The eclipse of Darwin

The eclipse of Darwinism was a phrase used by Julian Huxley 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. Historians of science such as Peter J. Bowler have used it to describe the history of evolutionary thought during the 1880s and 1890s when a number of alternatives to natural selection were developed and explored, and many evolutionary biologists considered natural selection to have been a wrong guess on Darwin's part. [wikiref|id=Quammen-2006|text=Quammen 2006 pp. 216-223] The four major alternatives to natural selection in the late 19th century, were theistic evolution, neo-Lamarckism, orthogenesis, and saltationism.

Reasons for the search for alternatives

Evolution was widely accepted in scientific circles within a few years after the publication of "Origin", but the acceptance of natural selection as its driving mechanism was much less widespread. There were a variety of reasons for this. Natural selection, with its emphasis on death and competition, did not appeal to many naturalists because they felt it was immoral and left little room for teleology (purpose) or the concept of "progress" in the development of life. In addition, some felt that natural selection would be too slow, given the estimates of the age of the earth and sun (10–100 million years) being made at the time by physicists such as Lord Kelvin. Another objection was that natural selection could not work because at the time the models for inheritance involved blending of inherited characteristics.wikiref|id=Larson-2004|text= Larson 2004 pp. 105–129] wikiref|id=Bowler-2003|text= Bowler 2003 pp. 196–253]

Theistic evolution

Theistic evolution was the idea that a God intervened in the process of evolution to guide it in such a way that the living world could still be considered to be designed. Some of its advocates included Asa Gray, who considered natural selection to be important but believed that God influenced the variations on which it acted, and George Jackson Mivart, and the Duke of Argyle who both rejected natural selection altogether. However, this idea rapidly fell out of favor among scientists, as they became more and more committed to the idea of methodological naturalism and came to believe that direct appeals to supernatural involvement were scientifically unproductive and a form of special pleading. By 1900 it had completely disappeared from mainstream scientific discussions, although it continued to be used as a way to reconcile religious belief with scientific discoveries among non-scientists.

Neo-Lamarckism

The term Lamarckism was used for the idea that characteristics acquired during the course of an organism's life, such as changes caused by the use or disuse of a particular organ, could be inherited by the next generation. Although Alfred Russel Wallace completely rejected the concept in favor of natural selection, Charles Darwin had 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.harv|Darwin|1872|p= [http://darwin-online.org.uk/content/frameset?itemID=F391&viewtype=text&pageseq=136 108] .] In the late 19th century the term neo-Lamarckism came to be associated with the position of naturalists who viewed the inheritance of acquired characteristics as the most important evolutionary mechanism. Advocates of this position included the British writer and Darwin critic Samuel Butler, the German biologist Ernst Haeckel, and the American paleontologist Edward Drinker Cope. They considered Lamarckism to be philosophically superior to Darwin's idea of selection acting on random variation. Butler and Cope both believed that this allowed organisms to effectively drive their own evolution, since organisms that developed new behaviors would change the patterns of use of their organs and thus kick-start the evolutionary process. In addition, Cope and Haeckel both believed that evolution was a progressive process. Cope looked for, and thought he found, patterns of linear progression in the fossil record. The idea of linear progress was also an important part of Haeckel's recapitulation theory of evolution, which held that the embryological development of an organism repeats its evolutionary history.

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.

As a consequence of the debate over the viability of neo-Lamarckism in the 1890s James Mark Baldwin, Henry Fairfield Osborne and C. Lloyd Morgan all independently proposed a mechanism where new learned behaviors could cause the evolution of new instincts and physical traits through natural selection without resort to the inheritence of acquired characteristics. They proposed that if individuals in a species benefited from learning a particular new behavior, the ability to learn that behavior could be favored by natural selection, and the end result would be the evolultion of new instincts and eventually new physical adaptations. This became known as the Baldwin effect and it has remained a topic of debate and research in evolutionary biology ever since. [ (Bowler 2006 pp. 243, 367)]

Orthogenesis

Orthogenesis or orthogenetic evolution was the hypothesis that life has an innate tendency to change, in a unilinear fashion, towards ever-greater perfection. It had a significant following in the 19th century, and its proponents included the Russian biologist Leo Berg, and the American paleontologist Henry Fairfield Osborn. Orthogenesis was particularly popular among some paleontologists, who believed that the fossil record showed a gradual and constant unidirectional change. Those who accepted this idea, however, did not necessarily accept that the mechanism driving orthogenesis was teleological (goal-directed). The orthogenesis hypothesis began to collapse when it became clear that it could not explain the patterns found by paleontologists in the fossil record, which were non-linear and contained many complications. A few hung on to the orthogenesis hypothesis 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.

altationism

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 very popular with early geneticists such as Hugo DeVries, William Bateson, and early in his career, T. H. Morgan. This later became the basis of the mutation theory of evolution.

Notes

References

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*Citation | last = Darwin | first = Charles | author-link =Charles Darwin | year = 1872 | title =The Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life | edition =6th | publication-place = London | publisher =John Murray | url =http://darwin-online.org.uk/content/frameset?itemID=F391&viewtype=text&pageseq=1 | accessdate =
*cite book|last=Larson|first=Edward J.|authorlink=Edward Larson|title=Evolution:The Remarkable History of Scientific Theory|publisher=Modern Library|year=2004|isbn=0-679-64288-9
*cite book|last=Quammen|first=David|authorlink=David Quammen|title=The Reluctant Mr. Darwin|publisher=Atlas Books|year=2006|isbn=0-393-05981-2