Lewin portrays the Darwinian and "Modern Synthetic" view of evolution as one in which evolution "moves at a stately pace, with small changes accumulating over periods of many millions of years yielding a long heritage of steadily advancing lineages. . . ." This is a serious, but unfortunately commonplace, distortion of Darwin and the Modern Synthesis.
Darwin claimed that adaptive changes induced by natural selection are gradual, but he clearly pointed out that this does not preclude sudden transitions appearing in the fossil record due both to the effect of time scale (for example, the classic example of gradual adaptive change- industrial melanism -would appear as a sudden transition if we observed the population only at intervals of 200 years, corresponding to an extraordinarily complete "fossil" record) and due to the fact that new adaptations initially evolve in a local population and then spread rapidly throughout the rest of the species so that "they appear as if suddenly created there, and will be simply classed as new species" ( 1 , p. 357).
Consequently, Darwin's meaning of the term "gradualism" was certainly compatible with the sudden appearance of forms in the fossil record-even for a very complete fossil record. Hence, the issue is not adaptive gradualism, but rather whether adaptive gradualism occurs continuously and slowly over long periods of geological time as the quote of Lewin given above implies. Darwin clearly pointed out that natural selection more often than not is a force preventing evolution and that only under relatively rare circumstances would it lead to episodes of adaptive change. He certainly did not embrace the view that adaptive changes are continuous over long periods of geological time. To illustrate this, juxtapose the following quotes from Darwin ( I , pp. 357, 373) with the quote from Lewin given above:
Many species when once formed never under go any further change but become extinct without leaving modified descendants; and the periods, during which species have undergone modification, though long as measured by years, have probably been short in comparison with the periods during which they retain the same form. . .
A number of species, however, keeping in a body might remain for a long period unchanged, whilst within the same period several of these species by migrating into new countries and coming into competition with foreign associates, might become modified; so that we must not overrate the accuracy of organic change as a measure of time.
Similarly, the Modern Synthesis is often treated as if it were a single, unified view of evolution, yet as is evident to anyone who has read and contrasted the works of Fisher, Haldane, and Wright (three of the principal contributors to the Modern Synthesis from the population genetics viewpoint), there never was a single evolutionary theory. Moreover, Fisher (2) discussed why many speciation events and morphological transitions follow a pattern of stasis punctuated with "sudden spurts of change"; Haldane (3) explicitly stated that the gradual, continuous changes in population genetics would occur "on a geological time scale, almost explosively"; and Wright (4), in his shifting balance theory of the 1920's and 1930's, explicitly stated that natural selection was an insufficient explanation for adaptive evolution and predicted that adaptive evolution would be characterized by periods of stasis interspersed with episodes of rapid adaptive transition. He also explicitly discussed the macroevolutionary implications of his theories, which are quite incompatible with the stereotyped "Modern Synthesis" presented by Lewin and others.
In summary, the macroevolution meeting at Chicago was not so much an historic challenge to evolutionary theory as it was a challenge to the history of evolutionary theory.
ALAN R. TEMPLETON
L. VAL GIDDINGS
Department of Biology, Washington
University, St. Louis, Missouri 63130
References
1. C. Darwin, The Origin of the Species (Modern Library-Random House, N e w York, 1936).
2. R. A. Fisher, The Genetical Theory of Natural Selection (Dover, N e w York, 1958). p. 153.
3. J. 9. S. Haldane, Am. Nut. 71, 337 (1937).
4. S. Wright, Proc. Am. Philos .Soc. 93, 471 (1949).