Punctuated Equilibrium and the Validation of Macroevolutionary Theory 917
times as well, keeping species together in "organic" ways that falsify the null
hypothesis of independent status.
As I write this chapter at the end of the 1990's, the implication of punctuated
equilibrium for higher-level theories about the pulsing and clumping of species in
putatively stable communities through considerable stretches of geological time
has become the most controversial and widely discussed "outreach" from the basic
theory that Eldredge and I formulated in 1972. Whatever my own opinions on the
major alternatives now under debate— and I confess to a somewhat cautious, if not
downright conservative, stance retaining a maximal role for the null hypothesis of
species as independent Darwinian individuals making their own way through
geological time—I take pride in the role that punctuated equilibrium has played in
building an intellectual context for making such a debate possible in the first place.
One can't even pose meaningful questions about higher-level aggregations of
species unless species themselves can be construed as stable and effective
ecological or evolutionary agents—a status best conferred by punctuated
equilibrium's recognition of species as true Darwinian individuals (see pp. 604-
608). Under Darwin's personal view of species as largely arbitrary names for
transitory segments of lineages in continuous anagenetic flux, such questions make
no sense, and the entire potential subject remains undefined. (Some tradition exists
for paleontological study of clumping in the distribution of named species through
time, but this literature has never achieved prominence because researchers could
not shake an apologetic feeling that they had based their studies on chimerical
abstractions. For reasons well beyond accident or non-causal correlation, the
beginning of serious and extensive research on this subject has coincided with the
development and acceptance of punctuated equilibrium, a theory that recognizes
these species as genuine evolutionary individuals.)
Precedents for studies of coordination above the species level can be found in
such formulations as Boucot's (1983) twelve EEU's (ecological evolutionary units)
for the entire Phanerozoic, or Sepkoski's (1988,1991) three successive EF's
(evolutionary faunas) for the same interval. But these works address the different
subject of how major environmental shifts, including (but not restricted to) the
substrates of mass extinctions, impact biotas defined at the family level and above.
The subject of temporal interactions among species as basic macroevolutionary
units raises a different set of questions about the nature of the "glue" that binds
such sets of Darwinian individuals together at time scales matching their average
durations (as contrasted with global geological changes that may coordinate—but
probably not actively "bind"— biotas for intervals greatly exceeding the average
lifespan of species). Some pioneering studies—most notably Olson's (1952)
remarkable work on "chronofaunas" of late Paleozoic terrestrial vertebrates—have
offered intriguing suggestions about the potential "glue" that may bind species into
evolutionary ecosystems. However, as stated above, the subject could not be
readily conceptualized until punctuated equilibrium provided a theoretical rationale
for viewing species as legitimate Darwinian individuals.