His research on divergence and speciation also lead him to study the difficulties of identifying species.
In 2004 Hey and Rasmus Nielsen produced the computer program IM,[1] which implements a method for fitting an isolation-with-migration model to a pair of closely related populations or species.
[3] Although a geneticist and evolutionary biologist, Hey has also published on philosophical and historical aspects of the species problem.
He has argued that the difficulties of defining species cannot be addressed without also appreciating people's motivations and tendencies with regard to categorization.
[4][5] Hey has also questioned the development of, and debate over, multiple species concepts that were inspired by Ernst Mayr.