals—represent an adaptation of the spinal column for propulsion in water. Unlike the tails of the mammal's marine ancestor, the Sarcopterygii, and of the teleosts, which move from side to side, the cetacean's tail moves up and down as it flexes its mammalian spine: the function of the tail in providing propulsion is remarkably similar.
Degeneration theory
Johann Friedrich Blumenbach and other monogenists such as Georges-Louis Leclerc, Comte de Buffon were believers in the "Degeneration theory" of racial origins the theory claims that races can degenerate into "primitive" forms. Blumenbach claimed that Adam and Eve were white and that other races came about by degeneration from environmental factors such as the sun and poor diet. Buffon believed that the degeneration could be reversed if proper environmental control was taken and that all contemporary forms of man could revert to the original Caucasian race.
Blumenbach claimed Negroid pigmentation arose because of the result of the heat of the tropical sun. The cold wind caused the tawny colour of the Eskimos and the Chinese were fair skinned compared to the other Asian stocks because they kept mostly in towns protected from environmental factors.
According to Blumenbach there are five races all belonging to a single species: Caucasian, Mongolian, Ethiopian, American and Malay. Blumenbach however stated:
I have allotted the first place to the Caucasian because this stock displays the most beautiful race of men.
According to Blumenbach the other races are supposed to have degenerated from the Caucasian ideal stock. Blumenbach denied that his "Degeneration theory" was racist, he also wrote three essays claiming non-white peoples are capable of excelling in arts and sciences in reaction against racialists of his time who believed they couldn't.
Creationist use
According to Christian creationists, devolution is:
A theory of origins based on scripture which begins with the ultimate complexity of all living things at the time of creation. This was followed by degeneration and the break down of all living things on the genetic level beginning at the Curse (Genesis 3) and continuing to this day with increased momentum.
The term was used in the play Inherit the Wind (a parable that fictionalizes the 1925 Scopes "Monkey" Trial), when the character of Matthew Brady (representative of William Jennings Bryan) argued that "Ladies and gentleman, devolution is not a theory but a cold fact ... the ape devolved from man", mocking evolutionary theory by offering an alternative he considers just as plausible. During the Scopes Trial itself, a report in The New York Times said "After flocking to view the monkeys, Dayton has decided that it was not man who evolved from the anthropoid, but the anthropoid which devolved from man; and it points now at the two chimpanzees and the "missing link" to prove the assertion". The suggestion of ape degenerating from "man" had already been brought up by the early young-earth creationist George McReady Price in a work published before the trial:
Accordingly, by every just rule of comparison and analogy, we may well declare that if there is any blood relationship between man and the anthropoid apes, it is the latter which have degenerated from the former, instead of the former having developed from the latter. I do not say that this is the true solution of this enigma; but I do say that there is far more scientific evidence in favour of this hypothesis than there ever has been in favour of the long popular theory that man is a developed animal.
An early creationist to discuss devolution was the ornithologist Douglas Dewar, writing about the subject of the fossil record for the carboniferous period Dewar wrote:
A few of the carboniferous insects were larger than any now existing; one of the dragon-flies had a wing-span of 28 inches. This suggests devolution rather than evolution!
The Young Earth creationist Ken Ham claims Adam and Eve were made into a state of perfection, with perfect DNA, no mistakes or mutations and that because of man sinning against God in Genesis of the Bible, that God cursed the ground and animals and sentenced man to die. Ham claims this is where mutations come from, and the incredible amount of genetic information that God had created at the beginning has been devolving ever since; according to Ham organisms in nature are losing genetic information.
Creationists like Ham claim that mutations lead to a loss of genetic information and this is evidence for devolution. Ken Ham for example has stated:
Observations confirm that mutations overwhelmingly cause a loss of information, not a net gain, as evolution requires.
Young Earth creationist Joseph Mastropaolo, argues that "Change over time, 'definition one' of evolution, actually describes devolution to extinction, the exact opposite of evolution.... actual epidemiological data from human genetic disorders and fatal birth defects, identify 'natural selection,' the alleged 'primary mechanism' for evolution, as actually a mechanism for devolution to extinction, the exact opposite of evolution." and elsewhere, "Evolution is the development of an organism from its chemicals or primitive state to its present state. Devolution is the sequence toward greater simplicity or disappearance or degeneration."
John C. Sanford, a plant geneticist and creationist, has argued for devolution; he has written a book entitled Genetic Entropy & the Mystery of the Genome (2005) in which he claims that the genome is deteriorating and therefore could not have evolved in the way specified by the Modern evolutionary synthesis. Sanford has published two peer reviewed papers detailing computer simulations that model genetic entropy.
The creationist author Lee Spetner is a critic of the role of mutations in the modern evolutionary synthesis, he has argued in his book Not by Chance: Shattering the Modern Theory of Evolution that mutations cause devolution.
Peter Stoner an old earth creationist claimed that the universe was immensely old, writing on astronomy he wrote that "every star is losing energy and mass", he claimed that the second law of thermodynamics proves "cosmic devolution".
Brian Regal associates devolution with the work of dentist and creationist Jack Cuozzo. Cuozzo in his book Buried Alive (1998) claims that Neanderthal dentition proves that the Neanderthals lived much longer than modern humans and that modern humans have devolved, according to Cuozzo "it seems as if human jaws are becoming smaller over time".
The concept of devolution is also found in the teachings of Hindu creationism. Michael Cremo a member of ISKCON has authored a book titled Human Devolution: A Vedic alternative to Darwin's theory, published by ISKCON's Bhaktivedanta Book Publishing, Cremo suggests that Darwinian evolution should be replaced with "devolution" from the original unity with Brahman.
Examples of devolution cited by creationists, include vestigial organs, Stickleback, Amblyopsidae and the Greta oto. Evolutionary biologists point out that examples such as this are not evidence for devolution and the creationists have misunderstood the mechanisms of evolution.
Streamlining evolution
"Devolution", the verb "devolve" and the past participle "devolved" are all common terms in science fiction for changes over time in populations of living things that make them less complex and remove some of their former adaptations. The terminology used herein is nontechnical, but the phenomenon is a real but counter-intuitive one, more accurately known as streamlining evolution. Since the development and maintenance of a feature such as an organ or a metabolite has an opportunity cost, changes in the environment that reduce the utility of an adaptation may mean that a higher evolutionary fitness is achieved by no longer using the adaptation, thus better using resources. This requires a mutation that inactivates one or more genes, perhaps by a change to DNA methylation or a methionine codon. Streamlining evolution allows evolution to remove features no longer of much/any use, like scaffolding on a completed bridge.
However, "devolution" in practice typically refers to changes that occur from a problem no longer existing rather than superior solutions existing. For instance, of the several hundred known species of animal that live their entire lives in total darkness, most have non-functional eyes rather than no eyes. This is due, for instance, to deterioration of the optic nerve. It occurs because mutations that prevent eye formation have low probability. However, several eyeless animal species, such as the Kauai cave wolf spider, who live in total darkness, and whose ancestry mostly had eyes, do exist. Together with gene duplication, streamlining evolution makes evolution surprisingly able to produce radical changes, despite being limited to successive, slight modifications.
In literature and popular culture
Speculative evolution had mentioned a few notable concepts about devolution, including transhuman and posthuman species.
Jonathan Swift's 1726 novel Gulliver's Travels contains a story about Yahoos, a kind of human-like creature regressed into a savage, animal-like state of society which the Houyhnhnms — descended from horses — are the dominant species.
Pierre Boulle's 1963 novel Planet of the Apes depicts humans as primitive animal-like beings which the apes take over as a dominant species and forming their own societies; orangutans are the politicians, gorillas are the security and police, and chimpanzees are the scientists, scholars, inventors and explorers.
The American band Devo is named after the concept of devolution.
Kurt Vonnegut's 1985 novel Galápagos is set a million years in the future, where humans have "devolved" to have much smaller brains.
H.P. Lovecraft's 1924 short story The Rats in the Walls also depicts devolved humans.
Robert E. Howard, in The Hyborian Age, an essay on the universe his Conan the Barbarian tales has been set in, stated the Atlanteans eventually devolved into "ape-men", and had once also been the Picts (distinct from the actual people; his are closely modeled on Algonquian Native Americans).
In the episode Genesis of Star Trek, devolution is caused by a virus.
DC Comics Aquaman has one of the seven races of Atlanteans called The Trench, similar to the Grindylow from British folklore, Cthulhu Mythos' Deep Ones, Universal Monsters' Gill-man, and Fallout's Mirelurk. They were regressed to survive the deepest, darkest places in the bottom of oceanic trenches they hide in — which they were named after — and are photophobic when they are in contact with light.
In the movie adaptation of Super Mario Bros., devolution is used as a means of punishment by King Koopa, turning the humanoid creatures of his world back into their primitive states (Goombas). In another part of the movie, he uses a "Devo gun" to turn a human into a monkey.
Lego's 2009 Bionicle sets include Glatorian and Agori. One of the six tribes include The Sand Tribe, which the Glatorian and the Agori of that tribe were regressed into scorpion-like beasts — The Vorox and The Zesk — by their creators, The Great Beings; whom are also the same species as the Glatorian and Agori.
Occultism
Helena Blavatsky, founder of Theosophy, believed that (contrary to standard evolutionary theory) apes had devolved from humans rather than the opposite, due to affected people "putting themselves on the animal level". Julius Evola, a later far-right Italian occultist, concurred (as "involution").
