Punctuated Equilibrium and the Validation of Macroevolutionary Theory 871
Most discussion on the linkage of differences in frequencies to distinctive
structural and functional characteristics of organisms (rather than to types of
environments per se) has focussed on claims for variation among broad taxonomic
groups (high frequency for gradualism in rodents vs. rarity in bovids, for example),
but I strongly suspect that this genealogical emphasis reflects traditions of
specialization in research more than any inherent preference for taxonomic parsing
in such a search. We should also consider more general features of organisms that
cut across taxonomic lines, and we should therefore examine broader differentia
potentially related to chosen environments or tendencies to speciate. Schoch
(1984), for example, suggested a link between high frequencies for punctuational
speciation and intense social competition, arguing that selection on such features
tends to proceed so rapidly, even in ecological time, that speciation would almost
surely occur in a geological instant. Breton (1996) linked punctuational modes to
evolution of "pioneer" structures (evolutionary novelties tied to morphological
reorganizations), and gradualism to "stabilization" and "settlement" structures
(refinements and improvements in local adaptations). I suspect that such arguments
may apply better to the different issue of average amounts of change per speciation
event, than to questions about the relative frequency of punctuational events (at
whatever degree of alteration) per se.
Most arguments about patterns of differences in relative frequencies have
invoked "externalist" claims about characteristic environments, rather than
"internalist" correlations with structural features of organisms (although the two
subjects may, of course, be correlated and need not stand in antithesis). In a first
attempt, Johnson (1975, 1982), working with Devonian brachiopods and conodonts
but generalizing more widely in an important set of papers, linked higher
frequencies of gradualism to pelagic environments and greater prevalence of
punctuated equilibrium to benthic habitats. He then justified the ecological
correlations by linking characteristic evolutionary modes with relative stability of
environments: "Among marine invertebrates, pelagic organisms are the most likely
to have inhabited extensive, gradually changing environments and are therefore the
most likely to have evolved by a rate and pattern that can be described as phyletic
gradualism... Post-larval, attached and stationary benthic organisms are the most
likely to have inhabited environments that are subject to relatively abrupt changes
and are therefore the most likely to have evolved by a rate and pattern that could be
described as punctuated equilibria."
In a series of papers, Parsons (well summarized in 1993) suggested a similar
linkage, while proposing a different, but generally concordant, explanation based
on a putative correlation between environmental "stress" and patterns of genetic
variation and available "metabolic energy." (I put Parsons's last factor in quotation
marks because I have trouble grasping both the definition and operationality of
such a concept.) Parsons writes (1993, p. 328): "In moderately stressed and
narrowly fluctuating environments, sufficient genetic variability and metabolic
energy should be available to permit adaptation. In these environments, phyletic
gradualism is expected. In highly stressed and