In 1930 Fisher published The Genetical Theory of Natural Selection, a book that did for evolutionary biology something like what his agricultural statistics did for experimental science: it gave a vague, qualitative idea a rigorous mathematical foundation. Darwin's natural selection and Gregor Mendel's particulate inheritance had seemed, to many biologists in the decades after Mendel's work was rediscovered in 1900, to sit in tension with one another. Fisher, working alongside J. B. S. Haldane and Sewall Wright in what became known as the modern evolutionary synthesis, showed that Mendelian genetics and Darwinian selection were not rivals at all but fit together mathematically, with selection acting on the discrete, gene-based variation that Mendel had described. His fundamental theorem of natural selection, relating a population's rate of fitness increase to the genetic variance in fitness present within it, remains a foundational, if still debated, result in population genetics.
Fisher's scientific legacy carries a shadow that later generations have had to reckon with directly. He was a committed and prominent eugenicist for his entire career, serving on the committee of the Cambridge University Eugenics Society as a student and remaining an active advocate for selective human breeding policies throughout his life, arguing as late as the 1950s against the emerging scientific consensus on race that group differences in ability had likely genetic contributions. His statistical innovations were developed partly in service of eugenic research questions, not as a separate, unrelated interest, which has complicated how institutions honor him, Gonville and Caius College, Cambridge, and University College London both removed his name and image from buildings and rooms in 2020, judging that his eugenics work was not incidental to his scientific reputation but bound up with it.