836 THE STRUCTURE OF EVOLUTIONARY THEORY
selection in an anagenetic sequence must be cumulated through each and every one
of these jiggles, not measured by calculating the coefficients needed simply to
change one endpoint into another. (Such a tactic would lead to the evidently false
conclusion that little or no selection had ever occurred in peppered moths.) In other
words, perhaps we must construe gradualism itself as a "higher level" phenomenon
of net accumulation through the jiggles, not as an expression of ordinary
directional selection summed through the ages.
But such a conclusion then raises a different (and broader) question: what,
then, is ordinary geological gradualism after all? How can such a minuscule
directional effect persist through all the swings and jiggles? And what does such a
phenomenon represent? Must we interpret such slow net change as caused by drift,
as Lande's models made conceivable? Such a conclusion would seem unlikely
given the common impression that certain features, size increase in particular,
occur preferentially and nonrandomly in gradualistic sequences (but see Jablonski,
1997, and Gould, 1997b, on the apparent falsity, and status as a psychological
artifact, of this venerable claim known as "Cope's Rule"). Can we even argue for
natural selection as the primary cause of classical gradualism at all? I am confident
that selection remains a good candidate, but of what sort, and at what level? The
selective basis of gradualism surely cannot be ascribed to the extrapolated
advantage at every given moment of traits so enhanced over the long run. Rather,
the selective edge must lie in some form of more general benefit not consistently
visible in ecological moments, but somehow skewed to a higher probability of
immediate occurrence that can then cumulate to a consistent trend in
macroevolutionary time.
One might be tempted to equate this skewing agent with some form of general
biomechanical improvement that might hold cumulative sway above the jiggling of
momentary advantage in any direction. But then the kinds of features that seem to
prevail in gradual anagenesis do not stand out for potential membership in this
category. Perhaps we need to consider selection on supra-organismal units, or
perhaps we should entertain nonselectionist alternatives, especially in the light of
Lande's modelling for drift. (Such hypotheses of random change would require a
far better knowledge of relative frequencies, both for characters within a taxon and
among taxa themselves, than we now possess or even know how to generate.) In
any case, I do not think we have even begun to explore the range of potential
explanations for the puzzling phenomenon of anagenetic gradualism. I, at least,
find the subject very confusing and challenging.
Finally, once we recognize gradualism as an interesting puzzle rather than a
dull expectation, we may be led to "dissect" the phylogenetic "anatomy" of such
trends more carefully, thus adding an operational benefit to the renewed theoretical
interest. In a striking example, Kucera and Malmgren (1998) published an elegant
study of morphological change in the late Cretaceous Contusotruncana lineage of
planktonic forams. After several million years of stasis, the defining feature of
"mean shell conicity" increased in a gradualistic manner (see Fig. 9-16) for 3.5
million years, beginning 68.5 million years ago, in this anagenetic lineage.