The Structure of Evolutionary Theory

of its own integrity; failure of this suppression leads to the pyrrhic victory of cell-
lineages that we call cancer. Interdemic selection, although once so widely
rejected, probably plays an essential role in the evolution of social cooperation in
general, and not only for such specific phenomena as human altruism. Species-
level selection, combined with other species-level properties of drive and drift,
establishes the independent basis for a distinctive speciational theory and
reformulation of macroevolution. The highest level of clade selection, although
sometimes operative, may be relatively weak by an extension of Fisher's argument
about low N.
7. I explore the distinctive differences between levels of selection by trying to
exemplify and "play out" the detailed disparities in a "grand analogy" between the
conventional operation of organismic selection and the relative conceptual novelty
of species selection. As an idiosyncratic sample of potential reforms and surprises,
consider the following claims: First, the formulation of a general taxonomy for
sources of change in hierarchically ordered systems, based on a primary distinction
of "drive" for directed changes arising within an individual, based on change
among lower-level individuals as constituent parts; and "sorting," with two
causally distinct subcategories of "selection" and "drift" for change based on
alterations of relative frequencies among individuals at the focal level itself.
Second, the recognition, by following the logic of the analogy, of some strikingly
counterintuitive comparisons that become both interesting and revealing upon
subsequent reflection—including the likeness of Lamarckian change, construed as
ontogenetic drive at the organismal level, with standard anagenetic transformation
as organismal drive at the species level (transformation by directional change of
constituent parts of a higher-level individual, in this case the organisms of a
species); this similarity may also highlight the rather different reasons for general
unimportance of both levels of drive—Lamarckism for the well-known reason of
theoretical non-occurrence in a Mendelian world, and anagenesis based on the
controversial claim for its evident plausibility in theory (as a basic Darwinian
process), but rarity in fact, given the dominant relative frequency of punctuated
equilibrium. Third, the establishment of a framework for distinguishing directional
speciation as a form of reproductive drive (inherently biased differences in
autapomorphies of descendant species vs. ancestral states) from true species
selection as a higher order sorting among daughter species that arise with
phenotypic differences randomly distributed about parental means. I believe that
we have missed this crucial distinction because the analog of directional speciation
at the organismal level—drives induced by mutation pressure—occur so rarely (for
conventional reasons of organismal selection's power to suppress them) that we
haven't considered the greater potency of analogous processes at other levels.
Fourth, the importance of testing "Wright's Rule"—the claim that speciation is
random with respect to the direction of evolutionary trends within clades—because
the major alternative of directional speciation as the cause of trends holds such
potential power at the species level, whereas its analog (drives of mutation
pressure) assumes so little importance at the organismal level. Fifth, the potentially
far greater im-
Defining and Revising the Structure of Evolutionary Theory 75