Punctuated Equilibrium and the Validation of Macroevolutionary Theory 875
must also apply mental screening to select "things" meriting our attention within
nature's potential infinity, and even to recognize a configuration of matter as a
"thing" in the first place. Therefore, phenomena without names, and without
theories marking them as worthy of notice, will probably not be recognized at all.
The phenomenon always existed "out there" in nature, of course, but
punctuated equilibrium largely "created" the category of stasis as an important item
in evolutionary theory through a four-step process of (1) defining stasis as a
positive "thing" with properties and boundaries, a phenomenon rather than an
unnamed and unrecorded absence of evolution; (2) bringing stasis to visibility as
the expectation of a particular theory of evolutionary modalities; (3) suggesting
methods for the active and rigorous study of stasis, so that the concept could be
operationalized as a subject for empirical research; and (4) granting interest and
importance to stasis as a controversial topic with broad implications for revising
traditional modes of thought in evolutionary biology.
Before Eldredge and I published our first paper in 1972, most paleontologists
treated stasis as an embarrassment, imposed by the poverty of the fossil record
upon hopes for recording evolution (defined as gradualistic anagenesis), and
therefore as not meriting active study, or even explicit recognition as a discrete
phenomenon. Just a decade later, the situation had changed so dramatically that
Wake, Roth and Wake (1983) could write, "perhaps no phenomenon is as
challenging to evolutionary biologists as what has been termed 'stasis'" (p. 212),
defined by them as "the maintenance of a standard morphology over vast periods
of time during which much environmental change has taken place" (p. 211).
Illustrating my claim that a phenomenon becomes interesting only in the light of
defining theories, Wake et al. (1983, p. 212) then stated: "With natural selection
operating in a changing environment as an agent of adaptation, we expect to see
changes at the organismal, ultimately physiological and morphological, level.
How, though, can we explain the paradoxical situation in which environments
change, even dramatically, but organisms do not?"
As I now survey the subject, a quarter century after our initial presentation
and definition, stasis has become an even more general and important issue in
evolutionary theory for three principal reasons:
FREQUENCY. Once the phenomenon had been named, and criteria established for
recognition and study, researchers documented stasis at far too high a relative
frequency to represent anything other than an evolutionary norm and expectation.
Such predominance also implicates stasis as a property actively maintained by
species—thus leading to a substantial literature (discussed at the end of this
subsection) on the causes of non-change. Several authors, notably Paul (1985) and
Jackson and Cheetham (1994, also Cheetham and Jackson, 1995), developed
models and data sets to prove that stasis occurs too frequently for explanation
under random models (including pure neutralism with no natural selection), and
therefore must be caused by active