Punctuated Equilibrium and the Validation of Macroevolutionary Theory 815
(Stanley's tests may be reduced to three, as his second "test of the Pontian cockles"
represents a particular instance of his first "test of adaptive radiations." Stanley
argued:
TEST OF ADAPTIVE RADIATION. After calculating average species durations
from the fossil record, one can affirm that pure anagenetic gradualism (or temporal
stacking of species end-to-end) cannot account for the magnitude of recorded
adaptive radiations in the time available—so rapid cladogenesis must be invoked.
TEST OF LIVING FOSSILS. Punctuated equilibrium associates realized amounts
of change primarily with frequency of speciation, anagenetic gradualism primarily
with elapsed time. If so-called "living fossils"—ancient groups with little recorded
change—also show unvarying low diversity through time, then we can affirm the
primarily prediction of punctuated equilibrium, and refute the corresponding
expectation of gradualism (for these groups are ancient). Stanley then documented
such a correlation between clades identified as "living fossils" and persistently low
diversity in these clades.
TEST OF GENERATION TIME. Under gradualism, amounts of realized evolution
should correlate strongly with generation time—for the time that should mark
accumulated evolutionary change does not tick by an abstract Newtonian clock,
but by number of elapsed generations, representing the number of opportunities for
natural selection to operate. But, under punctuated equilibrium, amount of change
correlates primarily with frequency of speciation—a property with no known
relationship to generation time. Stanley then cited the well-documented lack of
correlation between evolutionary rate and generation time as evidence for the
prevalence of punctuated equilibrium (fast-evolving elephants vs. stable
invertebrates with short generations).
Much as I regard Stanley's arguments as suggestive, I cannot accept them as
conclusive for two basic reasons. First, other plausible explanations exist for the
patterns noted. For example, many reasons other than the prevalence of punctuated
equilibrium might explain a lack of correlation between realized evolution and
generation time, even in a world of anagenetic gradualism. The correlation might
simply be weak or too easily overwhelmed (and therefore rendered invisible) by
such other systematic factors as variation in the intensity of selection. (Maybe
elephants, on average, experience selection pressures higher by an order of
magnitude than those affecting short-lived invertebrates; maybe population size
overwhelms the factor of generation time.)
Second, most of Stanley's tests (particularly his key claim about adaptive
radiation) don't really oppose punctuated equilibrium to gradualism, but rather
contrast a more general claim about the speciational basis of change (whatever the
mode of speciation) with anagenesis. Moreover, the tests employ a somewhat
unfairly caricatured concept of gradualism. I doubt that the most committed
gradualist ever tried to encompass the maximal change between ancestor and any
descendant in an adaptive radiation by stacking species end to end, and then
calculating whether the full effect could arise in the allotted time. A committed
gradualist might fairly say of an adaptive radiation: