Mutationism

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

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, 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 , a view that Nei has referred to as "neo-mutationism".

Contemporary status of mutationism
The contemporary view corresponding to mutationism could be called "neo-Mutationism" or the "new mutations" view, following Nei. The differences between neomutationism, neutralism and selectionism are all about the relative importance of mutations, genetic drift and natural selection. Neomutationism is competing against selectionism to explain such phenomenon as GC-content and the origin of isochore. 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 or UV radiation. Both hypotheses were lated disproved. Mutationists believe it is mostly the consequence of a mutational bias, called the GC mutational pressure. Other implications of a mutationist view are discussed by Stoltzfus.