Frederick Cohan

In graduate school I became interested in Ernst Mayr’s ideas on species and speciation, particularly his idea that a species is a cohesive body, whose divergence is opposed by various forces of evolution, and that the splitting of species marks the origin of lineages that no longer cohere. Species were thought to be cohesive mostly as a result of recurrent genetic exchange among them, but I was interested in finding whether cohesion might also operate through natural selection, even among isolated populations. Using Drosophila fruit flies, I tested whether populations from different parts of a species’ range might be channelled to respond in the same way to the same environmental challenge (Mayr’s prediction) or whether even geographically close populations might already be predisposed to diverge under uniform natural selection (my prediction, which proved to be correct). My experiments made me wonder whether there actually are important limits to cohesion across a large species.