Weismann barrier

A fundamental reason why Lamarckian mechanisms cannot act as an important, generally active evolutionary factor is that they could function only in organisms with unseparated germinal and somatic cell lines.In the 19th century, the foremost German biologist August Weismann already pointed out that Lamarckian evolution can mostly not function in multicellular animals because the germinal and somatic cellular lines are sharply separated basically by an impermeable genetic barrier.In the most important groups of animals, early ontogenesis differentiates the lines of cells from which the sex cells will be formed.Only sex cells are immortal in the evolutionary sense and transfer their genetic information to further generations through the progeny.In contrast, all the somatic cells forming the body of an animal are mortal; even if they were to undergo some useful modification or mutation, this feature would disappear with the death of the individual and could in no way affect the course of evolution.Thus, if thick skin is formed on the foot soles of a person, his germinal cells would never learn of this; if a gene for dihydrofolate reductase multiplies in the liver cells, this change will not be manifested in the progeny.

Weismann not only described the existence of the barrier between germinal and somatic cells, but also attempted to demonstrate it experimentally.However, he was not very fortunate in his choice of experimental system.His experiments, in which he cut off the tails of mice over a great many generations to demonstrate that mice would continue to be born with tails of the same length, yielded the expected result (i.e., no evolutionary response); however, only a very enthusiastic person could consider that this demonstrates the impermeability of the Weismann barrier.However, it should be recalled that, in the 19th century, a number of “experiments” tended to have the nature of demonstration of the existence of a phenomenon and Weismann’s experiments fulfilled their role very well in this sense.By the way, Weismann was a Jew, so he didn’t have to look far for clear empirical proof of the absence of heredity of a similar surgical operation, performed systematically over hundreds of generations ....

In conclusion, it should be recalled that this separation of the germinal and somatic lines is not so strict in the representatives of a great many groups of organisms, including the representatives of most animal species.The Weismann barrier does not exist at all for a number of taxa, or the germinal line is differentiated later in ontogenesis.However, it is interesting that species of animals with early differentiation of germinal cells, i.e. primarily arthropods and vertebrates, have enjoyed the greatest success in the evolution of animals, particularly in the number of split-off species.Some scientists are of the opinion that the actual function of early differentiation of the germinal line is to prevent intra-organism competition amongst the individual cell lines and thus permits the formation of large complicated organisms consisting of a great many cells that can accumulate a large level of genetic variability through somatic mutations during ontogenesis (Buss 1987).Differentiation of the cells of the germinal line does not occur at all in plants and fungi.This could be connected with the existence of cell walls that limit movement of the cells within the the body of the organism and thus substantially limit the scope for intra-organism competition.Because a flower can be formed in a plant through differentiation from somatic tissues, acquired traits can be inherited in plants.For example, if cells better adapted to growth in a certain temperature regime come, in time, to predominate in the tissues of woody species, this property can be transferred to further generations through the seeds from flowers formed by differentiation of these tissues (Pineda-Krch & Fagerstrom 1999; Flegr 2002) (Fig. III.11).However, it should be recalled that, once again, this is not Lamarckian evolution, as adaptation of cells to a certain temperature regime did not occur through adaptive mutations, which would occur as a reaction to conditions or to the behaviour of the organism, but through the Darwinistic mechanism of survival of those tissue cells (or branches of the tree) that are best adapted to the local environment as a consequence of a random change, somatic mutation, somatic recombination or epigenetic change (II.8.1).

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more