Tempo and mode in evolution.

Abstract

George Gaylord Simpson said in his classic Tempo and Mode in Evolution (1) that paleontologists enjoy special advantages over geneticists on two topics. One general topic, suggested by word tempo, has to do with evolutionary rates. . ., their acceleration and deceleration, conditions of exceptionally slow or rapid evolutions, and phenomena suggestive of inertia and momentum. A group of related problems, implied by word mode, involves the study of way, manner, or pattern of evolution, a study in which is a basic factor, but which embraces considerably more than tempo (pp. xvii-xviii). Simpson's book was self-consciously written in wake of Theodosius Dobzhansky's Genetics and Origin of Species (2). The title of Dobzhansky's book suggested its theme: role of genetics in explaining the origin of species-i.e., a synthesis of Darwin's theory of evolution by natural selection and maturing science of genetics. In introduction to his book, Simpson averred that an essential part of his study was an attempted synthesis of paleontology and genetics, an effort that pervaded whole book, but was particularly subject of first two chapters, which accounted for nearly half book's pages. Darwin believed that change occurs by natural selection of small individual differences appearing every generation within any species. Singly changes effected by selection are small but, given enough time, great changes can take place. Two of Darwin's most dedicated supporters, Thomas Huxley and Francis Galton, argued instead that evolution occurs by selection of discontinuous variations, or sports; evolution proceeds rapidly by discrete leaps. According to Huxley, if natural selection operates only upon gradual differences among individuals, gaps between existing species and in paleontological record could not be explained. For Galton, evolution was not a smooth and uniform process, but proceeded by jerks, some of which imply considerable organic change. This controversy was continued in latter part of nineteenth century by biometricians Karl Pearson and W. F. R. Weldon, who believed, like Darwin, in primary importance of common individual differences, and by geneticist William Bateson, who maintained primary importance of discontinuous variations. The rediscovery of Mendelian inheritance in 1900 provided what might have been common grounds to resolve conflict. Instead, dispute between biometricians and geneticists extended to continental Europe and to United States. Bateson was champion of Mendelians, many of whom accepted mutation theory proposed by De Vries, and denied that natural selection played a major role in evolution. The biometricians for their part argued that Mendelian characters were sports of little importance to process.