Punctuated Equilibrium and the Validation of Macroevolutionary Theory 783
than predictability by extrapolation—for the ordinary condition of stasis provides
little insight into when and how the next punctuation will occur, whereas the
fractal character of gradualism suggests that causes of change at any moment will,
by extrapolation, predict and explain the larger effects accumulated through longer
times.
On the practical side, punctuated equilibrium's formulation of tempo has
validated the study of stasis—paleontology's prevalent pattern within species—as a
source of insight about evolution, rather than a cause of chagrin best bypassed and
ignored as a testimony to an embarrassing poverty of evidence. Punctuated
equilibrium has broken "Cordelia's Dilemma" of silence about the supposed
"nothing" of stasis, and has established a burgeoning subfield of research in the
documentation of stability at several levels. In pursuing and valuing this
documentation, scientists then feel compelled to postulate explanations for the
puzzling frequency of this previously "invisible" phenomenon—and theoretical
inquiry about the "why" of stasis has also flourished following the prod from
punctuated equilibrium (see pp. 877-885 for fuller discussion).
Mode and the speciational foundation of macroevolution
For mode, as discussed throughout this chapter, punctuated equilibrium has
established a speciational basis for macroevolution. By supplying crucial data and
arguments for defining species as effective Darwinian individuals—that is, as basic
units for describing macroevolution in Darwinian terms as an outcome of patterns
in differential birth and death of species treated as stable individuals, just as
microevolution works by the same process applied to births and deaths of
organisms—punctuated equilibrium validates the hierarchical theory of selection.
This hierarchical theory (explicated in Chapter 8) establishes the independence of
macroevolution as a theoretical subject (not just as a domain of description for
accumulated microevolutionary mechanics), thereby precluding the full
explanation of evolution by extrapolation of microevolutionary processes to all
scales and times.
In practical terms, the implications of punctuated equilibrium for evolutionary
mode have strongly impacted two prominent subjects, heretofore almost always
rendered by extrapolation as consequences of adaptation within populations writ
large: evolutionary trends within clades, and relative waxing and waning of
diversity within supposedly competing clades through time. Punctuated
equilibrium suggests novel, and irreducibly macroevolutionary, explanations for
both phenomena (see pp. 885-916).
Finally, the role of punctuated equilibrium in establishing an independent
field of macroevolution includes both a weak and a strong version. The first,
undoubtedly valid as a generality, "uncouples" macro from microevolution as a
descriptive necessity, while not establishing independent causal principles of
macroevolution. The second clearly regulates many cases, but has not yet been
validated as commanding a high relative frequency; this second, or strong, version
establishes irreducible causal principles of macroevolution.
The weak version, based on "species sorting" rather than "species selection,"