886 THE STRUCTURE OF EVOLUTIONARY THEORY
under natural selection, is powered by the differential reproductive success of
organisms)—and not, as Darwin and his successors have long held, a
phenomenology ultimately built by, and extending causally from, the accumulating
consequences of continuous organismic adaptation in transforming populations.
In this sense, punctuated equilibrium—by crowning the case for stable species
as atoms of macroevolution—challenges all three legs of the essential Darwinian
tripod: the first leg of organismal focus most directly, by establishing the higher-
level species-individual as a potent causal agent of evolution as well; the second
leg of functionalism more indirectly by affirming, as generators of
macroevolutionary patterns, several modes of explanation that do not flow from
organismal adaptation, or even rest upon an adaptational base at all; and, most
comprehensively, the third leg of extrapolationism by validating a hierarchical
view of pattern and causality, and by denying that the mechanisms of
macroevolutionary change all flow from our uniformitarian understanding of how
natural selection, working in the organismal mode, can alter populations on the
scale of human observation in historical time.*
To illustrate the expansive and reformative potential of the species-organism
as a causal agent in macroevolution, I will discuss the three major topics that
punctuated equilibrium has helped to redefine during the past two decades:
TRENDS. In Chapter 8,1 proposed that trends among species in clades may differ
substantially from trends among organisms in populations as an "allometric" result
of varying weights attached to the three major causal processes at disparate scales
of organism-individuals and species-individuals—drives, and the two sources of
sorting, drift, and selection (see pp. 714-744 for full development of an argument
only summarized here). At the conventional organismic level, drives from below
assume little importance because the
*I also wish to reemphasize that I assert no exclusivistic claim in this formulation.
Supporters of the hierarchical theory must not repeat the parochial error of their forebears
by arguing that their newly specific, higher-level mechanisms can explain everything by
reaching down, just as Darwinian traditionalists tried to develop a complete causal theory
by extrapolating up. Thus, we do not challenge either the efficacy or the cardinal
importance of organismal selection. As previously discussed, I fully agree with Dawkins
(1986) and others that one cannot invoke a higher-level force like species selection to
explain "things that organisms do"—in particular, the stunning panoply of organismic
adaptations that has always motivated our sense of wonder about the natural world, and
that Darwin described, in one of his most famous lines (1859, p. 3) as "that perfection of
structure and coadaptation which most justly excites our admiration." But should we not
regard as equally foolish, and equally vain (in both senses of the word), any proposal that
insists upon explaining all "things that species and clades do" as extrapolated
consequences of organismic adaptation? I would not invoke species selection to explain
the marvelous mechanics of beetle elytra, but the same theme of appropriate scale also
leads me to equal confidence that the excellent adaptive design of beetle organisms
cannot fully explain why this order so vastly predominates in species diversity, even
among the most speciose of all metazoan classes—to the point of inspiring Haldane's
canonical quip about God's "inordinate fondness" for these creatures (see Gould, 1993a,
for an exegesis of this famous anecdote).