Punctuated Equilibrium and the Validation of Macroevolutionary Theory 889
evidence for the model's adequacy. By the expectations of all three central precepts
in Darwinian logic, and by our habits of restricting explanations of sustained
organismic trends to selectionist causes (given valid arguments for rejecting the
alternatives of drift and drive at the organismic level, as discussed above),
increasing adaptation of organisms must also propel macro-evolutionary trends
under extrapolationist premises.
(All Darwinians understand, of course, that natural selection only yields
adaptation to immediate environments—a notion not conducive to sustained
directional trends through geological time, given the effectively random fluctuation
of most environmental configurations through substantial geological intervals.
Consequently, most sustained trends have been interpreted as generalized
biomechanical improvements conferring advantages across most or all experienced
environments, and arising from Darwin's own preference for domination of biotic
over nonbiotic competition in the history of life. See the discussion of Darwin's
rationale for this defense of "progress" in evolution, Chapter 6, pp. 467-479.)
Discourse about trends dominates the traditional literature of evolutionary
paleontology, both at the most general level of universal phenomenology (Cope's
law of increase of size, Dollo's law of irreversibility, Williston's law of reduction
and specialization of modular segments, etc.), and as a dominating theme for the
history of almost any individual clade. We all know the particular tales for
textbook groups—increasing brain size in hominids; larger body size, fewer toes,
and higher crowned cheek teeth in horses; increasing symmetry of the cup in
Paleozoic crinoids, with eventual expulsion of the anal ray to the top of the calyx;
complexification in ammonite suture lines; reduction in number of stipes in
graptolite colonies. These summary themes for clades, all based on the concept of
general biomechanical improvement through time, distill the essence of traditional
paleontology.
I do not deny that generalized organismic advantage may explain some of
these classic trends. I do not, for example, challenge the traditional notion that
increasing perfection of radial symmetry may confer adaptive benefits in feeding
upon sessile organisms like stalked crinoids (see Moore and Landon, 1943, for the
classic statement). But I also note that other classic trends, apparently ripe for
explanation in biomechanical terms, have stubbornly resisted any reasonable
hypothesis ever proposed—most notably the complixifying ammonite suture,
which does not clearly confer greater resistance to shell crushing, and does not
evidently aid the growing animal by increasing surface area of tissues covering the
septa.
But other trends, despite their prominence, have never generated even a
plausible hypothesis of biomechanical advantage. Why should fewer-stiped
graptolite colonies "do better" in any usual sense of organismic (or, in this case,
astogenetic) advantage? In such circumstances, we need to expand our explanatory
net by considering alternative causal resolutions based on differential success of
species as Darwinian individuals engaged in processes of sorting. Instead of
focussing upon the putative biomechanical virtues of fewer stipes, we should be
asking how and why such a character might correlate