Perspectives in Biology and Medicine

HUMAN EVOLUTION: MANY SMALL STEPS, BUT NOT PUNCTUATED EQUIUBRIA JAMES V. NEEL* I. The Issue The issue of the tempo of evolution has been recurrent since the publication ofDarwin's Origin ofSpecies in 1859. With the emergence ofa mathematical framework for the genetics of populations, geneticists (and some paleontologists) have in general, following Darwin, favored what it is now fashionable to term "phyletic gradualism": evolution through gradual shifts in gene frequencies which in time transform one species into another, a thesis in the development ofwhich the writings of Dobzhansky [1] and Simpson [2, 3] have played a major role. That such transformations might be facilitated by the isolation of a subset of the species and/or genetic drift, especially at the periphery ofa species distribution , has been an integral component of phyletic gradualism (see discussion in [4, 5]). Poulton in 1903 [6] first applied the term "allopatric speciation" to these peripheral events, but the real development of the concept dates from Mayr [7]. "Allopatric speciation" (as contrasted with "sympatric speciation") is not in current genetic thought a type ofspeciation set apart in its genetic mechanisms from phyletic gradualism, but the extreme of a continuum of conditions for genetic change, a manifestation of a situation in which genetic change can occur with unusual rapidity (cf. [4,8-11]). The most detailed efforts to provide a precise theoretical underpinning for both allopatric and sympatric evolution are those of Wright [12-16]. Recognizing the subdivision which is so characteristic ofnatural populations, Wright proposed a "shifting-balance" theory of micro- and Expanded from a presentation at a Symposium on Genetic Epidemiology sponsored by the Genetics Society of America, June 16, 1983, at Saint Louis, Missouri. The original investigations described in this paper have been funded in part by the U.S. Department of Energy and the National Science Foundation. ?Department of Human Genetics, University of Michigan Medical School, Ann Arbor, Michigan 48109.© 1984 by The University of Chicago. AU rights reserved. 003 1-5982/85/2801-0419$01.00 Perspectives in Biology and Medicine, 28, 1 ¦ Autumn 1984 \ 75 macroevolution. Evolution was perceived as operating in three phases: "first, extensive local differentiation, with wide stochastic variability in each locality; second, occasional crossing of a saddle leading to a higher selective peak under mass selection; and third, excess proliferation of, and dispersion from, those local populations in which a peak-shift has occurred, leading to occupation of the superior selective peak by the species as a whole. The process is not alternative to mass selection but suppUmentary , since such selection is involved in all threephases" ([15], p. 8; italics mine). From the publication of The Origin, phyletic gradualism has had its dissenters, especially among paleontologists and biogeographers impressed by gaps in the fossil record and species discontinuities—notable among them in this century Osborn [17], Willis [18], and Schindewolf [19]. Nor should it be forgotten that the full tide of the 1894 publication for which Bateson is so well remembered was "Materials for the Study of Variation, Treated with Especial Regard to Discontinuity in the Origin of Species" [20]. On page 15 appears the statement: "The preliminary question, then, of the degree of continuity with which the process of Evolution occurs, has never been decided." Darwin, too, was troubled by apparent gaps in the fossil record, as often pointed out by advocates of "punctuated equilibrium," and in the fourth edition of TL· Origin he wrote that he does not suppose "that it [evolution] goes on continuously; it is far more probable that each form remains for long periods unaltered , and then again undergoes modification" (quoted from A. Huxley [21]). In more recent years, this dissent has been eloquently promulgated by, among others, Eldredge and Gould [22], Gould and Eldredge [23], Stanley [24], and Eldredge and Tattersall [25]. This alternative view reaffirming the validity of major discontinuities in the fossil record maintains that in addition to gradual processes, we must recognize a very significant role for sudden transitions, these followed by relatively long periods of evolutionary stasis, such spurts in an otherwise very leisurely time scale for change leading to the term "punctuated equilibria." A careful reading of these authors leaves the impression that the transitions...