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, Hugo De Vries and William Bateson. This theory contrasted with the “Natural Selection” theory of Charles Darwin by which hereditary variation arises by continuous “fluctuation” (not by “sports” arising from discrete events of mutation), evolutionary change accumulates in infinitesimal increments (see gradualism), and selection governs the course of evolution.
Though associated with Mendelian genetics, mutationism began in the 1890’s (prior to the rediscovery of Mendel’s laws) with the work 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 Brooks, Francis Galton, and Thomas Henry Huxley. Geneticists soon learned that discontinuous variations could arise by mutation and be transmitted to offspring: the rules of transmission constitute Mendel’s laws. This led to a view in which evolution was seen as a two-step process of mutation (seen as the creative process that introduces novelty) followed by acceptance or rejection.
Many evolutionists, however, were more concerned with the kind of continuous variations on which Darwin built his theory: how they arose, whether they were heritable, and how much (if heritable) they could contribute to evolutionary change. G. Udny Yule pointed out in 1902 that a trait reflecting effects of multiple Mendelian characters could show apparently continuous variation, thus there was no necessary conflict between Mendelian inheritance and a normal distribution of hereditary variation. Nevertheless, it was clear that much continuous variation is not heritable; it could be doubted that natural selection was sufficiently powerful to act on infinitesimal differences; and a common (erroneous) belief at the time (following Galton's notion of regression to the mean) held that even heritable fluctuations could not lead to large changes. A bitter controversy ensued between Mendelians, championed by William Bateson, and biometricians led by Karl Pearson, who did not accept the philosophical basis of Mendelism (i.e., postulating invisible entities) and who held that most variation in organisms was continuous rather than discontinuous. Perhaps the best way to describe the outcome of this debate is that both sides lost.
During the formulation of the modern evolutionary synthesis in the 1930's, population geneticists such as R. A. Fisher showed that selection could act on continuous variation with an underlying Mendelian basis. Also, change of allele frequency powered by mutations was shown mathematically to be much lower than changes due to natural selection or genetic drift. According to this theory, however, mutations are the initial source of genetic variation, but selection and/or genetic drift must increase their frequency in a population.
At the time of the Darwin centennial in Cambridge in 1909, Mutationism and Lamarckism were contrasted with “Natural Selection” (meaning Darwin’s fluctuation-selection mechanism, not just the principle of selection) as competing ideas; 50 years later, at the University of Chicago centennial of the publication of The Origin of Species (see Tax and Callendar, 1960), 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 (Sueoka, 1962; Nei, 1983, 2005), a view that Nei has referred to as "neomutationism".
Neomutationism vs. Selectionism
The differences between neomutationism, neutralism and selectionism are all about the relative importance of the main 3 mechanisms of evolution; mutations, genetic drift and natural selection. Neomutationism is competing against selectionism to explain such phenomenon as GC-content and the origin of isochores (Graur and Li, 2000). 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 (Argos et al., 1979; disproved by Galtier and Lobry, 1997) or UV radiation (Singer and Ames, 1970). While mutationists believe it is mostly the consequence of a mutational bias, called the GC mutational pressure (Sueoka, 1964; Muto and Osawa, 1987; Gu et al., 1998).
Relation with creationism
The concept of mutationism has been used by some creationists to create a straw man (or perhaps misunderstanding) of evolutionary theory, to say that the theory predicts that evolution happens only or primarily through mutations. However, neither mutationism nor the modern evolutionary synthesis takes a view that equates evolution with mutation.
References
- Argos, P., Rossmann, M.G., Grau, U.M., Zuber, A., Frank, G. and Tratschin, J.D. (1979). "Thermal stability and protein structure". Biochemistry. 18: 5698–5703.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - Dobzhansky, T. (1951). Genetics and the origin of species, 2nd edition. Columbia University Press, New-York.
- Galtier, N. and Lobry, J.R. (1997). "Relationships between genomic G+C content, RNA secondary structure, and optimal growth temperature in prokaryotes". Journal of Molecular Evolution. 44: 632–636.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - Gu, X., Hewett-Emmett, D. and Li, W-H. (1998). "Directional mutational pressure affects the amino acid composition and hydrophobicity of proteins in bacteria". Genetica. 102/103: 383–391.
{{cite journal}}: CS1 maint: multiple names: authors list (link) [1] - Graur, D. and [Wen-Hsiung Li (2000). Fundamentals of Molecular Evolution (second edition). Sinauer Associates. ISBN 0878932666.
{{cite book}}: Text "Li, W-H.]" ignored (help) - Muto, A. and Osawa, S. (1987). "The guanine and cytosine content of genomic DNA and bacterial evolution". PNAS USA. 84: 166–169.
{{cite journal}}: CS1 maint: multiple names: authors list (link) [2] - Nei, M. (1983). "Genetic polymorphism and the role of mutation in evolution". In In: Koehn, P.K. and Nei, M. (ed.). Evolution of genes and proteins. Sinauer Association, Mass. pp. 165–190.
{{cite book}}: CS1 maint: multiple names: editors list (link) - Nei, M. (2005). "Selectionism and Neutralism in Molecular Evolution". Molecular Biology and Evolution. 22(12): 2318–2342.
- Singer, C.E. and Ames, B.N. (1970). "Sunlight ultraviolet and bacterial DNA base ratios". Science. 170: 822–826.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - Sueoka, N. (1962). "On the genetic basis of variation and heterogeneity of DNA base composition". PNAS USA. 48: 582–592. [3]
- Sueoka, N. (1964). "On the evolution of informational macromolecules". In In: Bryson, V. and Vogel, H.J. (ed.). Evolving genes and proteins. Academic Press, New-York. pp. 479–496.
{{cite book}}: CS1 maint: multiple names: editors list (link)