Gould & Eldredge, Punctuated equilibrium comes of age


rates would fare just as well as groups with equally lower rates. But extinction intensities vary greatly, and the geological record features episodes of high dying. during which extinction-prone groups are more likely to disappear, leaving extinction-resistant groups as life's legacy. Valentine concludes that "these clade-characteristic rates are of course not adaptations per se, but effects flowing from clade properties that were established probably during the early radiations that founded the clades."

The most exciting direct extensions of punctuated equilibrium now involve the study of correlated punctuational events across taxa, and the ecological and environmental sources of such cohesion. Eminently testable are Vrba's49 "turnover-pulse hypothesis" of evolution concentrated in punctuational bursts at times of worldwide climatic pulsing, one of which, about 2.5 million years ago. may have stimulated the origin of the genus Homo; and Brett's50 hypothesis of "coordinated stasis" for the structuring of major palaeontological faunas. What might be the ecological source of such striking coherence across disparate taxa through such long times?51

The empirics of punctuated equilibrium
Like all major theories in the sciences of natural history, including natural selection itself, punctuated equilibrium is a claim about relative frequency, not exclusivity52. Phyletic gradualism has been well documented, again across all taxa from microfossils53 to maramals54,55. Punctuated equilibrium surely exists in abundance, but validation of the general hypothesis requires a relative frequency sufficiently high to impart the predominant motif and signal to life's history. The issue remains unsettled, but we consider (in our biased way) that four classes or evidence establish a strong putative case for punctuated equilibrium in this general sense.

Individual cases. Examples of stasis alone (cited earlier) and simple abrupt
replacement, although conforming to expectations of punctuated equilibrium, are not direct evidence for our mechanism: for stasis might just be a lull in anagenetic gradualism (though pervasive stasis for long periods in all species of a fauna (a common finding) would require special pleading from gradualists), and replacement might represent rapid transformation without branching, or migration of a distant (phyletic or geographic) relative rather than evolution in situ. A good test of punctuated equilibrium requires (in addition to the obvious need for documented rapidity in an interval known to be sufficiently short) both a phyletic hypothesis to assert sister-group relationship of the taxa involved, and survival of putative ancestors to affirm an event of true branching rather than rapid phyletic transformation.

Given these stringent requirements, and in the light of such an imperfect fossil record, we are delighted that so many cases have been well documented, particularly in the crucial requirement of ancestral survival after punctuated branching32.56-61. Williamson's discovery62 of multiple molluscan speciation events in isolated African lakes, with return of ancestral lineages upon reconnection with parental water bodies, has been most widely discussed63 and disputed64 (although all accept the punctuational pattern). Cheetham's elegant and meticulously documented65.66 story of evolution in the bryozoan Metrarabdotos from Tertiary strata of the Caribbean is particularly gratifying (Fig. 2) in the number of purely punctuational events, the full coverage of the lineage, and the unusual completeness of documentation, especially as Cheetham began his study expecting to reconfirm a gradualistic interpretation (writing to McKinney: "The chronoclinc I thought was represented . . . is perhaps the most conspicuous casualty or the restudy, which shows that the supposed cline members largely overlap each other in time. Eldredge and Gould were certainly right about the danger of stringing a series of chronologically isolated populations together with a gradualist's expectations.")

On the subject of punctuational corrections for received gradualistic wisdom. Prothero and Shubin 67 have shown that the most 'firmly' gradualistic part of the horse lineage (the general, and false, exemplar of gradualism in its totality), the Oligocene

FIG. 2 Reproduced with permission from ref. 65. Phylogenetic (stratophenetic) tree of Caribbean species of Metrarabdotos. Relationships were constructed by calculating euclidean distances between sampled populations using all canonical scores from discriminant analysis, and connecting nearest morphological neighbours in stratigraphic sequence. Morphological distances (horizontal axis) between inferred ancestor and descendant species are to scale; those between species of different pairs are not necessarily so (for example, M. kugleri is morphologically more distant from M. n. sp. 3 than from its ancestor, M. n. sp. 2, even though the diagram makes it appear otherwise). Within-species fluctuation in species-distinguishing morphology is based on pairwise discriminant scores scaled to the distances between species pairs.

NATURE - VOL 366 - 18 NOVEMBER 1993