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acquired character

August Weismann

[vahys-mahn]

Friedrich Leopold August Weismann (Birth. January 17, 1834 in Frankfurt am Main; Death. November 5 , 1914 in Freiburg im Breisgau, ) was a German biologist. Ernst Mayr ranked him the second most notable evolutionary theorist of the 19th century, after Charles Darwin.

Weismann advocated the germ plasm theory, according to which (in a multicellular organism) inheritance only takes place by means of the germ cells—the gametes such as egg cells and sperm cells. Other cells of the body—somatic cells—do not function as agents of heredity. The effect is one-way: germ cells produce somatic cells, and more germ cells; the germ cells are not affected by anything the somatic cells learn or any ability the body acquires during its life. Genetic information cannot pass from soma to germ plasm and on to the next generation. This is referred to as the Weismann barrier.

This idea, if true, would rule out the inheritance of acquired characteristics as proposed by Jean-Baptiste Lamarck, which Charles Darwin considered highly probable as the cause of change on which natural selection acts. However, it was only an idea, and as such required experimental proof. In fact, since it is a negative idea, final proof would consist in proving that all conceivable means by which so-called soma to germline transmission might occur do not in fact operate. Weismann, who of course knew nothing of the complexity of modern genetic theory, was in no position to provide such proof.

The Weismann barrier is often confused with the Central dogma of molecular biology which is incorrectly said to be a restatement of Weismann's idea by Francis Crick. In fact the central dogma states only that protein is not known to be capable of copying to either RNA or DNA . The two ideas are mistakenly linked because the central dogma has often been re-stated in broader terms, including a statement that RNA cannot copy to DNA (which is now known to be untrue). The inheritance of acquired characteristics, if it occurs, requires modification of DNA in the gametes, or germ cells, and one way in which this might occur is by copying from RNA. However, it would be wrong in any case to suppose that this is the only way, and the two ideas are quite separate, just as the concepts of natural selection and Lamarckism are not in conflict, since natural selection constitutes only half of Darwin's theory of evolution, the other half being genetic change, the mechanism of which was largely a mystery to Darwin, who knew nothing of random mutation or its causes.

Despite these observations, the idea of the Weismann barrier is central to what is now the generally accepted theory of evolution, founded on the Modern evolutionary synthesis. According to this theory, variations occur from time to time in the individuals of a population, as a result of genetic mutations which occur to some extent naturally, by copying errors, but can also be caused by chemicals known as mutagens or by ionising radiation, especially X-rays or cosmic rays. Some variations are reproductively more successful than others so that through this indirect process, aided by the further processes of crossing over, and assortative mating the genetic structure of a breeding group gradually changes. In Weismann's opinion this largely random process of mutation, which must occur in the gametes (or stem cells that make them) is the only source of change for natural selection to work on. Weismann was one of the first biologists to deny soft inheritance entirely.

Weismann's ideas preceded the rediscovery of Gregor Mendel's work, and though Weismann was cagey about accepting Mendelism, younger workers soon made the connection.

Weismann is much admired today. Ernst Mayr judged him to be the most important evolutionary thinker between Darwin and the evolutionary synthesis around 1930-40 (Mayr 1982).

Life

Youth and studies

Weismann was born a son of high school teacher Johann (Jean) Konrad Weismann (1804-1880), a graduate of ancient languages and theology, and his wife Elise (1803-1850), née Lübbren, the daughter of the county councillor and mayor von Stade, on January 17, 1834 in Frankfurt am Main. He had a typical 19th century bourgeois education, receiving music lessons from the age of four, and drafting and painting lessons from Jakob Becker (1810-1872) at the Frankfurter Städelsche Institut from the age of 14. His piano teacher was a devoted butterfly collector and introduced him to the collecting of imagos and caterpillars. But studying Natural Sciences was out of the question due to the cost involved and limited job prospects. A friend of the family, Friedrich Wöhler (1800-1882), recommended studying medicine. A foundation from the inheritance of Weismann's mother allowed him to take up studies in Göttingen. Following his graduation in 1856, he wrote his dissertation on the synthesis of hippuric acid in the human body.

Professional life

Immediately after university, Weismann took on a post as assistant at the Städtische Klinik (city clinic) in Rostock. Weismann successfully submitted two manuscripts, one about hippuric acid in herbivores, and one about the salt content of the Baltic Sea, and won two prizes. The paper about the salt content dissuaded him from becoming a chemist, since he felt himself lacking in apothecarial accuracy.

After a study visit to see Vienna's museums and clinics, he graduated as a medical doctor and settled in Frankfurt. During the war between Austria, France and Italy in 1859, he became Chief Medical Officer in the military. During a leave from duty, he walked Northern Italy and Tyrol. After a sabbatical in Paris, he worked with Rudolf Leuckart (1822-1898) at the University of Gießen, nonetheless to return to Frankfurt as personal physician to the banished Grand Duke Stephan of Austria, at Schaumburg Castle (from 1861 to 1863).

From 1863, he was lecturer, from 1865 professor and from 1873 to 1912 Ordinarius for zoology and director of the zoological institute at Albert Ludwig University of Freiburg in Breisgau.

His son, the composer Julius Weismann, was born in 1879.

Contributions to evolutionary biology

At the beginning of Weismann's preoccupation with evolutionary theory is his grappling with Christian creationism as a possible alternative. In his work Über die Berechtigung der Darwin'schen Theorie (On the justification of the Darwinian theory) he compares creationism and evolutionary theory, concluding that many biological facts can be seamlessly accommodated within evolutionary theory, but remain puzzling if considered the result of acts of creation.

After this work, Weismann accepts evolution as a fact on a par with the fundamental assumptions of astronomy (e.g. Heliocentrism). Weismann's position towards mechanism of inheritance and its role for evolution changed during his life. Three periods can be distinguished.

1868-1881/82

Weismann starts out believing, like many other 19th century scientists, among them Charles Darwin, that the observed variability of individuals of one species is due to the inheritance of sports (Darwin's term). He believed, as written in 1876, that transmutation of species is directly due to the influence of environment. He also wrote, "if every variation is regarded as a reaction (sic) of the organism to external conditions, as a deviation of the inherited line of development, it follows that no evolution can occur without a change of the environment". (Note that this is close to the Modern use of the concept that changes in the environment can mediate selective pressures on a population, in all but very few cases leading to evolutionary change.) Weismann also used the classic Lamarckian metaphor of use and disuse of an organ.

1882-1895

Weismann's first rejection of the inheritance of acquired traits was in a lecture in 1883, titled "On inheritance" ("Über die Vererbung"). Again, as in his treatise on creation vs. evolution, he attempts to explain individual examples with either theory. For instance, the existence of non-reproductive castes of ants, such as workers and soldiers, cannot be explained by inheritance of acquired characters. Germ plasm theory, on the other hand, does so effortlessly.

Even though Weismann used this theory to explain Darwin's original examples for "use and disuse", such as the tendency to have degenerate wings and stronger feet in domesticated waterfowl, he did not convert his contemporaries.

1896-1910

Weismann worked on the embryology of sea urchin eggs, and in the course of this observed different kinds of cell division, namely equatorial division and reductional division, terms he coined (Äquatorialteilung and Reduktionsteilung respectively).

His germ plasm theory states that multicellular organisms consist of germ cells containing heritable information, and somatic cells that carry out ordinary bodily functions.

The germ cells are influenced neither by environmental influences nor by learning or morphological changes that happen during the lifetime of an organism, and so this information is lost after each generation. This discovery eventually led to the rediscovery of Gregor Mendel's work.

Weismann barrier: Cutting off mice tails

The idea that germline cells contain information that passes to each generation unaffected by experience and independent of the somatic (body) cells, came to be referred to as the Weismann barrier, and is frequently quoted as putting a final end to the theory of Lamarck and the inheritance of acquired characteristics. While Weisman based the idea on his limited knowledge of cells and his (largely wrong) theory of Germ Plasm, he is also widely quoted as having 'proved' the non-existence of Lamarckian inheritance by the experiment of chopping of the tails of fifteen hundred mice, repeatedly over 20 generations, and reporting that no mouse was ever born in consequence without a tail. In fact he states that '901 young were produced by five generations of artificially mutilated parents and yet there was not a single example of a rudimentary tail or any other abnormality of the organ'. Despite the fact that this experiment has been repeatedly declared invalid, both then and today, many modern textbooks continue to declare that Weismann 'proved' that inheritance of acquired characteristics does not occur. It is also often reported that his experiments were not needed since generations of Jews and others had practised circumcision, without any observed effect on offspring. In fact Weismann was aware of the limitations of his experiment, and made it clear that he embarked on the experiment precisely because, at the time, there were many claims of animals inheriting mutilations (he specifically refers to a claim regarding a cat that had lost its tail having numerous tail-less offspring). There were also claims of Jews born without foreskins. None of these claims, he said, were backed up by reliable evidence that the parent had in fact been mutilated, leaving the perfectly plausible possibility that the modified offspring were the result of a mutated gene. He carried out his experiment in order to lay such claims, regarding mutilation specifically, to rest. What Lamarck and others actually claimed was the inheritance of characteristics acquired through necessity, or effort, or will, or environment, which is quite different.

In fact, Weismann, who knew almost nothing of the complexities of modern genetics, was in no position to declare such a barrier, especially since proving that something cannot happen requires investigation of every conceivable mechanism by which it might just happen. The philosopher Henri Bergson explained this well as long ago as 1911 in response to Weismann's assertion when he said, in his book Creative Evolution: After having been affirmed as a dogma, the transmissibility of acquired characteristics has been no less dogmatically denied, for reasons drawn a priori from the supposed nature of germinal cells. It is well known how Weismann was led, by his hypothesis of the continuity of the germ-plasm, to regard the germinal cells - ova and spermatozoa - as almost independent of the somatic cells. Starting from this, it has been claimed, and is still claimed by many, that the hereditary transmission of an acquired character is inconceivable. But if, perchance, experiment should show that acquired characteristics are transmissible, it would prove thereby that the germ-plasm is not so independent of the somatic envelope as has been contended, and the transmissibility of acquired characters would become ipso facto conceivable; which amounts to saying that conceivability and inconceivability have nothing to do with the case, and that experience alone must settle the matter. Bergson goes on to describe the difficulties that arise in such experiment, of separating the effect of habit from a natural aptitude that may have existed to induce the habit.

The dogmatic nature of Weismann's assertion was seriously challenged in the 1980's and 1990's by the Australian immunologist, Ted Steelewho presents considerable evidence for what is now known as soma to germline transfer in his book Lamarck's Signature. There has been a recent revival in what is known as Neo-Lamarckism

Some written work

  • Über die Berechtigung der Darwin'schen Theorie. Leipzig 1868
  • Über den Einfluß der Isolierung auf die Artbildung. Jena 1872
  • Studien zur Descendenztheorie: II. Ueber die letzten Ursachen der Transmutationen. Leipzig 1876
  • Die Continuität des Keimplasmas als Grundlage einer Theorie der Vererbung. Jena 1885
  • Zur Frage nach der Vererbung erworbener Eigenschaften. In: Biol. Zbl. 6 (1886):33-48
  • Über die Zahl der Richtungskörper und über ihre Bedeutung für die Vererbung. Jena 1887
  • Das Keimplasma - eine Theorie der Vererbung. Jena 1882
  • Aufsätze über Vererbung und angewandet biologische Fragen Jena 1892
  • Die Allmacht der Naturzüchtung: eine Erwiderung an Herbert Spencer. Jena 1893
  • Vorträge über Deszendenztheorie. 2 Bde. 1902

See also

Literature

  • Rolf Löther: Wegbereiter der Genetik: Gregor Johann Mendel und August Weismann. Verlag Harri Deutsch, Frankfurt am Main 1990, ISBN 3-8171-1130-4
  • Mayr, Ernst. The growth of biological thought. Harvard University Press 1982
  • H. Risler: August Weismann 1834-1914. In: Berichte der Naturforschenden Gesellschaft Freiburg im Breisgau, 1968, S. 77-93
  • H. Risler: August Weismanns Leben und Wirken nach Dokumenten aus seinem Nachlass. In: Freiburger Universitätsblätter, Heft 87/88, Freiburg 1985, S. 23-42

References

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