Saltation (biology)

For other uses, see Saltation (disambiguation).

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In biology, saltation (from Latin, saltus, "leap") is a sudden change from one generation to the next, that is large, or very large, in comparison with the usual variation of an organism. The term is used for occasionally hypothesized, nongradual changes (especially single-step speciation) that are atypical of, or violate, standard concepts - gradualism - involved in modern evolutionary theory.

History

Prior to Charles Darwin most evolutionary scientists had been saltationists. Jean-Baptiste Lamarck was a gradualist and 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." Geoffroy wrote that environmental pressures could produce sudden transformations to establish new species instantaneously. In 1864 Albert von Kölliker revived Geoffroy's theory that evolution proceeds by large steps, under the name of heterogenesis.

With the publication of On the Origin of Species in 1859 Charles Darwin had denied saltational evolution by writing that evolutionary transformation always proceeds gradually and never in jumps. Darwin insisted on slow accumlation of small steps in evolution and wrote "natural selection acts solely by accumulating slight successive favourable variations, it can produce no great or sudden modification; it can act only by very short steps".

From 1860 to 1880 saltation had a minority interest but by 1890 had become a major interest to scientists. This was because in the late 19th century due to discoveries in genetics, 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. 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).

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.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 1940's that demonstrated the explanatory power of natural selection that saltational views of evolution were largely abandoned.

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).

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.

Confusion with punctuated equilibrium

It is a popular misconception that punctuated equilibrium is a saltationist theory, often mistaken for Richard Goldschmidt's hypothesis of "Hopeful Monsters." However, punctuated equilibrium refers instead to a pattern of evolution where most speciation occurs relatively rapidly from a geological perspective (tens of thousands of years instead of millions of years), but through neo-Darwinian evolution, not by saltations.

Stephen Jay Gould

In 1977 Stephen Jay Gould argued that the recent discovery of regulatory genes offered new evidence which supported some of Goldschmidt's postulates. Gould argued that instances of rapid evolution neither undermine Darwinian theory (as Goldschmidt believed) nor await immediate discreditation (as many neo-Darwinians thought). Gould insisted that Darwin's belief in gradualism—which was largely inherited from the anti-catastrophic views of Charles Lyell—was never an essential component to Darwin's theory of evolution. Thomas Henry Huxley also warned Darwin that he had loaded his work "with an unnecessary difficulty in adopting Natura non facit saltum so unreservedly." Huxley feared this assumption could discourage naturalists who believed that major leaps and cataclysms played a significant role in the history of life. Gould continued:

As a Darwinian, I wish to defend Goldschmidt's postulate that macroevolution is not simply microevolution extrapolated, and that major structural transitions can occur rapidly without a smooth series of intermediate stages. . . . In his infamous book of 1940, Goldschmidt specifically invokes rate genes as a potential maker of hopeful monsters: 'This basis is furnished by the existence of mutants producing monstrosities of the required type and the knowledge of embryonic determination, which permits a small rate change in early embryonic processes to produce a large effect embodying considerable parts of the organism.' In my own, strongly biased opinion, the problem of reconciling evident discontinuity in macroevolution with Darwinism is largely solved by the observation that small changes early in embryology accumulate through growth to yield profound differences among adults.

Nevertheless, Gould argued that Goldschmidt's "hopeful monster" concept was incorrect:

The developmental theme of the 'hopeful monster' (despite its inappropriate name, virtually guaranteed to inspire ridicule and opposition), based on the important concept of 'rate genes,' came first in Goldschmidt's thought, and always occupied more of his attention and research. Unfortunately, he bound this interesting challenge from development, a partially valid concept that could have been incorporated into a Darwinian framework as an auxiliary hypothesis (and now has been accepted, to a large extent, if under different names), to his truly oppositional and ultimately incorrect theory of systemic mutation, therefore winning anathema for his entire system. Goldschmidt may have acted as the architect of his own undoing, but much of his work should evoke sympathetic attention today.

Macromutation theory

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.

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. 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 not entirely wrong.

Otto Schindewolf a German paleontologist also supported macromutations as part of his evolutionary theory. He was know 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.

Soren Lovtrup advocated a similar hypothesis of macromutation to Goldschmidt's in 1974. Lovtrup, a biochemist and embryologist from Denmark, believed that macromutations interfered with various epigenetic processes, that is, those which affect the casual 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. 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.

In 2008 evolutionary biologist Olivia Judson in her article The Monster Is Back, and It’s Hopeful listed some examples which may support the hopeful monster hypothesis and an article published in the nature journal in 2010 titled Evolution: Revenge of the Hopeful Monster reported that studies in stickleback populations in a British Columbia lake and bacteria populations in a Michigan lab have shown that large individual genetic changes can have vast effects on organisms "without dooming it to the evolutionary rubbish heap". According to the article "Single-gene changes that confer a large adaptive value do happen: they are not rare, they are not doomed and, when competing with small-effect mutations, they tend to win. But small-effect mutations still matter — a lot. They provide essential fine-tuning and sometimes pave the way for explosive evolution to follow."

A paper by (Page et al. 2010) have written that the Mexican axolotl (Ambystoma mexicanum) could be classified as a hopeful monster as it exhibits an adaptive and derived mode of development that has evolved rapidly and independently among tiger salamanders. According to the paper there has been an interest in aspects of the hopeful monster hypothesis in recent years:

“

Goldschmidt proposed that mutations occasionally yield individuals within populations that deviate radically from the norm and referred to such individuals as "hopeful monsters". If the novel phenotypes of hopeful monsters arise under the right environmental circumstances, they may become fixed, and the population will found a new species. While this idea was discounted during the Modern synthesis, aspects of the hopeful monster hypothesis have been substantiated in recent years. For example, it is clear that dramatic changes in phenotype can occur from few mutations of key developmental genes and phenotypic differences among species often map to relatively few genetic factors. These findings are motivating renewed interest in the study of hopeful monsters and the perspectives they can provide about the evolution of development. In contrast to mutants that are created in the lab, hopeful monsters have been shaped by natural selection and are therefore more likely to reveal mechanisms of adaptive evolution.

”

Guenter Theissen a professor of genetics has classified homeotic mutants as hopeful monsters and documented many examples for animal and plant lineages that may have originated as hopeful monsters in his scientific publications (Theissen, 2005 and Theissen et al. 2006).

Current status

Examples of saltational evolution include cases of stabilized hybrids that can reproduce without crossing (such as allotetraploids) and cases of symbiogenesis. Evidence of phenotypic saltation has been found in the centipede and some scientists have suggested there is evidence for independent instances of saltational evolution in Sphinx moths. Some processes of epigenetic inheritance can also produce changes that are saltational. There has been a controversy over if mimicry in butterflies and other insects can be explained by gradual or saltational evolution. According to (Norrstrom et al. 2007) there is evidence for saltation in some cases of mimicry. The endosymbiotic theory is considered to be a type of saltational evolution.

Specific cases of homeosis in flowers can be caused by saltational evolution. In a study of divergent orchid flowers (Bateman and DiMichele, 2002) wrote how simple homeotic morphs in a population can lead to newly established forms that become fixed and ultimately lead to new species. They described the transformation as a saltational evolutionary process, where a mutation of key developmental genes leads to a profound phenotypic change, producing a new evolutionary linage within a species. Polyploidy (most common in plants but not unknown in animals) is considered a type of saltation. Polyploidy meets the basic criteria of saltation in that a significant change (in gene numbers) results in speciation in just one generation. Mammalian liver cells are typically polyploidal, but they are not part of the germ line.

Use by creationists

Some creationists have associated Goldschmidt's "hopeful monsters" with the theory of punctuated equilibrium, as proposed by Eldredge and Gould. Punctuated equilibrium differs from hopeful monsters in that the former acts on populations rather than individuals, is theoretically more gradual (which proposes to take 50,000 to 100,000 years), functions by the evolution of reproductive isolation (through mechanisms such as allopatric speciation), and the latter says nothing of stasis. Creationists such as Luther Sutherland claim that both theories inadvertently appeal to the absence of fossil evidence for evolution and thereby undermining the theory of Darwinian evolution. This predicament is used by creationists to argue that "there are no transitional fossils." Paleontologists such as Niles Eldredge, Stephen Jay Gould, and Steven M. Stanley avoid this by explaining that transitional forms may be rare between species, but "they are abundant between larger groups", and none of these paleontologists support Goldschmidt's "hopeful monster" hypothesis. Steven M. Stanley argued that some of Goldschmidt's views err mainly in exaggerating the importance of "chromosomal rearrangements" leading to "rapid changes in growth gradients or developmental sequences, and on what we now call quantum speciation."

See also

• Catastrophism
• Phyletic gradualism
• Rapid modes of evolution
• Leo S. Berg
• History of evolutionary thought
• Eclipse of Darwinism