894 THE STRUCTURE OF EVOLUTIONARY THEORY
but that a similar variational evolution occurs at the level of species was
generally ignored. Transformational evolution of species (phyletic
gradualism) is not nearly as important in evolution as the production of a
rich diversity of species and the establishment of evolutionary advance by
selection among these species. In other words, speciational evolution is
Darwinian evolution at a higher hierarchical level. The importance of this
insight can hardly be exaggerated.As a most general statement, and extending Mayr's views from his specific
words (cited just above) to his most characteristic philosophical observation,
Darwinism's major impact upon western thinking transcends the replacement of a
fixed and created universe by an evolutionary flux. As thoughtful evolutionists
have always noted, and as Mayr has particularly stressed in our times by
contrasting "essentialist" and "populational" ways of thinking, a fundamental
revision in our concept of the essence of reality—from the Platonic archetype to
the variable population—may represent Darwin's most pervasive and enduring
contribution to human understanding. For what could be more profound or
portentous than our switch from "fixed essences" to "sensibly united groups of
varying items," as our explanation for the reality behind our names for categories
in our parsings of the natural world?
Yet however successful we have been in executing this great philosophical
shift at the level of microevolution—where we understand that no archetype for a
seahorse, a sequoia or a human being exists; where an enterprise named
"population genetics" stands at the core of an explanatory system; and where we
have all been explicitly taught to view change as the conversion of
intrapopulational variation into interpopulational differences—we have scarcely
begun to execute an equally important reconceptualization for our descriptions and
explanations of macroevolution. We still encapsulate the pageant of life's history
largely as a set of stories about the trajectories of abstracted designs through time.
Beetles and angiosperms flourish; trilobites and ammonites disappear. Horses build
bigger bodies and fewer toes; humans evolve larger brains and smaller teeth.
Meanwhile, in grand epitome, life itself experiences a general rise in mean
complexity as a primary definition (in popular culture at least) of evolution itself.
We know, of course, that flourishing implies more species, while extinction
marks an ultimate reduction to zero. But we still tend to visualize these patterns of
changing diversity as consequences of the status of designs, with the Darwinian
optimum standing for the old Platonic archetype as an ideal and guarantor. (And if
we manage to construe such quintessentially populational phenomena as the
waxing and waning of groups in this Platonic mode, then we will surely not be
tempted to reformulate anatomical trends in average form within groups—a
phenomenon far more congenial to our essentialist mythologies—in the Darwinian
language of changing frequencies in variable populations.) In this way, the two
major phenomena of macroevolution— phenotypic trends within groups and
changes in relative diversities among groups—have stubbornly resisted the
reformative power of Darwin's deepest