Punctuated Equilibrium and the Validation of Macroevolutionary Theory 885
Cheetham, 1995, strongly suggests in documenting stasis at too high a relative
frequency for models based on neutralism, directional selection, or any set of
assumptions that do not include some active force promoting stasis directly)—then
evolutionary change itself must be reconceptualized as the infrequent breaking of a
conventional and expected state, rather than as an inherent and continually
operating property of biological materials, ecologies and populations.
A phenomenon marking the disruption of normality holds a very different
philosophical status than a phenomenon representing the ordinary architecture of
biological space and time. Evolutionary change, regarded as an occasional
disrupter of stasis, requires a different set of explanatory concepts and
mechanisms—a different view of life, really—from evolutionary change, defined
as an anagenetic expectation intrinsically operating in most populations most of the
time. Punctuated equilibrium proposes that the macroevolutionary key to this new
formulation lies in speciation, or the birth of new higher-level individuals at
discrete geological moments (corresponding to long intervals at the scale of a
human lifetime). Macroevolution, in this view, becomes an inquiry into modes and
mechanisms for breaking the stasis of existing species, and generating new species,
conceived and defined as discrete higher-level Darwinian individuals—and not a
question about how species-individuals gradually change their parts and
constitutions through time (as in conventional Darwinism). But even if this
particular formulation at geological scales eventually yields more limited impact or
utility than proponents of punctuated equilibrium suspect, the more general
redefinition of evolution as a set of rare incidents in the breaking of stasis, rather
than the pervasive movement of an expected and canonical flow, still poses an
interesting challenge for rethinking a fundamental proposition about the nature and
history of life.
Punctuation, the origin of new macroevolutionary individuals,
and resulting implications for evolutionary theory
I have argued throughout this chapter that sets of related implications for
expanding and reformulating the structure of Darwinian theory, particularly in
applications to macroevolution, flow from each of the two major components of
punctuated equilibrium—stasis as a norm for the duration of paleospecies, and
punctuation (on geological scales) for their cladogenetic origin. The punctuational
origin of species by cladogenesis provides our strongest rationale for regarding
species as true evolutionary individuals in Darwin's causal world—rather than as
arbitrarily delineated segments of transforming continua, and therefore not as
genuine entities at all (a position maintained by both Darwin and Lamarck in some
of their most forceful passages). If, following what I called the "grand analogy,"
species represent "items" or "atoms" of macroevolution in the same sense that
organisms operate as fundamental interactors for natural selection in
microevolution (see pp. 714-744), then many features of the mechanics and
patterning of macroevolution must be reformulated. For macroevolution then
becomes a process irreducibly fueled by the differential birth and death of species
(just as microevolution,