Charles Darwin's theory of evolution generated scientific debate and discussion not only in Darwin's own time, but for decades afterward. In the latter part of the nineteenth century and the until the 1930s and '40s, scientists were puzzled and very divided over what drove evolution. Darwin's engine, natural selection, had few supporters: Many people found it distasteful because it seemed to be ruthless and wasteful of life and had no positive, uplifting goal.
Some of the critics argued, with Jean Baptiste Lamarck, that as creatures used certain body parts in response to a changing environment, they underwent changes that were inherited. Others had "orthogenesis" views, which held that species evolved in a predetermined direction toward a fixed goal but offered no explanation of how this ever emerged. Still others proposed that spontaneous, random mutations, or changes, in organisms accounted for the variety of species. Natural selection was not needed in this explanation.
It wasn't until the 1930s that a number of scientists in different fields began to reconcile the findings of genetics and inheritance with Darwin's theory and its emphasis on natural selection. The resolution was called the "evolutionary synthesis" or "modern synthesis," and one of its architects was Russian population geneticist Theodosius Dobzhansky, who lived from 1900 to 1975. The key revelation was that mutation, by creating genetic diversity, supplied the raw material for natural selection to act on. Instead of mutation and natural selection being alternative explanations, they were joined in this new synthesis.
Dobzhansky moved to the United States in 1927, and, through his lifelong experiments with fruit flies, showed that natural populations of the flies exhibited the same kinds of genetic variations that could be produced artificially by mutation in the laboratory. Moreover, he showed that natural populations included enough heritable variation for natural selection to work. Dobzhansky thus bridged the gap between laboratory experiments and field observation, and his 1937 book Genetics and the Origin of Species was one of the cornerstones of the modern synthesis.
Dobzhansky is remembered today not only for his strictly scientific achievements, but also for his deep concern about the possible misunderstanding and misuse by society of the concepts of genetic variation.