Punctuated Equilibrium and the Validation of Macroevolutionary Theory 861
accumulative directional effect. For example, Figure 9-25 (from Stanley and Yang,
p. 132) shows the temporal distribution of mean values for each of the 24
characters over 17 million years in the venerid bivalve Macrocallista maculata.
For most characters, the full temporal range lies within the variational scope of
living populations (noted by the "forks" for separate geographic samples at the top
of the trajectory). They conclude (p. 113): "We calculated net rates of evolution
separating pairs of populations that belong to single lineages. For all intervals of
time, the distribution of differences between population means for individual
variables is remarkably similar to a comparable distribution representing the
comparison of pairs of conspecific Recent populations from separate geographic
regions... Evolution has followed a weak zigzag course, yielding only trivial net
trends."
A particularly impressive study by Prothero and Heaton (1996) documents the
overwhelming dominance of punctuated equilibrium in a full tabulation of one of
the most prominent fossil faunas—a study that also gives us good insight into how
biased reporting in general, and Cordelia's dilemma in particular (p. 763), can so
strongly skew tabulated results to appearances of equal frequency or only mild
domination by punctuated equilibrium. These authors studied one of the world's
richest and best known mammalian sequences—the upper Eocene and Oligocene
White River Group of the American High Plains, particularly as exposed in the Big
Badlands of South Dakota—"one of the densest and most complete records of
mammalian evolution anywhere in the world... The spectacularly stark and
beautiful outcrops ... have been a Mecca for fossil collectors ever since the first
fossils were described in 1846... Enormous collections have accumulated, and
White River fossils are found in nearly every rock shop and mineral show across
the country" (p. 259). This large mammalian assemblage seems to possess
sufficient long-term coherence (from Duchesnean strata of the late middle Eocene
into Arikareean strata of late Oligocene times) for designation as the White River
Chronofauna (Emry, 1981).
The authors spent more than a decade conducting "an unbiased survey of
9 - 25. From Stanley and Yang, 1987. A history of change during 17 million years for each of 24
measured characters in the bivalve Macrocallista maculata. For the great majority of characters,
the entire temporal spread lies within the scope of variation in the geographic range of living
populations (represented by the "forks" for separate samples at the top of the trajectory). This
form of comparison provides an excellent documentation of stasis by the criterion of scaling to
the full range of geographic variation at a single time within the same taxon.