tributions to the overall pattern of evolution. The same ordinary form of testability
can be applied to any other contrast between strict Darwinism and the revised and
expanded formulations defended in this book.
As the most striking general contrast that might be illuminated by reference to
the different Zeitgeists of Darwin's time and our own, modern revisions for each
essential postulate of Darwinian logic substitute mechanics based on interaction for
Darwin's single locus of causality and directional flow of effects. Thus, for
Darwin's near exclusivity of organismic selection, we now propose a hierarchical
theory with selection acting simultaneously on a rising set of levels, each
characterized by distinctive, but equally well-defined, Darwinian individuals
within a genealogical hierarchy of gene, cell-lineage, organism, deme, species, and
clade. The results of evolution then emerge from complex, but eminently
knowable, interactions among these potent levels, and do not simply flow out and
up from a unique causal locus of organismal selection.
A similar substitution of interaction for directional flow then pervades the
second branch of selection's efficacy, as Darwin's functionalist formulation— with
unidirectional flow from an external environment to an isotropic organic substrate
that supplies "random" raw material but imposes no directional vector of its own to
"push back" from internal sources of constraint— yields to a truly interactive
theory of balance between the functionalist Darwinian "outside" of natural
selection generated by environmental pressures, and a formalist "inside" of strong,
interesting and positive constraints generated by specific past histories and timeless
structural principles. Finally, on the third and last branch of selection's range, the
single and controlling microevolutionary locus of Darwinian causality yields to a
multileveled model of tiers of time, with a unified set of processes working in
distinctive and characteristic ways at each scale, from allelic substitution in
observable years to catastrophic decimation of global biotas. Thus, and in
summary, for the unifocal and noninteractive Darwinian models of exclusively
organismal selection, causal flow from an environmental outside to an organismal
inside, and a microevolutionary locus for mechanisms of change that smoothly
extrapolate to all scales, we substitute a hierarchical selectionist theory of
numerous interacting levels, a balanced and bidirectional flow of causality between
external selection and internal constraint (interaction of functionalist and
structuralist perspectives), and causal interaction among tiers of time.
Among the many consequences of these interactionist reformulations,
punctuational rather than continuationist models of change (with stronger
structuralist components inevitably buttressing the punctuational versions) may
emerge as the most prominent and most interesting. The Darwinian mechanics of
functionalism yield an expectation of continuously improving local adaptation,
with longterm stability representing the achievement of an optimum. But
interactionist and multi-leveled models of causality reconcep-tualize stasis as a
balance, actively maintained among potentially competing forces at numerous
levels, with change then regarded as exceptional rather than intrinsically ticking
most of the time, and punctuational rather than
Defining and Revising the Structure of Evolutionary Theory 33