Saltation (biology)

For other uses, see Saltation (disambiguation).

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 neo-Darwinian evolution.

Saltation was the idea that new species arise as a result of 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.

Saltation does not fit into contemporary evolutionary theory, but there are some prominent proponents, 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.

Polyploidy (most common in plants but not unknown in animals) is considered a type of saltation, even though most polyploid individuals are sterile. 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.

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

German paleontologist Otto Schindewolf opposed the theory of gradual evolution, and in the 1930s suggested that major evolutionary transformations must have occurred in large leaps between species due to macromutation. This idea became known as the "Hopeful Monster" theory and was further taken and developed up by the German-born geneticist Richard Goldschmidt in the 1940s. Schindewolf was also the first to suggest, in 1950, that mass extinctions might have been caused by extraterrestrial impacts or nearby supernova.

Goldschmidt thought that small gradual changes could not bridge the hypothetical divide between microevolution and macroevolution. In his seminal work 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'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.

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.

Use by creationists

It has been common practice for creationists to associate 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