876 THE STRUCTURE OF EVOLUTIONARY THEORY
forces promoting such a result, either directly, or as a consequence of some
important linked property of organisms or populations.
This growth in emphasis has been so vigorous since 1972 that geological
gradualism, once the unquestioned expectation of evolution itself, is now generally
regarded as an infrequent, if not anomalous, phenomenon requiring a special
explanation in the light of anticipated stasis. Geary (1990, p. 507) after
documenting a case of gradualism within a clade showing a much higher frequency
of stasis, wryly noted: "Given that past studies were assumed complete only if
gradual change was apparent, it seems somewhat ironic that unseen mechanisms or
events, however realistic, must now be invoked in order to explain an instance of
gradual change!" Gradualism, in short, has become both a rarity and a puzzle.
(Much as I take a rather wickedly and secret personal pleasure in this sea change,
I'm not sure that I can, in good scientific conscience, regard such a priori mental
downgrading of gradualism as a "good thing." A posteriori downgrading based on
documented rarity represents nature's chief signal in my view, but I do think that
any study should begin with equal potential welcome for either result!)
GENERALITY. The interest in stasis, originally generated by punctuated
equilibrium for inquiries at the appropriate level of species durations through time,
has since expanded to other domains of size and time, and to more comprehensive
questions about the nature of change itself. Causes operating at punctuated
equilibrium's proper scale will not explain other forms of stasis, but the generalized
definition and inquiry did arise by expansion from our theory (at least as a
sociological phenomenon), while we may also anticipate the identification of some
common causes or constraints (see further discussion on conceptual "homology,"
pp. 928-931)—that is, in evolutionary parlance, causal parallelisms, based on
structural homologies, rather than convergences or mere analogies of appearance—
behind the deeper generality (with different immediate forces producing similar
and partly homologous results at various levels). I shall discuss some of these other
scales in Part B of this subsection. These extensions include: punctuational
anagenesis for directional changes in lineages of asexual organisms by clonal
sorting (in a domain below punctuated equilibrium, which, sensu stricto, only
operates at the level of speciation to explain trends in multicellular sexual lineages
by species sorting); longterm morphological stability for basic anatomical features
of larger clades (at a level above punctuated equilibrium and within monophyletic
lineages—see Chapter 10); putative "lock-step" stasis for the great majority of
defining species within larger faunas through significant geological intervals (at a
still higher level above punctuated equilibrium and across genealogical lineages to
a consideration of faunal dynamics—see pp. 916-922). Interest has also extended
beyond evolutionary systems to the meaning and causes of stasis in stairstep
patterns of ontogenetic growth, stubbornly persistent plateaus followed by
thresholds of rapid change in response to continuous input in human learning, and
active stasis followed by