January 17, 1834 (a Friday)

August Weismann

On this date, the German biologist August (Friedrich Leopold) Weismann was born in Frankfurt. Ernst Mayr ranked him the second most notable evolutionary theorist of the 19th century, after Charles Darwin. Weismann was one of the founders of modern genetics, who is best known for his opposition to the concept of the inheritance of acquired traits and for his “germ plasm” theory, the forerunner of DNA theory.

During his career, Weismann grappled with Christian creationism as a possible alternative to evolutionary theory. In his work Über die Berechtigung der Darwin’schen Theorie (On the Justification of the Darwinian Theory) published in 1868, he compared 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 he completed this work, Weismann accepted evolution as a fact on a par with the fundamental assumptions of astronomy (e.g., heliocentrism).

In a lecture in 1883 entitled “On inheritance” (“Über die Vererbung”), Weismann proposed the so-called germ-plasm theory of heredity. This theory states that a multicellular organism’s cells are divided into somatic cells (the cells that make up the body) and germ cells (cells that produce the gametes).  His great insight was to see that the two do not exchange information – variation must be produced in the germ cells. In other words, genetic information cannot pass from somatic cells to germ cells and on to the next generation. This was referred to as the Weismann barrier.  In addition, the germ cells are neither influenced by environmental conditions nor by learning or morphological changes that happen during the lifetime of an organism, which information is lost after each generation.  Thus, the germ-plasm theory ruled out the inheritance of acquired characteristics as proposed by Jean-Baptiste Lamarck.  In stating definitely seven years later that the material of heredity was in the chromosomes, Weismann anticipated the chromosomal basis of inheritance.

In October 1887, Weismann began the famous experiment of chopping off the tails of fifteen hundred white mice, repeatedly over 20 generations. He subsequently reported that no mouse was ever born in consequence without a tail, stating 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.

Weismann knew that it might be objected that the number of generations had been far too small:

Hence the experiments on mice, when taken alone, do not constitute a complete disproof of [inheritance of acquired characteristics]: they would have to be continued to infinity before we could maintain with certainty that hereditary transmission cannot take place. But it must be remembered that all the so-called proofs which have hitherto been brought forward in favour of the transmission of mutilations assert the transmission of a single mutilation which at once became visible in the following generation. Furthermore the mutilation was only inflicted upon one of the parents, not upon both, as in my experiments with mice. Hence, contrasted with these experiments, all such ‘proofs’ collapse; they must all depend on error.

Weismann made it clear that he embarked on the experiment precisely because, at the time, there were many claims of animals inheriting mutilations (he 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. The purpose of Weismann’s experiment was to lay the claims of inherited mutilation to rest.  Its results were consistent with Weismann’s germ-plasm theory.

Mitosis and meiosis compared.

Meiosis was discovered and described for the first time in sea urchin eggs in 1876, by noted German biologist Oscar Hertwig (1849-1922). It was described again in 1883, at the level of chromosomes, by Belgian zoologist Edouard Van Beneden (1846-1910) in Ascaris worms’ eggs. However, the significance of meiosis for reproduction and inheritance was grasped only in 1890 by Weismann, who noted that two cell divisions were necessary to transform one diploid cell into four haploid cells in order to maintain the correct number of chromosomes in the offspring.

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