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A Role for August Weismann In The Darwinian Revolution S.Subramanya Sastry 1 Introduction In 1859, Charles Darwin published the Origin Of Species in which he outlined a theory of evolution based on the mechanism of natural selection. Initially, it was well-received both by the public as well as the scientific community. This was in large part due to Darwin’s presentation of the theory as well as the efforts of Darwin’s friends and colleagues to publicize the theory. Though Darwin’s book ensured that a large number of people got converted to evolutionism, his mechanism of natural selection was not very popular. Yet, Darwinism gained widespread popularity in spite of the objections (technical and otherwise) raised against natural selection. Bowler ascribes this to the flexibility on the part of Darwin and Darwinism1 . Though Darwin proposed natural selection as the primary evolutionary mechanism, he accepted other auxiliary mechanisms too. The most important of these was the transmission of acquired characteristics (those acquired by use-disuse) first proposed by Lamarck in the early 1800s. In the rest of this essay, when Darwinism is referred to, it encompasses all these auxiliary evolutionary mechanisms in addition to natural selection. Between the 1860s to 1880s, Darwinism faced increasing objections that led to the proposal of a number of alternative evolutionary mechanisms. In this melee, August Weismann, a German naturalist and a staunch Darwinian, published his theory of the germ plasm in the 1880s. He proposed this theory to solve the problem of heredity, one of the outstanding problems of Darwinism 2 . As part of this theory, he attempted to purge Darwinism of its Lamarckian trimmings and expounded natural selection as the sole evolutionary mechanism. This stricter form of Darwinism has been dubbed Neo-Darwinism or Ultra-Darwinism3. This precipitated matters for Darwinism by attract1 Peter Bowler The Eclipse Of Darwinism, pages 13-15, 28 2 Ibid., page 41 3 People differ in their use of the term Neo-Darwinism. In one usage, Neo-Darwinism is used to indicate the Darwinian theory of evolution without resorting to any kind of indirect inheritance (Ralf-Dietmar Hegel, August Weismann: One of the First Synthetic Theorists of Evolutionary Biology, page 264). However, another usage of Neo- 1 ing much greater opposition than before. Around this time, Mendelian genetics was being revived in the context of discontinuous mutation that scientists were working on. In the period between 1890s and 1930s, even though Mendelian genetics was viewed as being contradictory with Neo-Darwinism, by the 1930s, both these theories were successfully reconciled to yield a theory that was able to explain evolution and heredity much more satisfactorily than all the other theories. Thus, around this time, Darwin’s theory of evolution achieved paradigm status in the field of evolutionary biology. Following Dobzhansky, in the rest of this essay, this revised Darwinian theory of evolution will be referred to as the synthetic theory of evolution4 . In this essay, I will examine August Weismann’s role in this success story of Darwin’s evolutionary theory (albeit in the form of the synthetic theory of evolution). I will initially portray Weismann as an ardent Darwinian. It is not very hard to support this claim if one examines his writings. More importantly, I will examine the role he played in laying down the groundwork for Mendelian genetics to take off which ultimately was responsible for a successful reception of Darwin’s theory in the 1930’s. Before proceeding to analyze Weismann’s role, I will provide a context for doing so. This is based on Bowler’s analysis of the Eclipse Of Darwinism in the 1900’s which examines the status of Darwinism around this period and the reasons for the eclipse. 1.1 Eclipse of Darwinism As mentioned previously, in the period between the 1860s and 1880s, a number of technical objections were raised against the mechanism of natural selection. There were also objections that were raised on theological grounds since Darwin’s theory reduced the status of man to yet another animal on this earth, and, more significantly, eliminated the need for divine intervention in evolution. As a result of these objections, a number of anti-Darwinian theories (orthogenesis, theistic evolution, Lamarckism) were proposed. Many of these were taken seriously by a large number of professional biologists, who were able to provide empirical arguments to support their position5. Darwinism refers to the evolutionary theory that emerged from the modern synthesis between the classical Darwinian theory of natural selection and Mendelian genetics (Timothy D.Johnston, The influence of germ-plasm theory on the distinction between learned and innate behavior, page 116). 4 Theodosius Dobzhansky: Evolution, Genetics and Man, pages 109-110. Discussing the conflicting use of the term Neo-Darwinism, he suggests the use of either synthetic theory or the biological theory of evolution to refer to the evolution theory of the 1930’s 5 Peter Bowler, The Eclipse of Darwinism, page 10 2 However, because of the flexibility and lack of dogmatism in early Darwinism and its allowance for a number of partially divergent views, it was fairly popular well into the 1880s. This flexibility was shared both by Darwin as well as by Darwin’s followers, including August Weismann. As a result, there were many naturalists who considered themselves Darwinians but did not necessarily believe in the mechanism of natural selection. In the 1880s, August Weismann published his theory of the germ plasm. Without going into the details of this theory at this point, it suffices to say that, if true, the theory implied that Lamarckism was infeasible. Also, Weismann viewed his theory as a solution to the problem of heredity and the source of heritable variations and thus expounded natural selection as an all-sufficient mechanism for evolution. This is viewed by Bowler as a decisive event in the debate between the Darwinians and antiDarwinians. Previously, people who straddled the fence without taking a stance with respect to selectionism were now forced to either go with or against Darwinism since Weismann made natural selection the sole evolutionary mechanism of Darwinism. This gave rise to another source of opposition for Darwinism because those who were explicitly committed to Lamarckism were now forced to oppose Weismann and hence Darwinism6 . Furthermore, geneticists found his theory to be incompatible with Mendelian laws of heredity, naturalists found his theory highly artificial and in the absence of experimental proof, opponents found his theory highly speculative7 . Thus, on the face of it, it appears that by publishing his theory of the germ plasm, Weismann actually harmed Darwinism. As Bowler puts it, “To a large extent, it can be argued that the eclipse of Darwinism was precipitated by movements within Darwinism that created a situation in which lukewarm supporters of the theory were turned into active opponents. 8 ” Given this viewpoint and the fact that Mendelian geneticists and Neo-Darwinians were not on the friendliest of terms in the early 1900s, why has it been claimed that Weismann, a NeoDarwinian, paved the way for Mendelian genetics? More to the point, even though the Mendelian geneticists initially opposed Darwinism, ultimately, they would have realized that genetics and Darwinism were complementary rather than contradictory. Hence, Darwinism would have come out of its eclipse of the 1900s and achieved paradigm status in spite of Weismann. Given this, what was Weismann’s role in the establishment of the Darwinian paradigm? I hope to provide some answers to these questions in this essay. 6 Ibid., page 7 Ibid., page 8 Ibid. 42 41 3 2 August Weismann: An Ardent Darwinian Ernst Mayr mentions that Darwin’s evolutionary theory had a very great impact in Germany9 . This might partially explain Weismann’s keen interest in Darwin’s theory. All his early evolutionary studies (on caterpillars and butterflies) of the 1870s were done assuming the truth of Darwin’s theory. In the following sections, I hope to show that Weismann was an ardent Darwinian, one who never relinquished his faith in Darwinian evolution and natural selection. 2.1 Weismann and Natural Selection Unlike many biologists of his time, Weismann was a firm believer in selectionism right from the beginning. Mayr cites Weismann’s first evolutionary publication of 1868 as a proof of this claim10 . He also considers Weismann to be the most consistent selectionist in the nineteenth century and quotes a number of Weismann’s statements as evidence for such a belief11 . In the 1870s, Weismann conducted a number of studies on butterflies and sphingid caterpillars to examine the efficacy of natural selection12 . These studies convinced him of the usefulness of natural selection in evolution. In his book, The Evolution Theory, Vol I, Weismann discusses the phenomena of coloration of animals in great detail13 . As part of this discussion, he presents numerous examples of color adaptation in a number of insects, eggs, caterpillars and other organisms. He explains how all these adaptations can be readily explained by resorting to natural selection. Dismissing alternative explanations for simple cases, he argues: But, all attempts at any other explanation are even more decidedly excluded when we turn our attention to more complicated cases of colour-adaptation, which are not confined to the simple, general coloration, but are helped by markings and colourpatterns, that is, by schemes of colour14 . That he strongly believed in natural selection as the sole explanation for color-adaptation can be seen from the following claim he makes: That all these extremely favourable protective colorations find their explanation in the 9 Ernst Mayr, Weismann and Evolution, page 296 305 11 Ibid., page 306 12 Ibid.; Frederick B. Churchill: August Weismann and a Break from Tradition, page 91 13 August Weismann: The Evolution Theory: Vol I, pages 57-90 14 Ibid. page 66 10 Ibid., page 4 slow and gradually cumulative effects of natural selection cannot be disputed; it is beyond doubt that they cannot be explained, so far as we know, in any other way15 . He also presents examples from the phenomena of mimicry (wherein certain organisms mimic the characteristics of other organisms in order to survive) to explain how only natural selection can satisfactorily explain this adaptation of organisms16 . He was also able to successfully address some objections raised against natural selection. For example, the presence of nonadaptive structures in organisms posed a problem for Darwinism since natural selection was a strictly utilitarian mechanism. An example of this was the loss of eyes of animals that inhabit dark caves. Weismann solves this problem by invoking the power of conservation exerted by natural selection. As long as an organ is under selection pressure, only those individuals that have the best organs survive. However, once the selection pressure is relaxed (for example, when the presence of sharp eyes is no longer crucial to the survival of an organism, as in the case of cave dwellers), perfection of that organ is no longer maintained (conserved) by natural selection. Coupled with competition for tissue substrate for other essential organs, he argued that, this can lead to degeneration of organs. Thus, Weismann was able to provide a satisfactory solution to the problem that was based solely on natural selection. Speaking of natural selection in general, Weismann goes so far as to claim that There is no part of the body of an individual or of any of its ancestors, not even the minutest and most insignificant part, which has arisen in any other way than under the influence of the conditions of life17 . In the light of the preceding discussion, it is accurate to characterize August Weismann as a devoted selectionist. However, one need not stop at that. He did more for Darwinism than merely defend natural selection. He set about to solve what he perceived were unsolved problems with Darwin’s theory of evolution. The foremost problem of these was the problem of source of variations that is so crucial to Darwin’s evolutionary theory. A related problem was that of heredity. The following subsection discusses how Weismann addressed this problem. 15 Ibid., page 87-88 16 Ibid., pages 91-118 17 Ibid., page 260; Ernst Mayr, Weismann and Evolution, page 307 5 2.2 Weismann and the problem of heredity In Darwin’s theory of evolution, speciation and introduction of varieties required that organisms undergo some form of heritable variations. However, Darwin did not have a satisfactory answer for what caused these variations. Given that such variations were introduced in organisms, natural selection could successfully explain adaptation and speciation in organisms. To some extent, the Lamarckian mechanism of inheritance of acquired characters provided a partial solution to this problem of variations18 . According to this Lamarckian mechanism, variations were introduced in organisms based on use/disuse of organs. Furthermore, the organism could transmit these acquired characters to its offspring. Darwin, probably, for lack of a better solution, accepted this and went so far as to come up with his theory of pangenesis to explain the mechanism of the inheritance of acquired characters. Like all other Darwinians, Weismann started his career by accepting the mechanism of inheritance of acquired characters. In all of his evolutionary writings prior to 1882, he believed it to be the major source of variation 19 . However, by 1883, he was ready to purge Darwinism of all Lamarckian connections and assert natural selection as the sole evolutionary mechanism. Having done this, he recognized the need to come up with an alternative mechanism for the production of genetic (heritable) variations20. Towards this end, he proposed his theory of the germ plasm. Before examining this theory and its impact, I briefly present Weismann’s attack on the mechanism of inheritance of acquired characters. 2.3 Weismann’s attack on Lamarckism Weismann did not disagree with Lamarck in that use/disuse of organs can be a source of variations in an organism’s lifetime. He says: “ Lamarck’s idea is at once clear: It is true that exercising an organ strengthens it, that disuse makes it weaker. ... Indeed, we may now go so far as to admit that every organ is strengthened by use and weakened by disuse, ... 21 ”. What he did not agree with was that this was the only source of variations22 . More importantly, he did not agree that these variations were heritable23 . He claims that inheritance of acquired characters has never 18 Ernst Mayr, Weismann and Evolution, page 312 19 Ibid. 20 Ibid., page 315, August Weismann, Essays Upon Heredity, pages 252-253 Weismann, The Evolution Theory, Vol I, page 20 22 Ibid., page 21; August Weismann, Essays Upon Heredity, “It is well known that Lamarck attempted to explain the structure of the organism as almost entirely due to this principle alone”, page 84 23 August Weismann, The Evolution Theory, Vol I, page 21; August Weismann, Essays Upon Heredity, pages 71-105 21 August 6 been proved, either by means of direct observation or by experiment. He was also very skeptical of descriptions where mutilations, scars and such suffered by an organism are transmitted to its offspring. Of these, he says, “... in these descriptions, the previous history is invariably obscure, and hence the evidence loses all scientific value”24 . Weismann argues that though exercise strengthens organs, perfection of an organism does not depend on the amount of exercise, but rather on the predisposition of the germ cells (reproductive cells) to produce a perfect organ. Thus, with the same amount of exercise, the organ that is destined to be strong will attain a higher degree of functional activity than one that is destined to be weak. Further, natural selection, by selecting the organism that possessed the stronger organ weeds out the germ cells that had a predisposition to produce the weaker organ 25 . By arguing thus, Weismann undercuts the importance of the Lamarckian mechanism because it is the germ cell coupled with natural selection that gives the impression that the acquired characters have been inherited. On a different note, this means that, in Weismann’s theory, natural selection operates on the genotypes (germ cells) rather than the phenotypes (external manifestations of the germ-cell traits). This is clear from what Weismann says: “... natural selection, while it apparently decides between individuals of various degrees of strength, is in truth operating upon the stronger and weaker germs”26 . Weismann brought arguments from color adaptations and mimicry to bear upon Lamarckism (use/disuse as well as inheritance of acquired characters). By doing so, he attempted to demonstrate that Lamarckism would be at a loss explaining these phenomena. To quote him: “But, even if we recognize the Lamarckian factor27 as a vera causa, it is easy to show that there are a great many characters which it is not in a position to interpret 28 ”. For example, speaking of the adaptation wherein certain insects possess exactly the same shade green as the plants that they feed on, he argues: “How could this green colour of the skin have been brought about by the activity of the skin, since the colour of the surroundings does not usually stimulate the skin to activity at all?29 ” Darwin had first explained color adaptation and mimicry in insects using natural selection. Bates claimed that these phenomena served as valuable proof of natural selection 30 . However, Weismann used these very same phenomena to claim that Lamarckism was not a complete theory since it could 24 August Weismann, 25 Ibid., page 84 Essays Upon Heredity, page 81 26 Ibid., page 85 27 He means the inheritance of 28 August 29 Ibid. 30 Peter acquired characters Weismann: The Evolution Theory: Vol I, page 21. Bowler The Eclipse Of Darwinism, pages 29-30 7 not explain these phenomena. There were a number of other cases (for example, how could certain complex behavior that is exhibited only once in an entire lifetime be learnt through use/disuse?) that Weismann used to demonstrate that Lamarckism was implausible. Since it was hard to obtain direct proof of a case of natural selection producing adaptations, Weismann hoped to best support natural selection by the principle of exclusion, i.e. by refuting all competing theories31 . Mayr best sums up Weismann’s strategy as follows: Weismann’s strategy was to show not only that an inheritance of acquired characters encounters formidable difficulties, but also that cases cited in its favor could be explained quite well through the theory of natural selection 32 . From the preceding discussion, it should be clear that Weismann went about supporting Darwinism by refuting all competing theories. Moreover, in publishing the theory of the germ plasm, he intended to solve the existing problems of Darwinism and make it a more perfect theory than what it was before. In the process, based on his germ-plasm theory, he also set about refuting competing theories of evolution. I am now ready to present some details of the germ plasm theory and how it influenced the outcome of the Darwinian Revolution. 3 The Germ Plasm Theory In a nutshell, the germ plasm, as opposed to somatoplasm, (ordinary body fluid), was responsible for transmitting the information of heredity. More importantly, the body of an organism served only as a host and could never affect this germinal material and hence could not affect its offspring. If true, this implied that Lamarckism was infeasible since characteristics that were acquired by an organism during its lifetime could not be transmitted to its offspring. But, as mentioned earlier, Weismann was now forced to come up with an alternative mechanism for producing heritable variations. Sexual reproduction came to his rescue here. Based on the progress of cytology in the 1870s and 1880s, he knew that during sexual reproduction, two distinct germ cells come together to “restablish the diploidy of the zygote”33 . Since these individual germ cells carried separate hereditary characteristics, he viewed this coming together of germ cells as a significant source of 31 Ernst Mayr, Weismann and Evolution, page 307 33 Ernst Mayr, Weismann and Evolution, page 316 32 Ibid., pages 313-314 8 genetic variation34 . For a long time, Weismann maintained in the “continuity of the germ plasm” wherein the germ plasm is perceived as a stable substance that has virtually unchanged through the evolution of multicellular organisms35. Thus, genetic recombination was the only means of introducing heritable variations in organisms. However, in later years, he realized that this limited the amount of variation possible and was hardpressed to come up with a source of novel genetic variations36. As a result, he allowed for an evolutionary change of the germ plasm. However, he was very hesitant to allow for “internalist” changes of the germ plasm (mutations) since he had consistently opposed orthogenesis37 . However, his theory faced a lot of criticism because of its many internal inconsistencies and also because most of his theory was based on pure speculation. Weismann very well recognized the speculative nature of his theories and says: ... even if it should have to be abandoned at a later period, it seems to me that, at the present time, it is a necessary stage in the advancement of our knowledge, and one which must be brought forward and passed through, whether it prove right or wrong, in the future.38 In any case, the most important aspect of his theory was his insistence on the constancy of germinal material and its isolation from the body. Further, he recognized the importance of sexual reproduction as a source of genetic variation. These ultimately made their appearance in the final synthetic theory of evolution of the 1930s. As discussed previously, in Weismann’s theory, natural selection acts on germinal traits rather than the external manifestations of these traits, i.e. he distinguished between the phenotypes and genotypes of organisms and argued that natural selection acted on genotypes only. This was different from earlier theories of evolution where this distinction was not made. Again, this distinction was crucial and was present in the synthetic theory of evolution. Mayr recognizes all these factors and also mentions that Weismann was also responsible for the “firm establishment of particulate inheritance” (as opposed to blending inheritance). He mentions 34 August Weismann, Essays Upon Heredity, pages 272-273; Ernst Mayr, Weismann and Evolution, pages 315-316 Weismann, Essays Upon Heredity, pages 165-248; Timothy D.Johnston, The influence of germ-plasm theory on the distinction between learned and innate behavior, page 117 36 Ernst Mayr, Weismann and Evolution, page 317 37 Ibid. 38 August Weismann, Essays Upon Heredity, page 174 35 August 9 that Weismann’s postulates on reduction division laid the foundation for Mendelian genetics39 . 4 Conclusions Based on the preceding discussion, it should be clear that simply characterizing Weismann as a Darwinian presents an incomplete picture of the man. He was much more than a Darwinian. He was responsible for sowing the seeds for the evolutionary theories that would ultimately come to be accepted. Though Weismann might have got many of the details of his theory wrong, he got many of the principles right which ultimately found their way in the synthetic theory of evolution. Churchill claims that more than simply achieving a breakthrough in the problem of heredity, Weismann was also responsible for shaking the assumptions of the scientific community of that time40 . It is instructive to examine what has been said about this subject before. According to Ernst Mayr, even though Weismann failed to solve the problem of heredity that he set out to solve, the intellectual preparation he gave to the area enabled the Mendelian theory to prosper41 . Ralf-Dietmar Hegel claims: “With the rejection of the inheritance of acquired characteristics and the assumption of the plasma as a substrate of inheritance of relatively high stability, Neo-Darwinism prepared the theoretical ground for the rediscovery of Mendelian laws of inheritance by Tschermak, Correns, and Bateson around 190042 .” These opinions (there are many more than just these two) should show the importance of Weismann and the role his theory had to play in the subsequent establishment of the Darwinian paradigm in the field of evolutionary biology. On a concluding note, here is what Michael Ruse has to say about Darwin’s role in the Darwinian Revolution: Had Darwin (and Wallace) never existed, scientists would still have become evolutionists – but it was Darwin who precipitated and expedited the change43 . A similar case can be made for August Weismann and the synthetic theory of evolution. 39 Ibid., page 40 Frederick 326 B. Churchill, The Weismann-Spencer Controversy Over the Inheritance Of Acquired Characters, page 457 41 Ernst Mayr, Weismann and Evolution, page 327 Weismann: One of the First Synthetic Theorists of Evolutionary Biology, page 264 43 Michael Ruse, The Darwinian Revolution: Science Red in Tooth and Claw, page 265 42 Ralf-Dietmar Hegel, August 10 References [1] Essays Upon Heredity, August Wiesmann [2] The Evolution Theory: Volume I, August Wiesmann [3] The Eclipse of Darwinism, Peter Bowler [4] Weismann and Evolution, Ernst Mayr, In the Journal of the History of Biology, Vol 18, No. 3, pages 295-329. [5] The Weismann-Spencer Controversy Over the Inheritance Of Acquired Characters, Frederick B. Churchill, In the Proceedings of the 15th International Congress of the History of Science, pages 451-468. [6] August Weismann and a Break from Tradition, Frederick B. Churchill, In the Journal of the History of Biology, 1968:1, pages 91-112. [7] The influence of germ-plasm theory on the distinction between learned and innate behavior, Timothy D.Johnston, In Journal of the History of the Behavioral Sciences, 1995, Vol 31, page 116 [8] August Weismann: One of the First Synthetic Theorists of Evolutionary Biology, Ralf-Dietmar Hegel, In World Views and Scientific Discipline Formation, page 264 [9] The Darwinian Revolution: Science Red in Tooth and Claw, Michael Ruse [10] Evolution, Genetics and Man, Theodosius Dobzhansky 11