872 THE STRUCTURE OF EVOLUTIONARY THEORY
widely fluctuating environments, a punctuated evolutionary pattern is expected
whereby stasis occurs most of the time."
In one of the most important recent papers on punctuated equilibrium,
Sheldon (1996) has generalized a superficially paradoxical link of morphological
stasis to highly fluctuating environments, and gradualism to more stable and
narrowly fluctuating environments, as the "plus ca change" model— citing the
sardonic French motto that "the more things change, the more everything's the
same," a reference to the proposed link of morphological stasis with highly
variable environments. Sheldon (1996, p. 772) explained the basis and resolution
of the paradox: "One might expect a changing environment to lead to changing
morphology, and a stable environment to stable morphology. But over long
intervals the opposite may often occur ... Perhaps gradual phyletic evolution can
only be sustained by organisms living in or able to track narrowly fluctuating,
slowly changing environments, whereas stasis, almost paradoxically, seems to
prevail in more widely-fluctuating, rapidly changing environments." Sheldon's
figure (reproduced here as 9-28) will make his argument clear. Species in highly
variable habitats must adapt to pervasive and rapid fluctuations, and generally do
so by evolving a stable and generalized morphology suited to the full
environmental range. But when external fluctuation exceeds a certain limit of
internal toleration, rapid speciation may be the only viable response. On the other
hand, mildly fluctuating environments may enhance selection for more precisely
tuned adaptations capable of tracking long-term climatic trends by gradual
adjustment.
Sheldon (1987) began his work by publishing one of the most widely
discussed empirical defenses of gradualism, based on several lineages of
Ordovician trilobites from the Builth Inlier, an environment interpreted as
generally stable and only narrowly fluctuating. (I appreciate the richness of
Sheldon's data, but regard his interpretations as ambiguous, for most of his
published trajectories seem to me—from my partisan standpoint (as I keep
repeating to
9 - 28. An epitome of Sheldon's argument (1996) that, paradoxically, highly fluctuating
environments may induce stasis and punctuation, with gradualism more commonly found in
environments undergoing slower but more steady change.