Punctuated Equilibrium and the Validation of Macroevolutionary Theory 781
moment. But we can also gain when operational invisibility at our scale (inability
to distinguish a small effect from measurement error) becomes palpable and
prominent in the large, or when the almost inconceivable rarity of an event that
averages one expression in ten thousand years achieves guaranteed repetition
across millions.
MACROEVOLUTIONARY IMPLICATIONSIf punctuated equilibrium has broader utility beyond the reform of paleontological
practice, then we must look to potential implications for macroevolutionary theory,
and for consequent enrichment in our general understanding of mechanisms that
regulate the history of life. I have linked my treatments of punctuated equilibrium
and the hierarchical theory of natural selection to form the longest section of this
book (presented as two chapters, 8 and 9) because I believe that punctuated
equilibrium supplies the central argument for viewing species as effective
Darwinian individuals at a relative frequency high enough to be regarded as
general—thereby validating the level of species as a domain of evolutionary
causality, and establishing the effectiveness and independence of macroevolution
by two of the three criteria featured throughout this book as indispensable
foundations of Darwinism.
First, punctuated equilibrium secures the hierarchical expansion of
selectionist theory to the level of species, thus moving beyond Darwin's preference
for restricting causality effectively to the organismic realm alone (leg one on the
essential tripod). Second, by defining species as the basic units or atoms of
macroevolution—as stable "things" (Darwinian individuals) rather than as arbitrary
segments of continua—punctuated equilibrium precludes the explanation of all
evolutionary patterns by extrapolation from mechanisms operating on local
populations, at human timescales, and at organismic and lower levels (leg three on
the tripod of Darwinian essentials). Thus, as emphasized in the last section,
punctuated equilibrium presents no radical proposal in the domain of
microevolutionary mechanics—in particular (and as so often misunderstood), the
theory advances no defenses for saltational models of speciation, and no claims for
novel genetic processes. Moreover, punctuated equilibrium does not attempt to
specify or criticize the conventional mechanisms of microevolution at all (for
punctuated equilibrium emerges as the anticipated expression, by proper scaling, of
micro-evolutionary theories about speciation into the radically different domain of
"deep," or geological time). But punctuated equilibrium does maintain, as the
kernel of its potential novelty for biological theory, that these unrevised
microevolutionary mechanisms do not hold exclusive sway in evolutionary
explanation, and that their domain of action must be restricted (or at least shared)
at the level of macroevolutionary pattern over geological scales— for punctuated
equilibrium ratifies an effective realm of macroevolutionary mechanics based on
recognizing species as Darwinian individuals. In other words, punctuated
equilibrium makes its major contribution to evolutionary theory, not by revising
microevolutionary mechanics, but by individuating