Tempo and Mode in Evolution WALTER M
Proc. Nati. Acad. Sci. USA Vol. 91, pp. 6717-6720, July 1994 Colloquium Paper This paper was presented at a colloquium entiled "Tempo and Mode in Evoluton" organized by Walter M. Fitch and Francisco J. Ayala, held Januwy 27-29, 1994, by the National Academy of Sciences, in Irvine, CA. Tempo and mode in evolution WALTER M. FITCH AND FRANCISCO J. AYALA Department of Ecology and Evolutionary Biology, University of California, Irvine, CA 92717 George Gaylord Simpson said in his classic Tempo andMode The conflict between Mendelians and biometricians was in Evolution (1) that paleontologists enjoy special advantages resolved between 1918 and 1931 by the work ofR. A. Fisher over geneticists on two evolutionary topics. One general and J. B. S. Haldane in England, Sewall Wright in the United topic, suggested by the word "tempo," has to do with States, and S. S. Chetverikov in Russia. Independently of "evolutionary rates. ., their acceleration and deceleration, each other, these authors proposed theoretical models of the conditions ofexceptionally slow or rapid evolutions, and evolutionary processes which integrate Mendelian inheri- phenomena suggestive of inertia and momentum." A group tance, natural selection, and biometrical knowledge. of related problems, implied by the word "mode," involves The work of these authors, however, had a limited impact "the study of the way, manner, or pattern of evolution, a on the biology of the time, because it was formulated in study in which tempo is a basic factor, but which embraces difficult mathematical language and because it was largely considerably more than tempo" (pp. xvii-xviii). theoretical with little empirical support.
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