Punctuated Equilibrium and the Validation of Macroevolutionary Theory 887
organism-individual so effectively suppresses the selective proliferation of lower-
level individuals within its own body. In most circumstances, the sorting process of
drift also contributes little to sustained trends because population sizes (of
organisms in demes) usually exceed the small numbers required for maximal
efficacy of such a stochastic force. Thus, of the three potential mechanisms, trends
at the organismal level usually arise by selection.
But this understandable, and theoretically defendable, domination of selection
at the focal level favored by traditional Darwinism does not extrapolate well to the
higher causal level of species (as Darwinian individuals) in clades (as populations).
When we shift our focus to this upper level, all three processes can claim
significant potential weight in theory. (We cannot yet estimate the actual empirical
weights due to paucity of research on a topic so recently defined—but see Wagner,
1996, for a breakthrough study based on quantitative and statistical discrimination
of all three modes for various trends in the evolution of Paleozoic gastropods—see
pp. 733-735 for a summary of his particular conclusions.) Since the species-
individual does not preferentially suppress its own transformation by directional
alteration of subparts (organisms), macroevolutionary trends may often be
propelled by drives from below. Such drives may arise either by the orthodox route
of anagenetic transformation in populations via organismic natural selection
("ontogenetic drive" in my terminology of Table 8-1), or by the unorthodox
process of directional speciation ("reproductive drive" in my terminology).
When Wright's Rule holds (see pp. 731-735), and species arise at random
with respect to the direction of a sustained trend in a clade, then we must invoke
sorting processes among species. Sorting by drift can be highly effective at the
species level because N tends to be small in relevant populations (species within
clades), in contrast with the traditional Darwinian level (organisms within demes),
where the magnitude of N usually precludes effective drift for major traits of
organismal phenotypes.
A traditional Darwinian perspective might therefore lead us to denigrate the
efficacy of the species level as a locus of causation for trends. If species do not
marshall sufficient "strength" to stifle their own transformation by drives from
below, or sufficient numbers to "prevent" the propagation of a cladal trend by
random sorting, then species must pale as evolutionary agents before the strength
of organisms (which manifest enough functional integrity to resist any differential
proliferation of subparts, and also maintain sufficient population size to forestall
random, and potentially nonadaptive, transformation of their collectivities).
I would, however, suggest that such an attitude stymies evolutionary theory as
a restricting bias in the category that Francis Bacon called idola theatri, or idols of
the theater, in his brilliant early 17th century analysis of mental impediments to
understanding the empirical world. Bacon defined idols of the theater as
constraining mental habits bred by allegiance to conventionalized systems of
thought. In the present case, we fall into the bad habit of reading susceptibility to
drive and drift as signs of weakness in an evolutionary individual