Punctuated Equilibrium and the Validation of Macroevolutionary Theory 835
neontological colleagues who, before then, had been unmoved by punctuated
equilibrium. How can geological gradualism be the extrapolated expression of
natural selection within populations? Surely, if a doubling of tooth size (say)
requires 2 million years to reach completion, then the process must be providing so
small an increment of potential advantage in each generation that natural selection
couldn't possibly "see" the effect in terms of reliably enhanced reproductive
success on a generational basis. Can a tooth elongated by a tiny fraction of a single
millimeter possibly confer any evolutionary advantage in a selective episode
during one generation of a population's history? Conversely, if bigger teeth provide
such sustained advantages, why stretch the process over millions of years?
Neontological studies have amply confirmed that natural selection can be a
powerful force—the lesson, after all, of our entire, and burgeoning, literature of
measurable change in Darwin's finches, anolid lizards, peppered moths, etc. So
why shouldn't such a doubling of tooth length be achieved over the palpable span
of a few human generations? Of course we all recognize a host of standard
arguments for reining in the speed of selective response: negative consequences
through discoordination with other parts of the body, slowing by networks of
correlated effects upon other anatomical features. But I doubt that even the
summation of all such effects could generate sufficient restraining power to spread
the blessings of a moment over two million years of plodding achievement. (See,
however, p. 540 for Mayr's confident assertion, a priori and without evidence, of
this evolutionary style and rate as canonical).
In other words, gradualism should be viewed as a problem and a potential
anomaly, not as an expectation. In an important early recognition of this principle,
Lande (1976), who (to say the least) is no friend of punctuated equilibrium,
calculated that Gingerich's measured trends confer such a small effect upon the
immediacy of ecological moments that, for one case, Lande calculated an
advantage corresponding to elimination of individuals four or more standard
deviations from the mean in regimes of truncation selection! However unrealistic
one might deem such a model, no one should miss the "bottom line": most
populations don't include any viable individuals four standard deviations from the
mean—and one can hardly imagine that the removal of such occasional misfits or
anomalies could slowly move the mean value of a population to new adaptive
heights over a million sustained years.
I do not mean to say that this paradox cannot be resolved to make gradualism
intelligible once again, but I do hold that any revalidation demands a substantial
reconceptualization for this venerable phenomenon. The obvious solution lies
embedded in results such as Bell and Haglund (1982) on the fine-scale structure of
stasis. Selection in the immediacy of ecological moments cannot be measured as
either the net nontrending of stasis or the steady accumulations of changing means
in anagenetic gradualism. Any local population constantly jiggles to and fro in
selective accommodation to changing local environment (as when mean coloration
for peppered moths becomes darker for a few centuries, but then lighter again, and
back to previous values, when lichens return to trees after abatement of industrial
pollution). The extent of