John Thomas Gulick was an American missionary and naturalist from Hawaii. He was one of the pioneers of modern evolutionary thinking based on his studies of Hawaiian snails of the genus Achatinella. He was among the first to describe the formation of species through geographic separation of breeding populations. He developed early ideas on the founder effect and what is now known as the Baldwin effect. He coined the term "divergent evolution".

Divergent evolution or divergent selection is the accumulation of differences between closely related populations within a species, sometimes leading to speciation. Divergent evolution is typically exhibited when two populations become separated by a geographic barrier and experience different selective pressures that cause adaptations. After many generations and continual evolution, the populations become less able to interbreed with one another. The American naturalist J. T. Gulick (1832–1923) was the first to use the term "divergent evolution", with its use becoming widespread in modern evolutionary literature. Examples of divergence in nature are the adaptive radiation of the finches of the Galápagos, changes in mobbing behavior of the kittiwake, and the evolution of the modern-day dog from the wolf.