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).
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.
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.
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.
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.
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