Punctuated Equilibrium and the Validation of Macroevolutionary Theory 865
about response to environmental change: "Evolutionary theory has come a long
way since Eldredge and Gould (1972) first pointed out that stasis is the norm in the
fossil record, and the data cannot be simply dismissed or explained away ... In fact,
stasis and resistance to change is so ingrained that species can actually pass
through the most significant climatic change of the last 65 million years as if
nothing happened."
In recent years, studies of stasis and punctuation in entire faunas have
blossomed, especially with the introduction and testing of two partly
complementary, but partly dissonant, explanations for apparently concerted
stability of entire faunas over substantial intervals—the turnover-pulse hypothesis
of Vrba (1985), and the theory of coordinated stasis, developed by Brett and Baird
(1995 and several other works), and extensively (often contentiously) treated in the
symposium of Ivany and Schopf, 1996. I shall treat the theory itself in the last
section of this chapter (pages 916-922), but will record here the convincing
documentation of extensive faunal stasis that established the evidentiary base for
these ideas.
The famous Middle Devonian (Givetian) Hamilton fauna of the Appalachian
Basin has provided a "type" case for coordinated stasis. The Hamilton fauna
includes more than 330 species of mostly typical Paleozoic invertebrates, ranging
through about 9 million years of strata in a series of about twenty identifiable
"communities" or biofacies. About 80 percent of these species persist throughout
the entire Hamilton, while fewer than 20 percent carry over from the fauna just
below. Ever since Cleland's original and wistful comment in 1903 (quoted on p.
750), students of the Hamilton fauna have recognized the overwhelming signal of
stasis presented by almost every species of the assemblage—although this original
wistfulness has now ceded to considerable positive interest! Brett and Baird write
(1995, p. 301): "Individual lineages within particular biofacies of the Hamilton
biotas appear to display very little morphological change, and that which is
observed is neither progressive nor directional."
Several taxa of this fauna have now been analyzed in great morphometric
detail, beginning with Eldredge's classic study of the trilobite Phacops rana, one of
the "founding" examples of punctuated equilibrium (see Eldredge, 1971; Eldredge
and Gould, 1972). Eldredge found stasis in more than 50 characters, and
directional, but punctuational, change only in one—reduction in rows of eye facets
in two punctuational steps during a 5-6 million year period otherwise marked by
stasis for this feature as well. Other quantitative studies of stasis in Hamilton
species include Pandolfi and Burke (1989) for tabulate corals, Lieberman (1995)
on trilobites, and Lieberman, Brett, and Eldredge (1994) on brachiopods. The last
study considered 8 characters in two species using principal components and
canonical discriminant analysis. The authors found fluctuating variation, correlated
neither with age nor facies, throughout the interval. However, and ironically given
past expectations of gradualism, the uppermost samples plotted closer to the lower-
most than to any intervening population. For the entire fauna, Brett and Baird
conclude (1995, p. 303): "Taken together, these studies indicate that a