The eclipse of Darwinism

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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.^[1][2] Historians of science such as Peter J. Bowler have used it 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 felt it was of relatively minor importance.^[3][4] Recently the term eclipse has been criticized as it inaccurately suggests that research on Darwinism paused during this period and the biological term interphase suggested as an alternative.^[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 was the belief that organisms were affected by internal forces or laws of development that drove evolution in particular directions, and saltationism was 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; it wasn't until developments in genetics made them seem increasingly untenable, and the development of population genetics and the modern evolutionary synthesis demonstrated the explanatory power of natural selection that they were largely abandoned. Ernst Mayr wrote that as late as 1930 most textbooks still emphasized such non-Darwinian mechanisms.^[6]

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 teleology or the concept of progress in the development of life.^[10][11] Some of these scientists and philosophers, like St. George Jackson Mivart and Charles Lyell, who came to accept evolution but disliked natural selection, raised religious objections.^[12] Others, such as Herbert Spencer, the botanist George Henslow (son of Darwin's mentor 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 age of the earth and sun (10–100 million years) being made at the time by physicists such as Lord Kelvin, and some felt that natural selection could not work because at the time the models for inheritance involved blending of inherited characteristics, an objection raised by the engineer Fleeming Jenkin in a review of Origin written shortly after its publication.^[13][14] Another factor at the end of the 19th century was the rise of new faction of biologists, typified by geneticists like Hugo DeVries and Thomas Hunt Morgan, who wanted to recast biology as an experimental laboratory science. They distrusted the work of naturalists like Darwin and Alfred Russel Wallace, dependent on field observations of variation, adaptation, and biogeography, as being overly anecdotal. Instead they focused on topics like physiology, and genetics that could be easily investigated with controlled experiments in the lab, and discounted things like natural selection and even the degree to which organisms were adapted to their environment, which could not easily be tested experimentally.^[15]

Theistic evolution

British science developed in the early 19th century on a basis of natural theology which saw the adaptation of fixed species as evidence that they had been specially created to a purposeful divine design. The philosophical concepts of German idealism inspired concepts of an ordered plan of harmonious creation, which Richard Owen reconciled with natural theology as a pattern of homology showing evidence of design. Similarly, Louis Agassiz saw the recapitulation theory as symbolising a pattern of the sequence of creations in which humanity was the goal of a divine plan. In 1844 Vestiges adapted Agassiz's concept into theistic evolutionism. Its anonymous author Robert Chambers proposed a "law" of divinely ordered progressive development, with transmutation of species as an extension of recapitulation theory. This popularised the idea, but it was strongly condemned by the scientific establishment. Agassiz remained forcefully opposed to evolution, and after he moved to America in 1846 his idealist argument from design of orderly development became very influential.^[16] In 1858 Owen cautiously proposed that this development could be a real expression of a continuing creative law, but distanced himself from transmutationists. Two years later in his review of Darwin's On the Origin of Species Owen attacked Darwin while at the same time openly supporting evolution,^[17] expressing belief in a pattern of transmutation by law-like means. This idealist argument from design was taken up by other naturalists such as George Jackson Mivart, and the Duke of Argyll who rejected natural selection altogether in favor of laws of development that guided evolution down preordained paths.^[18]

Many of Darwin's supporters accepted evolution on the basis that it could be reconciled with design. In particular, Asa Gray considered natural selection to be the main mechanism of evolution and sought to reconcile it with natural theology. He proposed that natural selection could be a mechanism in which the problem of evil of suffering produced the greater good of adaptation, but conceded that this had difficulties and suggested that God might influence the variations on which natural selection acted to guide evolution.^[19] For Darwin and Thomas Henry Huxley such pervasive supernatural influence was beyond scientific investigation, and George Frederick Wright, an ordained minister who was Gray's colleague in developing theistic evolution, emphasised the need to look for secondary or known causes rather than invoking supernatural explanations: "If we cease to observe this rule there is an end to all science and all sound science."^[20]

A secular version of this methodological naturalism was welcomed by a younger generation of scientists who sought to investigate natural causes of organic change, and rejected theistic evolution in science. By 1872 Darwinism in its broader sense of the fact of evolution was accepted as a starting point. Around 1890 only a few older men held onto the idea of design in science, and it had completely disappeared from mainstream scientific discussions by 1900. There was still unease about the implications of natural selection, and those seeking a purpose or direction in evolution turned to neo-Lamarckism or orthogenesis as providing natural explanations.^[21]

Neo-Lamarckism

Jean-Baptiste Lamarck

Jean-Baptiste Lamarck had originally proposed a theory on the transmutation of species that was largely based on a progressive drive toward greater complexity. Lamarck also believed, as did many others at the time, that characteristics acquired during the course of an organism's life could be inherited by the next generation, and he saw this as a secondary evolutionary mechanism that produced adaptation to the environment. Typically, such characteristics included changes caused by the use or disuse of a particular organ. It was this mechanism of evolutionary adaptation through the inheritance of acquired characteristics that much later came to be known as 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]

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, the American paleontologists Edward Drinker Cope and Alpheus Hyatt, and the American entomologist Alpheus Packard. They considered Lamarckism to be more progressive and thus philosophically superior to Darwin's idea of natural 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. The idea of linear progress was an important part of Haeckel's recapitulation theory of evolution, which held that the embryological development of an organism repeats its evolutionary history. Cope and Hyatt looked for, and thought they found, patterns of linear progression in the 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]

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 Britain the botanist George Henslow, the son of 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]

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]

Baldwin effect

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 inheritance 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 evolution 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.^[29]

Orthogenesis

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.

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]

Orthogenesis had a significant following in the 19th century, and its proponents included the Russian biologist Leo S. Berg, and Henry Fairfield Osborn, an American paleontologist.^[31] Orthogenesis was particularly popular among some paleontologists, who believed that the fossil record showed patterns of gradual and constant unidirectional change. Those who accepted this idea, however, did not necessarily accept that the mechanism driving orthogenesis was teleological (goal-directed). They also thought orthogenetic trends were not adaptive; in fact they felt that in some cases they led to developments that were detrimental to the organism, such as the large antlers of the Irish elk that they believed led to the animal's extinction.^[30]

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]

Saltationism and mutation theory

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 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.^[32][33]

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 Oenothera, which he started in 1886. The primroses de Vries worked with seemed to be constantly producing new vareties with striking variations in form and color, some of which appeared to be new species because plants of the new generation could only be crossed with one another, not with their parents. DeVries himself allowed a role for natural selection in determining which new species would survive, but some of those influenced by his work, including Morgan, felt that natural selection was not necessary at all. DeVries's ideas were very influential in the first two decades of the 20th century as some felt mutation theory could explain the sudden emergence of new forms in the fossil record and research on Oenothera spread all over the world. Though some of his critics, which included many field naturalists, wondered why no other organism seemed to show the same kind of rapid mutation.^[34]

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 Drosophila melanogaster in his lab in 1907. However, it was a researcher in that lab, Hermann Joseph Muller, who determined in 1918 that the new varieties de Vries had observed while breeding Oenothera were the result of polyploid hybrids rather than rapid genetic mutation.^[35] While they were doubtful of the importance of natural selection, the work of geneticists like Morgan, Bateson, DeVries and others from 1900 to 1915 established Mendelian genetics linked to chromosomal inheritance, which validated August Weismann's criticism of neo-Lamarckian evolution by discounting the inheritance of acquired characteristics. The work in Morgan's lab with Drosphila also undermined the concept of orthogenesis by demonstrating the random nature of mutation.^[36]

End of the eclipse

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]

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]

Developments in the field of genetics made many field naturalists such as Bernhard Rensch and Ernst Mayr abandon neo-Lamarckian ideas about evolution in the early 1930s.^[39] By the late 1930s biologists like Mayr and Theodosius Dobzhansky had synthesized the ideas of population genetics with the knowledge of field naturalists about the amount of genetic diversity in wild populations, and the importance of genetically distinct sub populations (especially sub populations partially or fully isolated from one another by geographical barriers) to begin the modern evolutionary synthesis.^[40] In 1944 George Gaylord Simpson integrated 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]

See also

• History of evolutionary thought
• Objections to evolution

Notes

References