His famous book, The Causes of Evolution (1932), was the first major work of what came to be known as the "modern evolutionary synthesis", reestablishing natural selection as the premier mechanism of evolution by explaining it in terms of the mathematical consequences of Mendelian genetics.
He was also a great science popularizer, and was perhaps the Stephen Jay Gould or Richard Dawkins of his day. His essay, Daedalus or Science and the Future (1923), was remarkable in predicting many scientific advances but has been criticized for presenting a too idealistic view of scientific progress.
Haldane was himself a very idealistic man, and in his youth was a devoted Communist and author of many articles in The Daily Worker. Events in the Soviet Union, such as the rise of the anti-Mendelian agronomist Trofim Lysenko and the crimes of Stalin, caused him to break with the Communist Party later in life.
He is also known for an observation from his essay, On Being the Right Size, which Jane Jacobs and others have since referred to as Haldane's principle. This is that sheer size very often defines what bodily equipment an animal must have: "Insects, being so small, do not have oxygen-carrying bloodstreams. What little oxygen their cells require can be absorbed by simple diffusion of air through their bodies. But being larger means an animal must take on complicated oxygen pumping and distributing systems to reach all the cells." The conceptual metaphor to animal body complexity has been of use in energy economics and secession ideas.
Haldane was friends with the author Aldous Huxley, and was the basis for the biologist Shearwater in Huxley's novel Antic Hay. Ideas from Haldane's Daedalus, such as ectogenesis (the development of fetuses in artificial wombs), also influenced Huxley's Brave New World.
He had many students, the most famous of whom, John Maynard Smith, is perhaps also the one most like himself.
Haldane was a very quotable man. Some of the things he said are probably more famous than he is: