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Friedrich Leopold August Weismann (b. January 17, 1834 in Frankfurt am Main; d. 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 that DNA makes DNA and RNA copies within an organism, but RNA cannot make DNA. This is now known not to be true. The two ideas are linked because 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 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 Weissman Barrier is central to what is now the generally accepted theory of evolution, more properly referred to as 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).
Additional recommended knowledge
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.
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. then he died
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.
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.
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.
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 Mendels 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. 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. In fact, he only proved that mutilations are not inherited, a fact that was well known without any need to cut off mouse tails, since generations of Jews and others had practised circumcision, without any observed effect on offspring. What Lamarck actually claimed was the inheritance of characteristics acquired through necessity, or effort, or will, which is quite different.
Some written work
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "August_Weismann". A list of authors is available in Wikipedia.|