How the Theory of Heredity Helped the Darwinian Revolution
In the decade following the publication of Origin of Species, the theory of evolution by natural selection became the primary working theory of biologists. By the turn of the century, however, natural selection had lost many of its first supporters. While Darwin was still held in high esteem by scientists, practicing researchers had begun to explore other theories of evolution that downplayed the importance of Darwin's mechanism of selection. The Darwinian Revolution, it seemed, had lost some of its steam.
Why had Darwinism lost so much of its initial strength? As well-argued as the theory of natural selection was, during the nineteenth century it lacked a model of heredity, a model of how characteristics get passed from generation to generation (from parents to offspring). In order for the theory of evolution via natural selection to work, organisms had to pass their traits on with some regularity. Otherwise, there would be no guarantee that those organisms with traits that promote survival would actually pass those traits on to the next generation. How did traits get passed from parents to offspring?
Scientists of the late nineteenth century recognized from general observations that some traits might be passed from generation to generation, but they did not possess a mature knowledge of genetics, or science of heredity, that could explain how and why this happened. Darwin recognized the difficulty that heredity posed to his theory, and after finishing Origin of Species he spent several years experimenting on heredity using the plants and animals around his estate. In 1868, he finished The Variation of Animals and Plants under Domestication (courtesy of Robert J. Robbins, Electronic Scholarly Publishing), a treatise intended largely to explain how heredity worked.
In Variation of Animals and Plants, Darwin proposed the hypothesis of pangenesis. According to this hypothesis, cells in an organism's body released gemmules when a particular action was performed. For example, the cells in an athlete's muscle tissue would release gemmules for increased muscle growth when the athlete lifted weights. These gemmules would collect in the organism's reproductive organs, and the parent organism could pass these gemmules on to its offspring. These gemmules would shape the next generation; the athlete's children might be stronger because of her regular exercise at the gym (Provine 9-10).
As you might notice, Darwin's hypothesis of pangenesis looks an awful lot like J.B. Lamarck's inheritance of acquired characteristics. Some scientists accepted Darwin's new model for heredity, but most did not. As a result, during the period from around 1870 to 1920, most biologists used some theory of evolution to guide their research, but no single theory of evolution united the discipline as a whole. Darwin had changed the field of biology, but there was a great deal of debate about how much of his theory should be accepted.
Genetics remained a vague and uncertain discipline until the work of an Austrian monk named Gregor Mendel was rediscovered in 1900, more than thirty years after it had first been published. In this lesson, we are going to explore Mendelian genetics, the theory of heredity that has become one of the foundations of modern biology. As you encounter Mendel's ideas, think about how his model of heredity might be brought together with Darwin's mechanism of selection to create a more robust theory of evolution.
