sis on Vrba's antelopes and, especially, Cheetham's rigorously quantitative and
multivariate data of evolution in the bryozoan genus Metrarabdotos, perhaps the
best documented and most impressive case of exclusive punctuated equilibrium
ever developed. Finally, we can learn much from variation in relative frequencies
among taxa, times, and environments—and interesting inferences have been drawn
from recorded differences, particularly in Sheldon's counterintuitive linkage of
stasis to rapidly changing, and gradualism to stable, environments.
- Among many reasons proposed to explain the predominance of stasis, a
phenomenon not even acknowledged as a "real" and positive aspect of evolution
before punctuated equilibrium gave it some appropriate theoretical space, habitat
tracking (favored by Eldredge), constraints imposed by the nature of subdivided
populations (favored by Lieberman), and normalizing clade selection (proposed by
Williams) represent the most novel and interesting proposals. - Among the implications of a predominantly punctuational origin of stable
species-individuals for macroevolutionary theory, we must rethink trends (the
primary phenomenon of macroevolution, at least in terms of dedicated discussion
in existing literature) as products of the differential success of certain kinds of
species, rather than as the adaptive anagenesis of lineages—a radical reformulation
with consequences extending to a new set of explanations no longer rooted (as in
all traditional resolutions) in the adaptive advantages conferred upon organisms,
but potentially vested in such structural principles as sequelae (by hitchhiking or as
spandrels) of fortuitous phenotypic linkage to higher speciation rates of certain
taxa. In further extensions, macroevolution itself must be reconfigured in
speciational terms, with attendant implications for a wide range of phenomena,
including Cope's rule (structurally ordained biases of speciation away from a lower
size limit occupied by founding members of the clade, rather than adaptive
anagenesis towards organismal benefits of large size), living fossils (members of
clades with persistently minimal rates of speciation, and therefore no capacity for
ever generating much change in a speciational scheme, rather than forms that are
either depauperate of variation, or have occupied morphological optima for untold
ages), and reinterpretation of cladal trends long misinterpreted as triumphs of
progressive evolution (and now reevaluated in terms of variational range in species
numbers, rather than vectors of mean morphology across all species at any time—
leading, for example, to a recognition that modern horses represent the single
surviving twig of a once luxurious, and now depleted, clade, and not the apex of a
continually progressing trend). By the same argument, generalized to all of life, we
understand the stability and continued domination of bacteria as the outstanding
feature of life's history, with the much vaunted progress of complexity towards
mammalian elegance reinterpreted as a limited drift of a minor component of
diversity into the only open space of complexity's theoretical distribution. But, to
encompass this reformulation, we need to focus upon the diversity and variation
among life's species, not upon the supposed vectors of its central tendencies, or
even its pe-
Defining and Revising the Structure of Evolutionary Theory 79