August Friedrich Leopold Weismann
August Friedrich Leopold Weismann
August Weismann, an early adherent to Darwin's theory of evolution, became famous for his studies on heredity. He denied that organisms could inherit acquired characteristics, touching off an important debate in the late nineteenth century between his own school, neo-Darwinism, and the opposing neo-Lamarckians who believed organisms could inherit acquired characteristics.
Weismann was born in Frankfurt-am-Main, Germany, and attended Göttingen University to study medicine. Beginning in 1866, he taught at the University of Freiburg, where he held a position as professor of Zoology until 1912. During his tenure, he convinced the university to establish a zoological institute and museum, and served as its director. As his vision began to deteriorate, Weismann turned his attention from microscopic studies to theoretical speculation concerning evolution and the mechanisms of hereditary transmission.
In the 1880s Weismann began to publish his views on evolution and heredity. His main works were Studies in the Theory of Descent (2 vols., 1882), The Germ-Plasm—a Theory of Heredity (1893), and The Evolution Theory (2 vols., 1904). Weismann believed that in order to understand heredity, one must distinguish between reproductive cells (containing hereditary material that Weismann called "germ plasm," but today would be called genes) and all other cells in an organism (somatoplasm). He believed that heredity was influenced solely by the reproductive cells without any influence from the somatic cells.
This view is often referred to as "hard heredity." When the function of the chromosome was discovered in the late nineteenth century, Weismann recognized that it might be the physical unit containing germ plasm, a hunch that was later confirmed.
Weismann rejected the Lamarckian theory—a view still popular in his day—that evolution was driven by the inheritance of acquired characteristics (such as the use or disuse of organs), also known as "soft heredity." For example, if called on to explain the evolutionary development of blind cave fish, Weismann would claim that it was not the disuse of the organs that directly caused the atrophy of their eyes, but rather natural selection favored the fish without eyes. Weismann believed that selection pressure—in this case, light to give the fish with eyes an advantage—was necessary to keep organisms from degenerating.
Weismann was not claiming that the environment has absolutely no influence on heredity. He acknowledged that alcohol, poisons, or other substances taken into the body might have deleterious effects on the germ plasm and result in degeneration. But he insisted that it was only effects on the germ plasm, not effects on the body, that could be inherited. As with Charles Darwin (1809-1882), Weismann never discovered how variations arose to produce evolutionary novelties, but he showed that they had to occur in the germ plasm or genetic material.
Weismann conducted a famous experiment to try to disprove the inheritance of acquired characteristics. He cut the tails off of several generations of mice and measured the tails in their offspring. There was no decrease in length, leading Weismann to conclude that the information passed on from an organism to its offspring is independent of the influence of the body cells.
Weismann's views on "hard heredity" spread rapidly, especially in Germany, England, and the United States. Many biologists, however, were not convinced by Weismann's theory of hard heredity. Ernst Haeckel (1834-1919), the most famous Darwinian biologist in Germany in the late nineteenth century, continued to believe that the inheritance of acquired characteristics produced hereditary change, though he, like Darwin, also insisted on the efficacy of natural selection. In the 1890s neo-Lamarckians vociferously opposed Weismann, since they did not think he solved the problem of the source of evolutionary variation. Around 1900 scientists in France, Spain, Latin America, and many other parts of the world leaned more toward Lamarck and soft heredity than to Weismann's hard heredity.
However, Weismann's views prevailed in the scientific world through the discovery of the chromosome, the rediscovery of Mendelian genetics around 1900, and, after his death in 1914, the subsequent neo-Darwinian synthesis of the 1930s and 1940s.