Tempo and Mode in Evolution

Tempo and Mode in Evolution (1944) was George Gaylord Simpson's seminal contribution to the evolutionary synthesis, which integrated the facts of paleontology with those of genetics and natural selection.
Simpson argued that the microevolution of population genetics was sufficient in itself to explain the patterns of macroevolution observed by paleontology. Simpson also highlighted the distinction between tempo and mode. "Tempo" encompasses "evolutionary rates … their acceleration and deceleration, the conditions of exceptionally slow or rapid evolutions, and phenomena suggestive of inertia and momentum," while "mode" embraces "the study of the way, manner, or pattern of evolution, a study in which tempo is a basic factor, but which embraces considerably more than tempo."
Simpson's "Tempo and Mode" attempted to draw out several distinct generalizations:
"Tempo and Mode" earned Simpson the Daniel Giraud Elliot Medal from the National Academy of Sciences in 1944. Fifty years after its publication, the National Academy of Sciences commissioned a book entitled "Tempo and Mode in Evolution: Genetics and Paleontology 50 Years After Simpson" edited by Walter M. Fitch and Francisco J. Ayala. It includes contributions by Ayala, Stephen Jay Gould, and W. Ford Doolittle.