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Dr. Orr is an evolutionary geneticist with several broad interests. Most of his work focuses on speciation, adaptation, and extinction. He studies these topics with both theory and experiment. In his theory work, he is primarily interested in adaptation in phenotypic models (e.g., Fisher’s geometric model) and in molecular models (e.g., molecular evolution over rugged versus smooth fitness landscapes). He also studies population genetic models of evolutionary rescue, in which a threatened population adapts to a changed environment sufficiently rapidly to avoid extinction. In his experimental work, Dr. Orr is primarily interested in the genetic basis of speciation. This work involves genetic analysis of various forms of reproductive isolation, e.g., prezygotic isolation, between taxa. This work takes advantage of the many genetic tools available in the genus Drosophila. Dr. Orr is also a frequent essayist and book reviewer and has written for The New York Review of Books and The New Yorker, among other publications.

The strength of prezygotic (sexual) isolation plotted against genetic distance (D) among allopatric vs. sympatric species of Drosophila.