Neo-Darwinism, Origin of

What is the theoretical framework of modern biology? If we would question a professional biologist, or even an eager reader of popular books on biology, we would await the answer: Darwinism, that is, Charles Darwin's theory of evolution by natural selection. But this answer is not entirely precise. Of course, Darwin's position in the history of biology can be compared to that of Galileo Galilei in the history of physics. But it would be surprising if the nearly 150 years since the first publication of Darwin's main work, On the Origin of Species (1859), had not brought forth new discoveries and fresh theoretical ideas that changed [Page 1728]the original version of Darwinism. And, indeed, this was the case. Therefore, a terminologically more precise answer to our initial question would be that Neo-Darwinism is the theoretical framework of modern biology.

What are the most important theoretical developments after Darwin that shaped Neo-Darwinism? The title Neo-Darwinism was used in the history of evolutionary thought to denominate, above all, the following three theoretical developments:

• August Weismann's panselectionism assumed that natural selection works, not only on the level of organisms, but also on the levels of chromosomes, cells, and populations (since the1870s).
• Theoretical population biology constructed mathe matical models for the workings of natural selection in the history of life (since the 1910s).
• The Modern Synthesis integrated all subdisciplines of biology into an overall picture of Darwinian evo lution (since the 1930s).

Contemporaries called the evolutionary thought of the German biologist August Weismann (1834–1914) Neo-Darwinism (or even Ultra-Darwinism) because he extended the range of the Darwinian principle of natural selection so that it comprised the struggle for existence, not only between organisms, but also between populations, cells, and chromosomes. Weismann did much experimental research on specific biological phenomena in the light of evolutionary theory, and he interpreted his experimental results with the help of courageous conceptualizations. That is why Weismann can be called the second most important Darwinian of the 19th century (excelled only by Darwin himself).

We shall give a first impression of the importance of Weismann's work by describing how he embedded the empirical falsification of a then widely accepted genetic theory into a new conceptual framework, and how he used this framework to sketch a radical Darwinian picture of the evolution of life.

Weismann tested experimentally, in the years 1875 to 1880, the theory of the inheritance of acquired characters. The idea that organisms transmit knowledge acquired by practice and learning genetically to their offspring was developed into a biological theory by the French biologist Jean Baptiste de Lamarck at the beginning of the 19th century. From edition to edition of On the Origin of Species,Darwin more and more became a Lamarckian in genetics. Weismann tested Lamarckism by experimenting with lines of mice, whose tails he cut generation after generation. But he did not succeed this way in breeding mice that have shorter tails as compared to mice of the same origin whose tails were never cut.

How to explain this result? Weismann developed, in the first half of the1880s, the theory of germ-plasm. He distinguished two types of cells in every organism: germ cells and somatic cells. The germ cells (in sexually reproducing organisms, the male sperm and the female egg)secure the reproduction of an organism by means of the germ-plasm they contain in their nuclei. It is, in modern terms, nothing else than the carrier of genetic information. All other functions of the organism are fulfilled by somatic cells.

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