Punctuated equilibrium comes of age

Stephen Jay Gould & Niles Eldredge
The intense controversies that surrounded the youth of punctuated equilibrium have helped it mature to a useful extension of evolutionary theory. As a complement to phyletic gradualism, its most important implications remain the recognition of stasis as a meaningful and predominant pattern within the history of species, and in the recasting of macroevolution as the differential success of certain species (and their descendants) within clades.

PUNCTUATED equilibrium has finally obtained an unambiguous and incontrovertible majority--that is, our theory is now 21 years old 1. We also, with parental pride (and, therefore, potential bias), believe that primary controversy has coded to general comprehension, and that punctuated equilibrium has been accepted by most of our colleagues (a more conventional form of majority) as a valuable addition to evolutionary theory. Kellogg 1 began the best book written to celebrate Darwinism's fiftieth birthday by noting how often (and how continually) various critics had proclaimed the Sterbelager (death bed) of natural selection. Punctuated equilibrium 2 has also prospered from announcements of its death or triviality 3 5 and has been featured in much recent discussions 6. 8.

As a neonate in 1972, punctuated equilibrium entered the world in unusual guise. We claimed no new discovery, but only a novel interpretation for the oldest and most robust of palaeontological observations: the geologically instantaneous origination and subsequent stability (often for millions of years) of palaeontological 'morphospecies'. This observation had long been ascribed, by Darwin and others, to the notorious imperfection of the fossil record, and was therefore read in a negative light--as missing information about evolution (defined in standard palaeontological textbooks of the time 9 as continuous anagenetic transformation or populations, or phyletic gradualism).

In a strictly logical sense, this negative explanation worked and preserved gradualism, then falsely equated with evolution itself, amidst an astonishing lack of evidence for this putative main signal of Darwinism. But think of the practical or heuristic dilemma for working paleontologists: if evolution meant gradualism, and imperfection precluded the observation of such steady change, then scientists could not access the very phenomenon that both motivated their interest and built life's history. As young, committed and ambitious parents, we therefore proposed punctuated equilibrium, hoping to validate our profession's primary data as signal rather than void. We realized that a standard biological account. Mayr's 10 peripatric theory or speciation in small populations peripherally isolated from a parental stock, would yield stasis and punctuation when properly scaled into the vastness of geological time--for small populations speciating away from a central mass in tens or hundreds of thousands of years, will translate in almost every geological circumstance as a punctuation on a bedding plane, not gradual change up a hill of sediment, whereas stasis should characterize the long and recoverable history of successful central populations.

Punctuated equilibrium then grew during its childhood and adolescence, in ways both unruly and orderly. Unruly accidents of history included the misunderstandings of colleagues (who, for example, failed to grasp the key claim about geological scaling, misread geological abruptness as true suddenness, and then interpreted punctuated equilibrium as a saltational theory), and the purposeful misuses of creationist foes as this political issue heated up in the United States during the late 1970s (although we took pride in joining with so many colleagues for a successful fight against this philistine scourge, as one of us testified in the Arkansas 'monkey' trial in 1981 11 and the other wrote a book on creationist distortions 12).

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But orderly extensions, implicit in the undeveloped logic of our original argument, fuelled the useful growth of punctuated equilibrium to fruitful adulthood. (We now realize how poorly we initially grasped the implications of our original argument; we thank our colleagues, especially S. M. Stanley 13 and E. S. Vrba 14, for developing several extensions). We originally focused on tempo, but more important theoretical arguments flowed from implications concerning evolution's mode 15-17--particularly the causes surrounding our two major claims for equilibrium, or stasis of established species, and the need to reformulate macroevolution, notably the key phenomenon of trends, as an accumulation of discrete speciation events treated as entities rather than indefinable segments of continua--a subject encompassed by debate about species selection 13 or species sorting 18.

Punctuated equilibrium and macroevolution

Stasis and its meaning. We opened our original paper with a section on what philosopher N. R. Hanson called "the cloven hoofprint of theory" 19, or the structuring of all supposedly objective observation by expectations of prevailing general views. Stasis, as palpable and observable in virtually all cases (whereas rapid punctuations are usually, but not always, elusive), becomes the major empirical ground for studying punctuated equilibrium. Putting together the philosophical insight of ineluctable theoretical bias, with the empirical theme of the tractability of stasis, we devised a motto: "stasis is data." For no bias can be more constricting than invisibility--and stasis, inevitably read as absence of evolution, had always been treated as a non-subject. How odd, though, to define the most common of all palaeontological phenomena as beyond interest or notice! Yet paleontologists never wrote papers on the absence of change in lineages before punctuated equilibrium granted the subject some theoretical space. And, even worse, as paleontologists didn't discuss stasis, most evolutionary biologists assumed continual change as a norm, and didn't even know that stability dominates the fossil record. Mayr has written 20: "Of all the claims made in the punctuationalist theory of Eldredge and Gould, the one that encountered the greatest opposition was that of 'pronounced stasis as the usual fate of most species', after having completed the phase of origination... I agree with Could that the frequency of stasis in fossil species revealed by the recent analysis was unexpected by most evolutionary biologists."

As the most important change in research practice provoked by punctuated equilibrium, stasis has now exited from its closet of non-definition to become a subject of quantitative investigation in all major fossil groups--from microfossils 21,22 (27,000 measured specimens from 400 closely spaced samples spanning 8 million years in the latter study), to molluscs 23-27, to mammals 28 30. Although punctuated equilibrium deals directly only with stability of species through time, the higher-level analogue of non-trending in larger clades has also graduated from an undefined non-subject to a phenomenon worth documenting 31. Moreover, because species often maintain stability through such intense climatic change as glacial cycling 32, stasis must be viewed as an active phenomenon, not a passive response to unaltered environments. Many leading evolutionary

NATURE - VOL 366 - 18 NOVEMBER 1993