918 THE STRUCTURE OF EVOLUTIONARY THEORY
The intense discussion of punctuational patterns at the level of species
aggregations or ecosystems (extensive stability of species composition in regional
faunas, followed by geologically rapid overturns and replacements of large
percentages of these species) has centered upon two theoretical schemes and their
proposed exemplars in the fossil record (see extensive symposium of 18 articles,
edited by Ivany and Schopf, 1996, and entitled "New perspectives on faunal
stability in the fossil record"). Working with the famous and maximally
documented Hamilton faunas (Devonian) of New York State, and then extending
their work up and down the stratigraphic record for a 70 million year interval of
Paleozoic time in the Appalachian Basin, Brett and Baird (1995) documented 13
successive faunas, each including 50 to 335 invertebrate species, and each showing
considerable stability both for the history of any species throughout its range (the
predicted stasis of punctuated equilibrium, see Lieberman, Brett and Eldredge,
1995, for quantitative evidence), and, more importantly in this context, in the
virtually constant composition of species throughout the fauna's range.
Each fauna persists for 5 to 7 million years until replaced, with geological
rapidity, by another strikingly different fauna including only 20 percent or fewer
carryovers from the preceding unit. As a defining attribute, 70 to 85 percent of
species in the fauna persist from the earliest strata to the very end, remaining in
apparently stable ecological associations (with characteristic numerical
dominances of taxa) to forge a pattern that Brett and Baird call "coordinated
stasis." Eldredge (1999, p. 159) writes of coordinated stasis: "It is a true, repeated
pattern, the most compelling and at the same time underappreciated pattern in the
annals of biological evolutionary history."
Vrba (1985) found a similar pattern in the maximally different ecosystem of
vertebrates in Pliocene terrestrial environments of southern and eastern Africa. The
geologically rapid faunal replacement, following an extensive period of previous
stability, occurred in conjunction with a 10-15 degree drop in global temperatures
that lasted some 200,000 to 300,000 years, and began about 2.7 to 2.8 million years
ago. In generalizing this pattern as the "turnover-pulse hypothesis," Vrba
emphasizes the role of environmental disruption in prompting the transition and,
especially, the coordinated effects of both extinction and speciation as
consequences of disruption—extinction by rapid change and removal of habitats
favored by species of the foregoing fauna, and origination by fragmentation of
habitats and resulting opportunities for speciation by geographic isolation of
allopatric populations. As an example of the role granted to increased propensities
for speciation, as promoted by the same environmental events that decimated the
previous faunas, Vrba links the origin and initially rapid speciation (at least three
taxa) of the genus Homo to this Pliocene turnover-pulse, a proposition that has
generated substantial interest and debate.
The two propositions—Brett and Baird's coordinated stasis, and Vrba's
turnover-pulse—identify similar patterns in prolonged stasis and punctuational
replacement for linked groups of species. However, the two formulations