In biology, saltation () is a sudden and large mutational change from one generation to the next, potentially causing single-step speciation. This was historically offered as an alternative to Darwinism. Some forms of mutationism were effectively saltationist, implying large discontinuous jumps.
Speciation, such as by polyploidy in plants, can sometimes be achieved in a single and in evolutionary terms sudden step. Evidence exists for various forms of saltation in a variety of organisms.
History
Prior to Charles Darwin most evolutionary scientists had been saltationists.Henry Fairfield Osborn. (1894). From the Greeks to Darwin: An outline of the development of the evolution idea. New York, London, Macmillan and Co. Jean-Baptiste Lamarck was a gradualist but similar to other scientists of the period had written that saltational evolution was possible. Étienne Geoffroy Saint-Hilaire endorsed a theory of saltational evolution that "monstrosities could become the founding fathers (or mothers) of new species by instantaneous transition from one form to the next."Hallgrímsson, Benedikt; Hall, Brian K. (2011). Variation: A Central Concept in Biology. Academic Press. p. 18 Geoffroy wrote that environmental pressures could produce sudden transformations to establish new species instantaneously.Peter J. Bowler. (2003). Evolution: The History of an Idea. University of California Press. p. 127 In 1864 Albert von Kölliker revived Geoffroy's theory that evolution proceeds by large steps, under the name of heterogenesis.Wright, Sewall. (1984). Evolution and the Genetics of Populations: Genetics and Biometric Foundations Volume 1. University of Chicago Press. p. 10
With the publication of On the Origin of Species in 1859 Charles Darwin wrote that most evolutionary changes proceeded gradually.
From 1860 to 1880 saltation had a minority interest but by 1890 had become a major interest to scientists.Radick, Gregory. (2008). The Simian Tongue: The Long Debate about Animal Language. University Of Chicago Press. p. 368 In their paper on evolutionary theories in the 20th century Levit et al wrote:
The advocates of saltationism deny the Darwinian idea of slowly and gradually growing divergence of character as the only source of evolutionary progress. They would not necessarily completely deny gradual variation, but claim that cardinally new 'body plans' come into being as a result of saltations (sudden, discontinuous and crucial changes, for example, the series of macromutations). The latter are responsible for the sudden appearance of new higher taxa including classes and orders, while small variation is supposed to be responsible for the fine adaptations below the species level.Levit, G. S; Meister, K.; Hossfeld, U. (2008). Alternative Evolutionary Theories: A Historical Survey. Journal of Bioeconomics 10.1. pp. 71–96.
In the early 20th century a mechanism of saltation was proposed as large mutations. It was seen as a much faster alternative to the Darwinian concept of a gradual process of small random variations being acted on by natural selection. It was popular with early geneticists such as Hugo de Vries, who along with Carl Correns helped rediscover Gregor Mendel's laws of inheritance in 1900, William Bateson, a British zoologist who switched to genetics, and early in his career Thomas Hunt Morgan. Some of these geneticists developed it into the mutation theory of evolution.Peter J. Bowler. (2003). Evolution: The History of an Idea. University of California Press. pp. 265–270 There was also a debate over accounts of the evolution of mimicry and if they could be explained by gradualism or saltation. The geneticist Reginald Punnett supported a saltational theory in his book Mimicry in Butterflies (1915).Punnett, Reginald. (1915). Mimicry in Butterflies. Cornell University Library.
The mutation theory of evolution held that species went through periods of rapid mutation, possibly as a result of environmental stress, that could produce multiple mutations, and in some cases completely new species, in a single generation. This mutationist view of evolution was later replaced by the reconciliation of Mendelian genetics with natural selection into a gradualistic framework for the neo-Darwinian synthesis.Peter J. Bowler. (2003). Evolution: The History of an Idea. University of California Press. It was the emergence of population thinking in evolution which forced many scientists to adopt gradualism in the early 20th century. According to Ernst Mayr, it wasn't until the development of population genetics in the neo-Darwinian synthesis in the 1940s that demonstrated the explanatory power of natural selection that saltational views of evolution were largely abandoned.Ernst Mayr. (2007). What Makes Biology Unique?: Considerations on the Autonomy of a Scientific Discipline. Cambridge University Press; 1 edition
Saltation was originally denied by the "modern synthesis" school of neo-Darwinism which favoured gradual evolution but has since been accepted due to recent evidence in evolutionary biology (see the current status section).Bateman, R. M.; DiMichele, W. A. (1994). Saltational evolution of form in vascular plants: a neoGoldschmidtian synthesis. In Shape and Form in Plants and Fungi (eds D. S. Ingram and A. Hudson), Academic Press, London. pp. 61-100.Gregory, T. R.; Hebert, P. D. N. (1999). The modulation of DNA content: proximate causes and ultimate consequences. Genome Res. 9, 317–324.Jablonka, Eva; Lamb, Marion J. (2005). Evolution in Four Dimensions: Genetic, Epigenetic, Behavioral, and Symbolic Variation in the History of Life. A Bradford Book. Serres, M. H.; Kerr, A. R.; McCormack, T. J.; Riley, M. (2009). Evolution by leaps: gene duplication in bacteria. Biology Direct 4: 46. In recent years there are some prominent proponents of saltation, including Carl Woese. Woese, and colleagues, suggested that the absence of RNA signature continuum between domains of bacteria, archaea, and eukarya constitutes a primary indication that the three primary organismal lineages materialized via one or more major evolutionary saltations from some universal ancestral state involving dramatic change in cellular organization that was significant early in the evolution of life, but in complex organisms gave way to the generally accepted Darwinian mechanisms.Roberts, E.; Sethi, A.; Montoya, J.; Woese, C. R.; Luthey-Schulten, Z. (2008). Molecular signatures of ribosomal evolution. Proc. Natl. Acad. Sci. USA, 105: 13953–13958. The geneticist Barbara McClintock introduced the idea of "jumping genes", chromosome transpositions that can produce rapid changes in the genome.McClintock, B. (1984). The significance of responses of the genome to challenge. Science Vol. 226, pp. 792-801.
Saltational speciation, also known as abrupt speciation, is the discontinuity in a lineage that occurs through genetic mutations, chromosomal aberrations or other evolutionary mechanisms that cause reproductively isolated individuals to establish a new species population. Polyploidy, karyotypic fission, symbiogenesis and lateral gene transfer are possible mechanisms for saltational speciation.Ogunseitan, Dele. (2004). Microbial Diversity: Form and Function in Prokaryotes. Wiley-Blackwell. p. 13.
Macromutation theory
The botanist John Christopher Willis proposed an early saltationist theory of evolution. He held that species were formed by large mutations, not gradual evolution by natural selection.Willis, J. C. (1923). The Origin of Species by Large, rather than by Gradual, Change, and by Guppy's Method of Differentiation. Annals of Botany. Vol. 37, No. 148. pp. 605-628.Beal, J. M. (1941). The Course of Evolution by J. C. Willis. Botanical Gazette, Vol. 102, No. 3. p. 638.
The German geneticist Richard Goldschmidt was the first scientist to use the term "hopeful monster". Goldschmidt thought that small gradual changes could not bridge the hypothetical divide between microevolution and macroevolution. In his book The Material Basis of Evolution (1940) he wrote "the change from species to species is not a change involving more and more additional atomistic changes, but a complete change of the primary pattern or reaction system into a new one, which afterwards may again produce intraspecific variation by micromutation." Goldschmidt believed the large changes in evolution were caused by macromutations (large mutations). His ideas about macromutations became known as the hopeful monster hypothesis which is considered a type of saltational evolution.Grant, Verne. (1963). The Origin of Adaptations. Columbia University Press.
Goldschmidt's thesis however was universally rejected and widely ridiculed within the biological community, which favored the neo-Darwinian explanations of R.A. Fisher, J. B. S. Haldane and Sewall Wright.Gould, S. J. (1982). "The uses of heresy; an introduction to Richard Goldschmidt's The Material Basis of Evolution." pp. xiii-xlii. New Haven: Yale University Press. However, there has been a recent interest in the ideas of Goldschmidt in the field of evolutionary developmental biology as some scientists are convinced he was partially correct.Scott F. Gilbert. (2000). Developmental Biology Sinauer Associates; 6th edition.
Otto Schindewolf, a German paleontologist, also supported macromutations as part of his evolutionary theory. He was known for presenting an alternative interpretation of the fossil record based on his ideas of orthogenesis, saltational evolution and extraterrestrial impacts opposed to gradualism but abandoned the view of macromutations in later publications.Schindewolf, Otto. (1969). Über den “Typus” in morphologischer und phylogenetischer Biologie. Mainz: Akademie der Wissenschaften und der Literatur.
Søren Løvtrup, a biochemist and embryologist from Denmark, advocated a similar hypothesis of macromutation to Goldschmidt's in 1974.Hood, Kathryn E.; Halpern, Carolyn Tucker; Greenberg, Gary. (2010). Handbook of Developmental Science, Behavior, and Genetics. Wiley-Blackwell. p. 70 Lovtrup believed that macromutations interfered with various epigenetic processes, that is, those which affect the causal processes in biological development. This is in contrast to the gradualistic theory of micromutations of Neo-Darwinism, which claims that evolutionary innovations are generally the result of accumulation of numerous very slight modifications. Lovtrup also rejected the punctuated equilibria of Stephen Gould and Niles Eldredge, claiming it was a form of gradualism and not a macromutation theory. Lovtrup defended many of Darwin's critics including Schindewolf, Mivart, Goldschmidt, and Himmelfarb.Review of Lovtrup's book in the New Scientist, Oct 15, 1988 Mae Wan Ho described Lovtrup's theory as similar to the hopeful monster theory of Richard Goldschmidt.
Goldschmidt presented two mechanisms for how hopeful monsters might work. One mechanism, involved "systemic mutations", rejected the classical gene concept and is no longer considered by modern science; however, his second mechanism involved "developmental macromutations" in "rate genes" or "controlling genes" that change early development and thus cause large effects in the adult phenotype. These kind of mutations are similar to the ones considered in contemporary evolutionary developmental biology.Theissen, Guenter. (2010). Homeosis of the angiosperm flower: Studies on three candidate cases of saltational evolution. Palaeodiversity 3, Supplement: 131-139.
On the subject of Goldschmidt Donald Prothero in his book Evolution: What the Fossils Say and Why It Matters (2007) wrote: