Theodosius Dobzhansky studied genetics and evolutionary biology, primarily using fruit flies, in the Soviet Union and the United States during the twentieth century. Scholars credit his widely read 1937 book Genetics and the Origin of Species with providing one of the earliest and most impactful integrations of Charles Darwin’s theory of evolution by natural selection with Gregor Mendel’s theory of genetics and therefore contributing significantly to the Modern Synthesis. In his research, Dobzhansky studied wild populations of organisms such as fruit flies and found that populations of the same species can differ in the frequency of one or more genetic traits. Understanding the role of such genetic diversity in evolution became a central theme of his research. Later in his career, he wrote about the social implications of his research, including the relationship between biological and cultural notions of race. Through both his scientific papers and popular writings, Dobzhansky shaped the course of evolutionary biology in the twentieth century and influenced public discussions of race in the years following World War II.