The Structure of Evolutionary Theory

thodox formulations for all three branches of essential Darwinian logic. To cite just
one relevant example for each branch, theoretical development and accumulating
data on punctuated equilibrium allowed us to reconceptualize species as genuine
Darwinian individuals, fully competent to participate in processes of selection at
their own supraorganismic (and suprademal) level— and then to rethink
macroevolution as the differential success of species rather than the extended
anagenesis of organismal adaptation (see Chapter 9). This validation of the
species-individual aided the transformation of what had begun as a particular
argument about group (or interdemic) selection into a fully generalized hierarchical
theory, with good cases then documented from the genie to the cladal level (see
Chapter 8).
On the second branch of full efficacy for natural selection as an externalist
and functionalist process, the stunning discoveries of extensive deep homologies
across phyla separated by more than 500 million years (particularly the vertebrate
homologs of arthropod Hox genes)—against explicit statements by architects of the
Modern Synthesis (see p. 539) that such homologies could not exist in principle, in
a world dominated by their conception of natural selection—forced a rebalancing
or leavening of Darwinian functionalism with previously neglected, or even
vilified, formalist perspectives based on the role of historical and structural
constraints in channeling directions of evolutionary change, and causing the great
dumpings and inhomogeneities of morphospace—phenomena that had previously
been attributed almost exclusively to functionalist forces of natural selection.
On the third branch of extrapolation, the discovery and relatively quick
validation, beginning in 1980, of a truly catastrophic trigger for at least one great
mass extinction (the K-T event of 65 million years ago), fractured the
uniformitarian consensus, embraced by a century of paleontological complacency,
that all apparent faunal overturns could be "spread out" into sufficient time for
explanation by ordinary causes under plausible intensifications that would not alter
conventional modes of evolution and extinction.
Moreover, as we shall see, these three apparently rather different kinds of data
and their attendant critiques cohere into a revised general structure for evolutionary
theory—thus marking our age as a time for building up and not only as a time for
breaking down.
CONCEPTS. Following the Kantian dictum that percepts without concepts are
blind, but concepts without percepts empty, these two categories interpenetrate as
"pure" data suggest novel ideas (how can one not rethink the causes of mass
extinction when evidence surfaces for a bolide, 7-10 km in diameter, and packing
104 the megatonnage of all the earth's nuclear weapons combined), whereas
"abstract" concepts then taxonomize the natural world in different ways, often
"creating" data that had never been granted enough previous intellectual space
even to be conceived (as when punctuated equilibrium made stasis a theoretically
meaningful and interesting phenomenon, and not just an embarrassing failure to
detect "evolution," in its traditional definition of gradual change—and
paleontologists then began active studies of a subject that had previously been
ignored as uninteresting, if conceptu-
Defining and Revising the Structure of Evolutionary Theory 27