Bernhard Rensch (21 January 1900–4 April 1990) was a German evolutionary biologist, and ornithologist who did field work in Indonesia and India. He is probably best known as one of the architects of the modern evolutionary synthesis, which he popularised in Germany. His work on the evolution of geographically isolated populations, and on evolution above the species level, contributed to the modern synthesis. He also worked extensively on animal behavior (ethology).