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The eclipse of Darwinism

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The eclipse of Darwinism

Julian Huxley used the phrase "the eclipse of Darwinism" to describe the state of affairs prior to the modern evolutionary synthesis when evolution was widely accepted in scientific circles but relatively few biologists believed that natural selection was its primary mechanism.[1][2] Historians of science such as Peter J. Bowler have used the same phrase as a label for the period within the history of evolutionary thought from the 1880s through the first couple of decades of the 20th century when a number of alternatives to natural selection were developed and explored - as many biologists considered natural selection to have been a wrong guess on Charles Darwin's part, and others regarded natural selection as of relatively minor importance.[3][4] Recently the term eclipse has been criticized for inaccurately implying that research on Darwinism paused during this period, Paul Farber and Mark Largent have suggested the biological term interphase as an alternative metaphor.[5]

There were four major alternatives to natural selection in the late 19th century:

  • Theistic evolution was the belief that God directly guided evolution. (This should not be confused with the more recent use of the term theistic evolution, referring to the theological belief about the compatibility of science and religion.)
  • The idea that evolution was driven by the inheritance of characteristics acquired during the life of the organism was called neo-Lamarckism.
  • Orthogenesis involved the belief that organisms were affected by internal forces or laws of development that drove evolution in particular directions
  • Saltationism propounded the idea that evolution was largely the product of large mutations that created new species in a single step.

Theistic evolution largely disappeared from the scientific literature by the end of the 19th century as direct appeals to supernatural causes came to be seen as unscientific. The other alternatives had significant followings well into the 20th century; mainstream biology largely abandoned them only when developments in genetics made them seem increasingly untenable, and when the development of population genetics and the modern evolutionary synthesis demonstrated the explanatory power of natural selection. Ernst Mayr wrote that as late as 1930 most textbooks still emphasized such non-Darwinian mechanisms.[6]

Contents

  • Extent 1
  • Motivation for alternatives 2
  • Theistic evolution 3
  • Neo-Lamarckism 4
    • Baldwin effect 4.1
  • Orthogenesis 5
  • Saltationism and mutation theory 6
  • End of the eclipse 7
  • See also 8
  • Notes 9
  • References 10

Extent

Evolution was widely accepted in scientific circles within a few years after the publication of On the Origin of Species, but acceptance of natural selection as its driving mechanism was much less.[7] By the end of the 19th century criticism of natural selection had reached the point that in 1903 the German botanist, Eberhardt Dennart, wrote that "We are now standing at the death bed of Darwinism", and in 1907 the Stanford University entomologist Vernon Lyman Kellogg who supported natural selection had to admit "... the fair truth is that the Darwinian selection theory, considered with regard to its claimed capacity to be an independently sufficient mechanical explanation of descent, stands today seriously discredited in the biological world." [8] Though he added that there were problems preventing the widespread acceptance of any of the alternatives, as large mutations seemed too uncommon, and there was no experimental evidence of mechanisms that could support either Lamarckism or orthogenesis.[9] Ernst Mayr wrote that a survey of evolutionary literature and biology textbooks showed that as late as 1930 the belief that natural selection was the most important factor in evolution was a minority viewpoint with only a few population geneticists being strict selectionists.[6]

Motivation for alternatives

There were a variety of different factors that motivated people to propose other evolutionary mechanisms as alternatives to natural selection. Natural selection, with its emphasis on death and competition, did not appeal to some naturalists because they felt it was immoral, and left little room for John Stevens Henslow also a botanist), and Samuel Butler, felt that evolution was an inherently progressive process that natural selection alone was insufficient to explain. Still others, including the American paleontologists Edward Drinker Cope and Alpheus Hyatt, had an idealist perspective and felt that nature, including the development of life, followed orderly patterns that natural selection could not explain.[13]

Some felt that natural selection would be too slow, given the estimates of the

References

  1. ^ (Huxley 1942, pp. 22–28)
  2. ^ (Bowler 2003, pp. 196, 224)
  3. ^ (Bowler 1983)
  4. ^ (Quammen 2006, pp. 216–223)
  5. ^
  6. ^ a b c (Mayr & Provine 1998, p. x)
  7. ^ (Quammen 2006, p. 205)
  8. ^ (Endersby 2007, pp. 143,453)
  9. ^ (Larson 2004, p. 128)
  10. ^ (Bowler 2003, p. 197)
  11. ^ (Larson 2004, pp. 119–120)
  12. ^ (Quammen 2006, pp. 209–210)
  13. ^ a b c (Bowler 2003, pp. 196–253)
  14. ^ a b (Larson 2004, pp. 105–129)
  15. ^ (Endersby 143–147,182)
  16. ^ (Bowler 1983, pp. 44–49)
  17. ^ (Secord 2001, pp. 424, 512)
  18. ^ (Bowler 1983, pp. 46, 49–50)
  19. ^ (Bowler 2003, pp. 203–206)
  20. ^ (Larson 2004, pp. 110–111)
  21. ^ (Bowler 1983, pp. 26–27, 44–45, 54–55)
  22. ^ (Bowler 2003, pp. 86–95)
  23. ^ (Darwin 1872, p. 108.)
  24. ^ a b c (Bowler 2003, pp. 236–244)
  25. ^ a b (Larson 2004, pp. 125–129)
  26. ^ a b (Quammen 2006, pp. 217–219)
  27. ^ (Bowler 2003, pp. 239–240)
  28. ^ (Bowler 2003, pp. 253–255)
  29. ^ (Bowler 2003, pp. 243, 367)
  30. ^ a b (Quammen 2006, p. 221)
  31. ^ (Bowler 2003, p. 249)
  32. ^ (Bowler 2003, pp. 265–270)
  33. ^ (Larson 2004, pp. 127–129, 157–167)
  34. ^ (Endersby 2007, pp. 148–162)
  35. ^ (Endersby 2007, pp. 202–205)
  36. ^ (Bowler 2003, pp. 269–272)
  37. ^ (Larson 2004, p. 223)
  38. ^ (Mayr & Provine 1998, pp. xi–xii)
  39. ^ (Mayr & Provine 1998, pp. 124–127, 296)
  40. ^ (Mayr & Provine 1998, pp. xii–xiii)
  41. ^ (Bowler 2003, p. 337)

Notes

See also

Developments in the field of paleontology into the synthesis by statistically analyzing the fossil record to show that it was consistent with the branching non-directional form of evolution predicted by the modern synthesis, and in particular that the linear trends cited by earlier paleontologists in support of Lamarckism and orthogenesis did not stand up to careful analysis.[41] Mayr wrote that by the end of the synthesis natural selection together with chance mechanisms like genetic drift had become the universal explanation for evolutionary change.[6]

During the period 1916–1932, the discipline of population genetics developed largely through the work of the geneticists Ronald Fisher, J.B.S. Haldane, and Sewall Wright. Their work recognized that the vast majority of mutations produced small effects that served to increase the genetic variability of a population rather than creating new species in a single step as the saltationists assumed, and they were able to produce statistical models of population genetics that included Darwin's concept of natural selection as the driving force of evolution.[38]

Biston betularia f. typica is the white-bodied form of the peppered moth.
Biston betularia f. carbonaria is the black-bodied form of the peppered moth. In 1924 J.B.S. Haldane published a statistical analysis of the evolution of industrial melanism in peppered moths as an example of natural selection in a real world population.[37]

End of the eclipse

[36] Morgan was a supporter of de Vries's mutation theory and was hoping to gather evidence in favor of it when he started working with the fruit fly

[34] 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. Its originator was the Dutch botanist Hugo de Vries. De Vries looked for evidence of mutation extensive enough to produce a new species in a single generation and thought he found it with his work breeding the evening primrose of the genus

Saltationism 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 mutation theory of evolution.[32][33]

Saltationism and mutation theory

Support for the orthogenesis hypothesis began to decline during the modern evolutionary synthesis in the 1940s when it became apparent that it could not explain the complex branching patterns of evolution revealed by statistical analysis of the fossil record by paleontologists. A few hung on to the idea of orthogenesis as late as the 1950s by claiming that the processes of macroevolution, the long term trends in evolution, were distinct from the processes of microevolution.[13][14]

Orthogenesis had a significant following in the 19th century, and its proponents included the Russian biologist Irish elk that they believed led to the animal's extinction.[30]

Orthogenetic evolution was the hypothesis that life has an innate tendency to change, in a unilinear fashion in a particular direction. The term orthogenesis was popularized by a Theodor Eimer a German zoologist. Eimer also believed in Lamarckian inheritance of acquired characteristics, but he felt that internal laws of growth determine which characteristics would be acquired and guided the long term direction of evolution down certain paths.[30]

Henry Fairfield Osborn's 1918 book Origin and Evolution of Life claimed the evolution of Titanothere horns was an example of an orthogenetic trend in evolution.

Orthogenesis

As a consequence of the debate over the viability of neo-Lamarckism in the 1890s, Baldwin effect and it has remained a topic of debate and research in evolutionary biology ever since.[29]

Baldwin effect

Critics of neo-Lamarckism pointed out that no one had ever produced solid evidence for the inheritance of acquired characteristics. The experimental work of the German biologist August Weismann resulted in the germ plasm theory of inheritance. This led him to declare that inheritance of acquired characteristics was flatly impossible, since the Weismann barrier would prevent any changes that occurred to the body after birth from being inherited by the next generation. Despite these criticisms, neo-Lamarckism remained the most popular alternative to natural selection at the end of the 19th century, and would remain the position of some naturalists well into the 20th century.[25][28]

In Britain the botanist John Stevens Henslow who had been Darwin's mentor at the University of Cambridge, was an important advocate of neo-Lamarckism. He studied how environmental stress affected the development of plants, and he wrote that the variations induced by such environmental factors could largely explain evolution. The science historian Peter J. Bowler writes that, as was typical of many 19th century Lamarckians, Henslow did not appear to understand the need to demonstrate that such environmentally induced variations would be inherited by descendants that developed in the absence of the environmental factors that produced them, but merely assumed that they would be.[27]

Many American proponents of neo-Lamarckism were strongly influenced by Louis Agassiz and a number of them, including Hyatt and Packard, were his students. Agassiz had an idealistic view of nature, connected with natural theology, that emphasized the importance of order and pattern. Agassiz never accepted evolution; his followers did, but they continued his program of searching for orderly patterns in nature, which they considered to be consistent with divine providence, and preferred evolutionary mechanisms like neo-Lamarckism and orthogenesis that would be likely to produce them.[24][26]

In the late 19th century the term fossil record.[24][25] Packard argued that the loss of vision in the blind cave insects he studied was best explained through a Lamarckian process of atrophy through disuse combined with inheritance of acquired characteristics. Packard also wrote a book about Lamarck and his writings.[24][26]

Lamarckism.[22] Although Alfred Russel Wallace completely rejected the concept in favor of natural selection, Charles Darwin always included what he called Effects of the increased Use and Disuse of Parts, as controlled by Natural Selection in On the Origin of Species, giving examples such as large ground feeding birds getting stronger legs through exercise, and weaker wings from not flying until, like the ostrich, they could not fly at all.[23]

Jean-Baptiste Lamarck

Neo-Lamarckism

A neo-Lamarckism or orthogenesis as providing natural explanations.[21]

Many of Darwin's supporters accepted evolution on the basis that it could be reconciled with design. In particular, [20]

British science developed in the early 19th century on a basis of Duke of Argyll who rejected natural selection altogether in favor of laws of development that guided evolution down preordained paths.[18]

Theistic evolution

[15]

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