884 THE STRUCTURE OF EVOLUTIONARY THEORY
against all but the conservative marine form... The appearance of stasis in the
fossil record would result from an enormous variability in the persistence of
ecological niches."
I am more attracted to Lieberman's suggestions, based on averaging among
demes with no net change among persistent demes adapted to differing habitats,
than to Williams's hypothesis, based on differential survival of one stable deme in
a persistent habitat—if only because Lieberman has generated empirical evidence
for longterm survival in several habitats within his two brachiopod species,
whereas Williams's stickleback example may represent an unusual situation in the
drastically different habitats (fresh vs. marine waters) of his transient vs. persistent
demes. Still, I applaud these two suggestions for stasis based on the structuring of
species-individuals as collections of deme-individuals, with differential selection
acting upon demes in an irreducibly macroevolutionary mode. These proposals
therefore occupy the heterodox end in a spectrum of proposed explanations for
stasis—for they challenge the Darwinian orthodoxy of primacy or exclusivity for
organismal selection. I especially appreciate Williams's openness towards
explanations in this form, given his previous and highly influential preferences for
formulating all evolutionary explanation, except when absolutely unavoidable, at
the level of genie selection (in his famous book, Williams, 1966, as discussed on
pp. 550-554).
In summary, then, the assertion of predominant stasis in the geological history
of most paleospecies—one of the two primary claims of punctuated equilibrium—
has provoked an interesting debate in evolutionary theory, with implications for
some of the most basic concepts and perspectives in our science. First, and if only
as a comment about the contemporary sociology of science, the recognition of
stasis as a norm of controlling relative frequency at the level of punctuated
equilibrium (at least for conventional sexual species of Metazoa), has spurred
general interest in phenomena of stability and non-change throughout other levels
of evolutionary inquiry (see, for example, Maynard Smith, 1983). We do not yet
know (see fuller discussion on pp. 928-931) whether or rather how much, stasis
across all scales might be attributed to structural similarity in nature's materials and
processes—thus rendering this common pattern as an interesting parallelism (to
use our evolutionary jargon) with genuinely homologous causal elements across
scales, rather than a fortuitous convergence of similar overt patterns for disparate
and merely analogous reasons. But at least we stand at the threshold of such an
inquiry.
Second, and even more generally, the validation of predominant stasis as a
norm would impel us to recast the basic problematic of evolution itself. If,
following our conventional assumptions from Darwin to now, change represents
the norm for a population through time, then our task, as evolutionary biologists,
lies in specifying how this expected and universal phenomenon operates. But if, as
punctuated equilibrium suggests, stasis represents the norm for most populations at
most times; and if, moreover, stasis emerges as an active norm, not merely a
passive consequence (as the modelling of Jackson and