Robert Stebbins publishes a monograph describing the ring speciation of Ensatina salamanders in California. The different forms are capable of interbreeding in most neighboring locations, creating hybrids, but not everywhere, with the intersecting Sierra foothill and Coast Range populations unable to interbreed and thus acting as distinct species. This has become a widely used textbook example of evolutionary processes.

Ensatina eschscholtzii eschscholtzii at Tilden. Credit: Natasha Stepanova.