Punctuated Equilibrium and the Validation of Macroevolutionary Theory 827
32 species in stasis. Johnson's title for his excellent article also records this bias in
degrees of relative interest—for he sets the unmentioned but overwhelmingly
predominant theme of stasis in opposition to his label for the entire work: "The rate
of evolutionary change in European Jurassic scallops."
The most brilliantly persuasive, and most meticulously documented, example
ever presented for predominant (in this case, exclusive) punctuated equilibrium in
a full lineage—Cheetham's work on the bryozoan Metrarabdotos, more fully
treated on pp. 843-845 and 868- 870 —began as an attempt to illustrate apparent
gradualism. Cheetham wrote (in litt. to Ken McKinney, and quoted with
permission from both colleagues): "The chronocline I thought was represented ... is
perhaps the most conspicuous casualty of 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." Cheetham's
biometry led him to the opposite conclusion of exclusive stasis: "In nine
comparisons of ancestor-descendant species pairs, all show within-species rates of
morphological change that do not vary significantly from zero, hence accounting
for none of the across-species difference" (Cheetham, 1986, p. 190).
The establishment of stasis as an operational and quantifiable subject
behooves us to develop methods and standards of depiction and characterization.
Several studies have simply presented mean values for single characters in a
vertical succession, but such minimalism scarcely seems adequate. At the very
least, variances should be calculated (and included in published diagrams in the
form of error bars, histograms, etc.)—if only to permit statistical assessment of
significances for mean differences between levels, and for correlations of mean
values with time.
Smith and Paul (1985), for example, presented both ontogenetic regressions
and histograms for samples from each meter of sediment to illustrate stasis in
relative size of the peristome in Discoides subucula (Fig. 9-14). Cronin (1985) also
used both central tendency and variation to illustrate stasis throughout 200,000
years of intense climatic fluctuation (during Pleistocene ice cycles) for the
ostracode Furiana mesacostalis. Cronin (Fig. 9-15) encircled all specimens of the
species at three expanding levels of time in a multivariate plot of the first two
canonical axes (encompassing 92 percent of total information): (1) variation in a
single sample spanning 100 to 1000 years; (2) in one formation encompassing
20,000 to 50,000 years; and (3) across two formations, representing 100,000 to
200,000 years. Two features of this pattern provide insight into the anatomy of
stasis: first, relatively small increase in the full range of variation over such marked
extensions in lengths of time; second, the concentric nature of the enlarging
ellipses, indicating no preferred direction in added variation, but merely the regular
expansion anticipated in any random system with increasing sample size. As
Cronin notes, this lack of directionality seems all the more surprising when we
recognize that this lineage persisted in stasis through several ice-age cycles. Stasis
must