Punctuated Equilibrium and the Validation of Macroevolutionary Theory 789
alongside their putative ancestors. Moreover, the tight correlation between
phenetic, cladistic, and genetic distances among living Stylopoma species suggests
that changes in all three variables occurred together during speciation. All of these
observations support the punctuated equilibrium model of speciation."
Despite the encouragement provided by these and other cases, problems
continue to surround the definition of paleontological species—a subject of central
importance to punctuated equilibrium, given our invocation of speciation as the
quantum of change for life's macroevolutionary history, and the source of raw
material for higher-level selection and sorting. These problems center upon three
main issues (in both the inherent logic of the case and by recorded debate in the
literature): the first untroubling, the second potentially serious, and the third largely
resolved in empirical terms. All three issues raise the possibility that paleospecies
systematically misrepresent the nature and number of actual biospecies. (If
paleospecies don't correspond with biospecies in all cases—an undeniable
proposition of course—but if these discrepancies show no pattern and produce no
systematic bias, then we need not be troubled unless the relative frequency of no
correspondence becomes overwhelmingly high, an unlikely situation given the
excellent alignments found in the few studies explicitly done to investigate this
problem, as discussed just above.) The following three subsections treat these three
remaining issues seriatim.
Reasons for a potential systematic underestimation of biospecies
by paleospecies
Might we be missing a high percentage of actual speciation events because
paleontologists can only recognize a cladogenetic branch with clear phenotypic
consequences (for characters preserved as fossils), whereas many new species arise
without substantial morphological divergence from their ancestors? In the clearest
case, paleontologists (obviously) cannot detect sibling species, a common
phenomenon in evolution (see Mayr, 1963, for the classic statement). Moreover,
we may also miss subtle changes in phenotype, or substantial alterations (of color,
for example) in features that are often important in recognizing species, but do not
achieve expression in the fossil record.
Our harshest critics have urged this point as particularly telling against
punctuated equilibrium. Levinton, for example (1988, p. 182), holds that "the vast
majority of speciation events probably beget no significant change." He then views
the consequences as effectively fatal for punctuated equilibrium (1988, p. 211):
"The punctuated equilibrium model argues that morphological change is associated
with speciation and that species are static during their history due to some internal
stabilizing mechanism. There is no evidence coming from living species to support
this. If anything, recent research has demonstrated that speciation occurs typically
with little or no morphological change; hence the large-scale occurrence of sibling
species."