Punctuated Equilibrium and the Validation of Macroevolutionary Theory 873
remind readers to be especially critical)—more consistent with expectations of
fluctuating stasis with little or no net change, see Eldredge and Gould, 1988.) I am
particularly grateful that Sheldon, even while developing one of the most famous
data sets against the theory, has always accepted the importance of establishing
relative frequencies for different groups and situations, and has consistently
regarded punctuated equilibrium as a valuable theory (to which he has made major
contributions), with important implications for our understanding of
macroevolution.
Such broad arguments about environmental correlations have been
notoriously difficult to document because, even when the effect can be validated as
both real and pervasive, so many other factors will be operating in any particular
case (including immediate and local influences able to overwhelm the smaller
impact of the generality under test) that the signal may be lost in surrounding
noise. But I am strongly attracted to Sheldon's plus ca change hypothesis for two
primary reasons. First, the concept makes good sense of patterns that have often
been noted empirically, but regarded as confusing in interpretation—particularly
the common finding of pronounced stasis through major climatic fluctuations,
including Pleistocene ice age cycles (Cronin, 1985, on ostracodes; Coope,
1980,1994 on beetles), and the largest climatic crash in Tertiary North America
(Prothero and Heaton, 1996). The presentation of a hypothesis like Sheldon's
prompts researchers to focus studies on interesting issues, and to seek wider
implications. For example, Wei (1994) used Sheldon's hypothesis to explain the
link of stasis to intensification of ice-age climatic fluctuations in the planktonic
foram Globoconella inflata. Wei (1994, p. 81) suggests that stasis may represent "a
compromise for the species as an attempt to meet with both glacial and interglacial
extremes."
Second, Sheldon's hypothesis predicts a large suite of definite correlations
subject to empirical test. Plus ca change predicts linkages of different relative
frequencies for punctuated equilibrium and gradualism to geographic gradients
(with more punctuated equilibrium expected in temperate areas, and more
gradualism in the topics), environmental distinctions (with more punctuated
equilibrium in near shore shallow-water strata and more gradualism in offshore
regions, as Johnson had earlier predicted), and evolved responses of organisms and
populations (with, ceteris paribus, more punctuated equilibrium in eurytopes and r-
strategists, and more gradualism for stenotopes and K-strategists). Needless to say,
ceteris paribus does not always hold—but with so many expected consequences,
the probability of finding patterns (if they exist) does rise substantially.
Finally, as for any good hypothesis in science, Sheldon's plus ca change
suggests several interesting extensions. For example, Sheldon raises an intriguing
argument for linking these putative correlations with patterns of genuine selection
at the species level or above:
Perhaps the most important (and perhaps the most controversial)
mechanism I am suggesting here is a type of lineage selection with two
stages: