August Weismann, a 19th-century German evolutionary biologist, made profound contributions that reshaped the understanding of heredity and evolution. He became a leading supporter of Charles Darwin’s theory of natural selection when many contemporaries held alternative views on how traits were passed down. His work helped establish a framework for modern genetics.
The Germ Plasm Theory
Weismann’s germ plasm theory proposed a fundamental division within the cells of multicellular organisms. He asserted that an organism’s cells are separated into two types: somatic cells and germ cells. Somatic cells constitute the body’s tissues and organs, but they are mortal and their characteristics are not passed to offspring.
Conversely, germ cells, found in the gonads (ovaries and testes), are the immortal lineage responsible for transmitting heritable information across generations. These cells, such as sperm and egg cells, contain the “germ plasm,” which Weismann considered the material basis of heredity. The germ plasm can be imagined as a master blueprint for an organism, while the somatic cells are the individual buildings constructed from that blueprint. Modifications to a building do not alter the original master blueprint.
The Weismann Barrier
The Weismann Barrier, a direct consequence of the germ plasm theory, states that hereditary information flows in a singular direction: from the germ plasm to the somatic cells. Information cannot transfer from somatic cells back to the germ plasm, meaning that changes acquired by an organism during its lifetime cannot be transmitted to its offspring. This concept challenged the prevailing idea of Lamarckian inheritance, which suggested that acquired characteristics, such as developed muscles or the loss of a limb, could be inherited.
According to Weismann, any physical alterations an individual experiences are confined to the somatic cells. These changes have no mechanism to modify the genetic information in the germ cells, which are the sole carriers of heredity. This one-way flow of information meant that evolution must proceed through changes arising within the germ plasm itself, rather than through modifications of the body.
Experimental Refutation of Acquired Traits
Weismann provided evidence to support his theories through experiments. His most famous investigation involved systematically removing the tails of mice over multiple generations. He amputated the tails of mice and observed their offspring for five generations.
Despite the repeated mutilations, each new generation of mice was born with full-length tails, showing no reduction in tail size or any other abnormality. This result directly contradicted the idea of acquired characteristics being inherited and supported his concept of the Weismann Barrier, demonstrating that changes to the soma do not influence the germline.
Impact on Evolutionary Thought
Weismann’s work provided a foundational framework for heredity, which strengthened Charles Darwin’s theory of natural selection. Darwin’s theory explained the mechanism of evolution but lacked a robust explanation for how traits were inherited. By ruling out Lamarckian inheritance, Weismannism cleared the path for a more accurate understanding of genetic transmission.
His ideas were introduced before the rediscovery of Gregor Mendel’s work, yet they laid the groundwork for integrating genetics with evolutionary theory. Weismann’s view that random mutation in germ cells was the only source of variation for natural selection became a central tenet. This perspective set the stage for the Modern Synthesis in the early 20th century, a unification of Darwinian natural selection with Mendelian and population genetics, which remains the accepted framework for evolutionary biology today.