864 THE STRUCTURE OF EVOLUTIONARY THEORY
the punctuated equilibrium debate began, how would an evolutionary
paleontologist have treated the White River Fauna? Almost surely, any expert on
these strata would have selected the cases of apparent gradualism for study and
publication, while ignoring the others as negative instances of no evolution, worth
only a side comment at best, if noted at all, and suited for explicit mention (but
without any quantitative analysis) only in formal taxonomic treatises. Thus, readers
with no personal knowledge of the entire fauna—especially non-paleontological
readers unaware of strong signals for stasis and punctuation in virtually all
faunas—would almost surely assume that the three reported studies characterized
the usual situation for the history of fossil species, rather than representing the only
examples of a rare phenomenon.
Prothero and Heaton (1996, p. 258) raise the important point that the
examples of gradualism most widely featured, and most frequently cited to urge
the general case against punctuated equilibrium, derive from such faunas— where
they stand as unusual examples against an unstudied (or simply non-discussed) but
overwhelming prevalence for stasis and punctuation among all species. They
remind us, for example, that Gingerich's most famous half dozen or so cases from
lower Eocene beds of the northern Bighorn Basin "are just part of a fauna of over a
hundred genera. Detailed monographs by Bown (1979), Schankler (1980), and
Gingerich (1989) [in his very own taxonomic work] have shown that stasis is
prevalent among most of the taxa not featured by Gingerich (1976, 1980, 1987)"
(p. 258). Of another famous claim for gradualism (one that I do not challenge as a
single case, while asking that relative frequencies also be acknowledged), Prothero
and Heaton (1996, p. 258) write: "Krishtalka and Stucky (1985, 1986) reported a
gradualistic transformation in the early Eocene artiodactyl Diacodexis. However,
this is a single lineage from the same faunas described by Schankler, Gingerich,
and Bown, so these studies do not address the overall prevalence of gradualism vs.
stasis."
Finally, although this issue belongs more to the forthcoming discussion of
faunal stasis as an extension of punctuated equilibrium (see pp. 916-922), Prothero
and Heaton's (natural) experimental design in choosing the White River
Chronofauna for such intensive study included the existence, in the midst of the
fauna's duration, of one of the most profound and rapid climatic changes in
Tertiary North America—"the earliest Oligocene climatic crash" (p. 257) at 33.2
million years ago, where "vegetation changed from dense forests to open forested
grassland, mean annual temperatures dropped 13°C, and conditions got much drier
and more seasonal" (p. 257). The nondisturbance of stasis—indeed, the virtual
"ignorance" of this event by most species, at least by observable changes in
diversity or skeletal anatomy—also illustrates the strength of stasis and the
apparently active (rather than merely passive) sources of its maintenance. Prothero
and Heaton write (1996, p. 257): "Only a few mammalian lineages speciated, a few
more went extinct, and the vast majority (62 out of 70) persisted through this
climatic event with no observable response whatsoever." The authors then end
their paper by throwing down the gauntlet to supporters of traditional evolutionary
views