878 THE STRUCTURE OF EVOLUTIONARY THEORY
debate on the theoretical novelty of punctuated equilibrium lies in the legitimate
weights that will eventually be assigned to the various proposals of this array.)
STABILIZING SELECTION. For most evolutionists who chose to see nothing new
in punctuated equilibrium, the previously unacknowledged frequency of stasis
(admitted, albeit sometimes begrudgingly, as an unexpected finding) could only
indicate a stronger role than previously envisaged for the conventional mechanism
of stabilizing selection. Although this putative explanation of stasis within
paleospecies achieved an almost canonical status among evolutionists who tried to
forge complete compatibility between punctuated equilibrium and the Modern
Synthesis, and although we all acknowledge stabilizing selection as too important
and pervasive a phenomenon to hold no relevance for this issue, a complete
explanation of stasis in these conventional terms seems implausible both on
empirical grounds, and also by the basic logic of proper scaling.
As often emphasized in this chapter, if stasis merely reflects excellent
adaptation to environment, then why do we frequently observe such profound
stasis during major climatic shifts like ice-age cycles (Cronin, 1985), or through
the largest environmental change in a major interval of time (Prothero and Heaton,
1996)? More importantly, conventional arguments about stabilizing selection have
been framed for discrete populations on adaptive peaks, not for the totality of a
species—the proper scale of punctuated equilibrium—so often composed of
numerous, and at least semi-independent, subpopulations. A form of stabilizing
selection acting among rather than within subpopulations may offer more
promise—as Williams (1992) has proposed (see discussion under point 6)—but
such forms of supraorganismal selection fall into a domain of heterodoxies, not
into this category of conventional explanations that would leave the Modern
Synthesis entirely unaffected by the recognition of stasis as a paleontological
norm.
DEVELOPMENTAL AND ECOLOGICAL PLASTICITY. If stabilizing selection holds
that species don't change because they have achieved such excellence in current
adaptation, this second proposal (of Wake, Roth and Wake, 1983) proposes that
species don't change (in an evolutionary and genetic sense) because they can
usually accommodate to environmental alteration by exploiting the plasticity
(behavioral and developmental) permitted within their existing genetic and
ontogenetic system—thus calling upon the physiologist's entirely different
meaning of the term "adaptation" (improvement in functionality by exploiting
possibilities within a norm of reaction, as in the enlarged lungs of people who
inhabit the high Andes), rather than the usual evolutionary meaning in our
profession.
(Although I have roughly ordered this list of proposed explanations for stasis
from Darwinian conventionality towards more challenging proposals, I don't
regard any item as excluding any other—indeed, I would be surprised if all cannot
claim at least some measure of validity, for once again we deal with an issue of
relative frequencies and differential circumstances—and I don't regard any pair as
establishing a contradiction. In particular, these first two