A Reflection on 50 Years of John Maynard Smith's "Protein Space".

In 1970, John Maynard Smith published a letter, entitled "Natural Selection and the Concept of a Protein Space," that proposed a simple analogy for the incremental process of adaptive evolution. His "Protein Space" analogy contains the substrate for many central ideas in evolutionary genetics, and has motivated important discoveries within several subdisciplines of evolutionary science. In this Perspectives article, I commemorate the 50th anniversary of this seminal work by discussing its unique legacy and by describing its intriguing historical context. I propose that the Protein Space analogy is not only important because of its scientific richness, but also because of what it can teach us about the art of constructing useful and subversive analogies.