Mutationism
Encyclopedia
Mutationism refers to the theory
emphasizing mutation as a creative principle and source of discontinuity in evolution
ary change, particularly associated with the founders of modern genetics.
and be transmitted to offspring via stable non-mixing factors: the rules of transmission of these factors constitute Mendel’s laws. A more revolutionary discovery, from the perspective of evolutionary theory, was that slight variations in quantitative traits that emerge reliably every generation— like the "fluctuations" on which Charles Darwin
built his theory— were not heritable. This result was shown in a series of breeding experiments carried out by the Danish biologist Wilhelm Johannsen
. From mixtures of different true-breeding varieties of beans of different sizes, selection on a breeding population could be used to sort out the large from the small varieties, but would not change their heights, even though fluctuations in size continued to appear each generation, following the familiar normal distribution.
This result was understood widely as a direct threat to the "Natural Selection" theory of Darwin, who argued in The Origin of Species
that
Darwin knew that discontinuous variations or "sports" occurred, and that their effects were heritable, but he argued that such changes would not be important in evolution, which must occur gradually according to the doctrine of natura non facit salta (see gradualism
). In Darwin's theory, infinitesimal hereditary variation arises automatically in response to the effect of "altered conditions of life" on "the sexual organs"; whenever conditions change, adaptation happens automatically (and by infinitesimal increments) as selection preserves fluctuations that fit the new conditions. That is, Darwin proposed a mechanism of automatic evolution, based on automatic variation that would always be present when needed.
However, genetics showed that the kind of variation that arises automatically in response to altered conditions is not genetic variation, but non-heritable environmental variation. Heritable variation, by contrast, arises spontaneously by events of "mutation". This is how the discovery of genetics forced a re-appraisal of the mechanism of evolution — a re-appraisal that led to the rise of "mutationism".
and William Bateson
on naturally occurring discontinuous variations; their thoughts concerning the role of discontinuity in evolution drew on earlier ideas of William Keith Brooks
, Francis Galton
, and Thomas Henry Huxley.
The "mutationist" view began by abandoning Darwin's idea of automatic fluctuation, embracing instead the concept that variation emerges by rare events of mutations. This view was expressed in the writings of key founders of genetics, including Thomas Hunt Morgan
, Reginald Punnett
, Wilhelm Johannsen
, Hugo de Vries
, William Bateson
and others. Assuming that heritable variation cannot be taken for granted, the mutationists saw evolution as a two-step process of the chance occurrence of a mutation
, followed by its persistence or elimination (selection). The mutationists denied that selection is creative, and they gave mutation a certain measure of control over the course of evolution.
A common misconception is that the mutationists denied selection. Instead, mutationists such as Morgan simply understood its role differently. In the following passage, Morgan (writing in 1916 ) displays a clear understanding of the concept of the probability of fixation of a new mutation, which might be deleterious, neutral, or advantageous:
Morgan resisted calling this process "Natural Selection" because it differed so much from Darwin's view.
, refused to accept Mendelian genetics. A key conceptual innovation of the Modern Synthesis, crucial for its acceptance, was the "gene pool" concept, which argued that natural populations "maintain" abundant heritable variation through a combination of recombination, mixis, recessivity, heterosis and balancing selection.
At the time of the Darwin centennial in Cambridge in 1909, Mutationism and Lamarckism
were contrasted with Darwin's “Natural Selection” as competing ideas; 50 years later, at the University of Chicago centennial of the publication of The Origin of Species
, mutationism (like Lamarckism) was no longer seriously considered.
Nevertheless, after another 50 years, evolutionary biologists are re-considering the mutationist view.
, scientists studying "molecular evolution" began to suggest mutational explanations for patterns such as genomic nucleotide composition, and eventually it became a characteristic of the field of molecular evolution to emphasize the role of mutation in evolution.
Contemporary interest in mutationism is revealed by articles in mainstream research journals that advocate mutationist ideas, referring to Goldschmidt's concept of the Hopeful Monster, or using the label "mutationism" or "neo-mutationism". Phrases such as "new mutations" or "mutation-driven evolution" also indicate a departure from the "shifting gene frequencies" view of the Modern Synthesis, in which evolution consists of establishing a new multi-locus equilibrium for the frequencies of pre-existing alleles, without new mutations.
These contemporary writings suggest that mutation plays a role in evolution that was proposed by the "mutationists" but rejected in the Modern Synthesis. A key aspect of this role, advocated forcefully in mainstream journals, is "how single mutations can have huge effects that drive evolution", a result that is difficult to argue from retrospective analyses, but that appears clearly in laboratory evolution experiments, such as those of Lenski and colleagues (cited in ). According to Takahata,
Another aspect of this role, discussion of which is restricted largely to trade journals in molecular evolution and genomics, is that patterns of diversity and rates of change reflect systematic biases in mutation. Stoltzfus and Yampolsky list examples in which mutation-biased evolution is either a plausible hypothesis or the received view. A case that has received much attention is in regard to genomic GC-content
and the origin of isochore
. The mutational hypothesis is that systematic differences in GC content emerge slowly due to the cumulative effect of mutational biases favoring GC or AT. However, because the bond is stronger and more resilient between the G:C pairs than between A:T pairs, a commonly proposed selective explanation is that a high GC-content
is an adaption to harsh conditions, either high temperature
or UV radiation. Both hypotheses were later disproved.
Theory
The English word theory was derived from a technical term in Ancient Greek philosophy. The word theoria, , meant "a looking at, viewing, beholding", and referring to contemplation or speculation, as opposed to action...
emphasizing mutation as a creative principle and source of discontinuity in evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
ary change, particularly associated with the founders of modern genetics.
The discovery of genetics challenges Darwin's theory
As the 20th century dawned, geneticists learned that discontinuous variations could arise by mutationMutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...
and be transmitted to offspring via stable non-mixing factors: the rules of transmission of these factors constitute Mendel’s laws. A more revolutionary discovery, from the perspective of evolutionary theory, was that slight variations in quantitative traits that emerge reliably every generation— like the "fluctuations" on which Charles Darwin
Charles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
built his theory— were not heritable. This result was shown in a series of breeding experiments carried out by the Danish biologist Wilhelm Johannsen
Wilhelm Johannsen
Wilhelm Johannsen was a Danish botanist, plant physiologist and geneticist. He was born in Copenhagen. While very young, he was apprenticed to a pharmacist and worked in Denmark and Germany beginning in 1872 until passing his pharmacist's exam in 1879...
. From mixtures of different true-breeding varieties of beans of different sizes, selection on a breeding population could be used to sort out the large from the small varieties, but would not change their heights, even though fluctuations in size continued to appear each generation, following the familiar normal distribution.
This result was understood widely as a direct threat to the "Natural Selection" theory of Darwin, who argued in The Origin of Species
The Origin of Species
Charles Darwin's On the Origin of Species, published on 24 November 1859, is a work of scientific literature which is considered to be the foundation of evolutionary biology. Its full title was On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the...
that
"Natural selection can act only by the preservation and accumulation of infinitesimally small inherited modifications, each profitable to the preserved being; and as modern geology has almost banished such views as the excavation of a great valley by a single diluvial wave, so will natural selection, if it be a true principle, banish the belief of the continued creation of new organic beings, or of any great and sudden modification in their structure." (Ch. 4, OOS)
Darwin knew that discontinuous variations or "sports" occurred, and that their effects were heritable, but he argued that such changes would not be important in evolution, which must occur gradually according to the doctrine of natura non facit salta (see gradualism
Gradualism
Gradualism is the belief in or the policy of advancing toward a goal by gradual, often slow stages.-Politics and society:In politics, the concept of gradualism is used to describe the belief that change ought to be brought about in small, discrete increments rather than in abrupt strokes such as...
). In Darwin's theory, infinitesimal hereditary variation arises automatically in response to the effect of "altered conditions of life" on "the sexual organs"; whenever conditions change, adaptation happens automatically (and by infinitesimal increments) as selection preserves fluctuations that fit the new conditions. That is, Darwin proposed a mechanism of automatic evolution, based on automatic variation that would always be present when needed.
However, genetics showed that the kind of variation that arises automatically in response to altered conditions is not genetic variation, but non-heritable environmental variation. Heritable variation, by contrast, arises spontaneously by events of "mutation". This is how the discovery of genetics forced a re-appraisal of the mechanism of evolution — a re-appraisal that led to the rise of "mutationism".
Mutationism
Though later associated with Mendelian genetics, mutationism began in the 1890s (prior to the rediscovery of Mendel’s laws) with the studies of Hugo De VriesHugo de Vries
Hugo Marie de Vries ForMemRS 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 unaware of Gregor Mendel's work, for introducing the term "mutation", and for developing a mutation...
and William Bateson
William Bateson
William Bateson was an English geneticist and a Fellow of St. John's College, Cambridge...
on naturally occurring discontinuous variations; their thoughts concerning the role of discontinuity in evolution drew on earlier ideas of William Keith Brooks
William Keith Brooks
William Keith Brooks, LL.D., Ph.D. was an American zoologist, born in Cleveland, Ohio, March 25, 1848. He was educated at Williams College and at Harvard . He was employed at Johns Hopkins University from 1876 onward...
, Francis Galton
Francis Galton
Sir Francis Galton /ˈfrɑːnsɪs ˈgɔːltn̩/ FRS , cousin of Douglas Strutt Galton, half-cousin of Charles Darwin, was an English Victorian polymath: anthropologist, eugenicist, tropical explorer, geographer, inventor, meteorologist, proto-geneticist, psychometrician, and statistician...
, and Thomas Henry Huxley.
The "mutationist" view began by abandoning Darwin's idea of automatic fluctuation, embracing instead the concept that variation emerges by rare events of mutations. This view was expressed in the writings of key founders of genetics, including Thomas Hunt Morgan
Thomas Hunt Morgan
Thomas Hunt Morgan was an American evolutionary biologist, geneticist and embryologist and science author who won the Nobel Prize in Physiology or Medicine in 1933 for discoveries relating the role the chromosome plays in heredity.Morgan received his PhD from Johns Hopkins University in zoology...
, Reginald Punnett
Reginald Punnett
Professor Reginald Crundall Punnett FRS 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...
, Wilhelm Johannsen
Wilhelm Johannsen
Wilhelm Johannsen was a Danish botanist, plant physiologist and geneticist. He was born in Copenhagen. While very young, he was apprenticed to a pharmacist and worked in Denmark and Germany beginning in 1872 until passing his pharmacist's exam in 1879...
, Hugo de Vries
Hugo de Vries
Hugo Marie de Vries ForMemRS 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 unaware of Gregor Mendel's work, for introducing the term "mutation", and for developing a mutation...
, William Bateson
William Bateson
William Bateson was an English geneticist and a Fellow of St. John's College, Cambridge...
and others. Assuming that heritable variation cannot be taken for granted, the mutationists saw evolution as a two-step process of the chance occurrence of a mutation
Mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...
, followed by its persistence or elimination (selection). The mutationists denied that selection is creative, and they gave mutation a certain measure of control over the course of evolution.
A common misconception is that the mutationists denied selection. Instead, mutationists such as Morgan simply understood its role differently. In the following passage, Morgan (writing in 1916 ) displays a clear understanding of the concept of the probability of fixation of a new mutation, which might be deleterious, neutral, or advantageous:
"If through a mutation a character appears that is neither advantageous nor
disadvantageous, but indifferent, the chance that it may become established
in the race is extremely small, although by good luck such a thing may occur rarely.
It makes no difference whether the character in question is a dominant or a recessive
one, the chance of its becoming established is exactly the same.
If through a mutation a character appears that has an injurious effect, however slight
this may be, it has practically no chance of becoming established.
If through a mutation a character appears that has a beneficial influence on the
individual, the chance that the individual will survive is increased, not only for
itself, but for all of its descendants that come to inherit this character. It is
this increase in the number of individuals possessing a particular character, that
might have an influence on the course of evolution."
Morgan resisted calling this process "Natural Selection" because it differed so much from Darwin's view.
Demise of Mutationism and Rise of the Modern Synthesis
While the mutationist view was very popular in the first 3 decades of the 20th century, it was replaced eventually by the Darwinian view expressed in the Modern Synthesis. In 1902 G. Udny Yule argued that a trait reflecting effects of multiple Mendelian characters could show a normal distribution. Thus, even though variations that arise in response to altered conditions are environmental and non-heritable (contrary to Darwin's assumptions), some of the continuous variation in natural species could have a genetic basis, and could serve as the Mendelian basis for Darwinian gradualism. Nevertheless, the synthesis of Mendelian genetics and Darwinism later put forth by R. A. Fisher and others did not develop immediately, for various reasons: it could be doubted that natural selection was sufficiently powerful to act on infinitesimal differences; it could be doubted that natural populations had enough heritable variation to support a Darwinian view; a common (erroneous) belief at the time (following Francis Galton's notion of regression to the mean) held that even heritable fluctuations could not lead to large or qualitative changes; and some advocates of Darwinism, such as Karl PearsonKarl Pearson
Karl Pearson FRS was an influential English mathematician who has been credited for establishing the disciplineof mathematical statistics....
, refused to accept Mendelian genetics. A key conceptual innovation of the Modern Synthesis, crucial for its acceptance, was the "gene pool" concept, which argued that natural populations "maintain" abundant heritable variation through a combination of recombination, mixis, recessivity, heterosis and balancing selection.
At the time of the Darwin centennial in Cambridge in 1909, Mutationism and Lamarckism
Lamarckism
Lamarckism is the idea that an organism can pass on characteristics that it acquired during its lifetime to its offspring . It is named after the French biologist Jean-Baptiste Lamarck , who incorporated the action of soft inheritance into his evolutionary theories...
were contrasted with Darwin's “Natural Selection” as competing ideas; 50 years later, at the University of Chicago centennial of the publication of The Origin of Species
The Origin of Species
Charles Darwin's On the Origin of Species, published on 24 November 1859, is a work of scientific literature which is considered to be the foundation of evolutionary biology. Its full title was On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the...
, mutationism (like Lamarckism) was no longer seriously considered.
Nevertheless, after another 50 years, evolutionary biologists are re-considering the mutationist view.
Contemporary status of mutationism
With the arrival of molecular biologyMolecular biology
Molecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...
, scientists studying "molecular evolution" began to suggest mutational explanations for patterns such as genomic nucleotide composition, and eventually it became a characteristic of the field of molecular evolution to emphasize the role of mutation in evolution.
Contemporary interest in mutationism is revealed by articles in mainstream research journals that advocate mutationist ideas, referring to Goldschmidt's concept of the Hopeful Monster, or using the label "mutationism" or "neo-mutationism". Phrases such as "new mutations" or "mutation-driven evolution" also indicate a departure from the "shifting gene frequencies" view of the Modern Synthesis, in which evolution consists of establishing a new multi-locus equilibrium for the frequencies of pre-existing alleles, without new mutations.
These contemporary writings suggest that mutation plays a role in evolution that was proposed by the "mutationists" but rejected in the Modern Synthesis. A key aspect of this role, advocated forcefully in mainstream journals, is "how single mutations can have huge effects that drive evolution", a result that is difficult to argue from retrospective analyses, but that appears clearly in laboratory evolution experiments, such as those of Lenski and colleagues (cited in ). According to Takahata,
"Unlike neo-Darwinism, which regards mutation as merely raw material and natural selection as the creative power, Nei's mutationism assumes that the most fundamental process for adaptive evolution is the production of functionally more efficient genotypes by mutation (especially birth and death of duplicated genes) and by recombination."
Another aspect of this role, discussion of which is restricted largely to trade journals in molecular evolution and genomics, is that patterns of diversity and rates of change reflect systematic biases in mutation. Stoltzfus and Yampolsky list examples in which mutation-biased evolution is either a plausible hypothesis or the received view. A case that has received much attention is in regard to genomic GC-content
GC-content
In molecular biology and genetics, GC-content is the percentage of nitrogenous bases on a DNA molecule that are either guanine or cytosine . This may refer to a specific fragment of DNA or RNA, or that of the whole genome...
and the origin of isochore
Isochore (genetics)
In genetics, an isochore is a large region of DNA with a high degree uniformity in G-C and C-G which tends to have more genes, higher local melting or denaturation temperatures, and different flexibility...
. The mutational hypothesis is that systematic differences in GC content emerge slowly due to the cumulative effect of mutational biases favoring GC or AT. However, because the bond is stronger and more resilient between the G:C pairs than between A:T pairs, a commonly proposed selective explanation is that a high GC-content
GC-content
In molecular biology and genetics, GC-content is the percentage of nitrogenous bases on a DNA molecule that are either guanine or cytosine . This may refer to a specific fragment of DNA or RNA, or that of the whole genome...
is an adaption to harsh conditions, either high temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
or UV radiation. Both hypotheses were later disproved.
See also
- Molecular evolutionMolecular evolutionMolecular evolution is in part a process of evolution at the scale of DNA, RNA, and proteins. Molecular evolution emerged as a scientific field in the 1960s as researchers from molecular biology, evolutionary biology and population genetics sought to understand recent discoveries on the structure...
- History of evolutionary thoughtHistory of evolutionary thoughtEvolutionary 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...
- History of molecular evolutionHistory of molecular evolutionThe history of molecular evolution starts in the early 20th century with "comparative biochemistry", but the field of molecular evolution came into its own in the 1960s and 1970s, following the rise of molecular biology...