John H. Gillespie is an evolutionary biologist interested in theoretical population genetics and molecular evolution.
In molecular evolution, he emphasized the importance of advantageous mutations and balancing selection.
He is widely considered the main proponent of natural selection in molecular evolution.
Gillespie had only four Ph.D. students during his career: Richard Hudson, James N. McNair, David J. Cutler, and Andrew Kern, but mentored many more.
Gillespie was a professor in the College of Biological Sciences at the University of California, Davis until his retirement in 2005.