Mutationism

Mutationism refers to the theories of evolution where mutations are the main driving force.

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History

Mutationism (sometimes, “Mendelism”) refers to the theory emphasizing mutation as a creative principle and source of discontinuity in evolutionary change, particularly associated with the founders of modern genetics such as Thomas Hunt Morgan, Reginald Punnett, Wilhelm Johannsen, Hugo de Vries and William Bateson. Though later associated with Mendelian genetics, mutationism began in the 1890’s (prior to the rediscovery of Mendel’s laws) with the studies of Hugo De Vries 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.

As the 20th century dawned, geneticists learned that discontinuous variations could arise by mutation 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 led to an altered understanding of how evolution works. In Darwin's “Natural Selection” theory, hereditary variation arises by continuous “fluctuation”, and evolutionary change accumulates automatically in infinitesimal increments (see gradualism) as selection preserves fluctuations in the favorable direction. In the new "mutationist" view, infinitesimal heritable variation could not be taken for granted. As a result, evolution was seen as a two-step process of the chance occurrence of a mutation, followed by its persistence or elimination (selection), where both steps are important but have different roles. The mutationists denied that selection is creative, and they gave mutation a certain measure of control over the course of evolution.

The resolution that led to the Modern Synthesis, which resurrected Darwin's view using a different mechanism, came slowly. In 1902 G. Udny Yule argued that a trait reflecting effects of multiple Mendelian characters could show a normal distribution. Even though most fluctuations are environmental, some of the continuous variability of natural species could have a genetic basis, and if sufficiently abundant, this could serve as the basis for a Mendelian mechanism allowing Darwinian gradualism. Nevertheless, the synthesis of Mendelian genetics and Darwinism (later put forth by R. A. Fisher and others, and known as the Modern Synthesis) did not develop immediately, for various reasons: it could be doubted that natural selection was sufficiently powerful to act on infinitesimal differences; 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 Pearson, refused to accept Mendelian genetics.

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^[1], mutationism (like Lamarckism) was no longer seriously considered. However, with the arrival of molecular biology, some scientists proposed that mutational pressure was the basic process of evolution ^{[2] [3]}, a view that Nei has referred to as "neo-mutationism".^[4]

Contemporary status of mutationism

Contemporary interest in mutationism is revealed by articles in mainstream journals that advocate mutationist ideas, using the label "mutationism" or "neo-mutationism" or using terms such as "new mutations" or "mutation-driven evolution" ^[5][4]. The argument is that, since the molecular revolution in the 1960’s and 1970’s, evidence has been accumulating that evolution depends on mutation in a way that was not envisioned in the Modern Synthesis. The dependence is sufficiently sensitive that rates of evolution reflect even subtle biases in mutation such as transition:transversion bias or GC:AT bias phenomenon.

Examples in which mutation-biased evolution is not just plausible but seems to be the received view are in regard to genomic GC-content and the origin of isochore^[6]. In the case of the GC-content, because the bond is stronger and more resilient between the G:C pairs than between A:T pairs, selectionists have speculated that a high GC-content was an adaption to harsh conditions, either high temperature ^[7] or UV radiation^[8]. Both hypotheses were lated disproved. ^[9][10] Mutationists believe it is mostly the consequence of a mutational bias, called the GC mutational pressure.^[11][12][13]

Nevertheless, although mutation-driven is real, and though it is not part of the Modern Synthesis, most evolutionary biologists do not seem to know about this or to consider it a fundamental challenge. Perhaps this is because molecular evolution is seen as kind of side issue, e.g., Li advocates mutation-driven evolution but then notes that "it should be emphasized that we are concerned with molecular evolution, not the evolution of form, function, or fitness, where presumably selection reigns".

The boldest mutationist claims in the contemporary literature argue against such restrictions. For instance, Nei argues that we need a new mutations theory of phenotypic evolution; Stoltzfus argues that mutation-biased adaptation is theoretically possible and has been observed in the laboratory. Citing the concept of developmental biases in variation, Stoltzfus argues that a mutationist perspective may provide the answer to the “evo-devo” problem of “integrating development into evolutionary theory” ^[14]. That is, contrary to what Li suggests, these other authors are arguing that the mutationist or "new mutations" view is relevant to phenotypes and to adaptation, not just to invisible (and neutral) changes in molecular features.

See also

• Evolution
• Molecular evolution
• History of molecular evolution

References