890 THE STRUCTURE OF EVOLUTIONARY THEORY
with the propensities of species for branching or for resistance to extinction—the
"birth" and "death" processes that regulate sorting at this higher level.
I am not even confident that we should preferentially attribute traits with more
plausible organismic advantages—including the enlarging brain of hominids as an
obvious example—to conventional microevolutionary explanations, without
seriously considering unorthodox possibilities based on causal correlations of such
traits with propensities for speciation, or on the sheer good fortune of nonadaptive
hitchhiking due to fortuitous presence in the subclade growing to domination for
other reasons. We should be paying more attention to interesting and plausible
proposals like Sacher's (1966) on lifespan and developmental timing as the primary
target of selection in hominid evolution, with large brains on a facilitating causal
pathway to advantageous retardations of development (Gould, 1977b).
In any case, and most generally, the need to describe trends—when
punctuated equilibrium dominates the geometry of evolutionary change within a
clade—as differential success of stable species, rather than as extrapolated
anagenesis of populations, requires, in itself and as a "one liner" of extensive
reformatory power, a radical reformulation (in the literal sense of reconstruction
from the very radices, or roots, of the subject on up) of the primary topic in our
macroevolutionary literature. Jackson and Cheetham (1999, p. 76) conclude:
"Granted the prevalence of punctuated equilibria, macroevolutionary trends must
arise through differential rates of origination and extinction, and not by adaptive
evolution within single species. All of this is compatible with traditional
neodarwinian evolutionary biology, but was unexpected before the theory of
punctuated equilibria."
In summary, the efficacy of drifts and drives, in addition to selection, for
generating trends at the hierarchical level of species as Darwinian individuals,
suggests a rich, and virtually unexploited, domain of alternative explanations that
might break through the disabling paradox of our current inability to resolve such a
salient phenomenon in our preferred mode of adaptive advantages to organisms.
Species-level explanations of trends in organismal phenotypes add at least two
categories of potential resolution to our usual search for organismic benefit.
First, the trending character may be causally significant not for its phenotypic
consequences to the organism, but for its role in influencing rates and directions of
speciation in populations of organisms bearing the trait. If fossorial features of
burrowing rodents (Gilinsky, 1986), or nonplanktonic lifestyles of marine
molluscan larvae (Hansen, 1978, 1980), help to generate populational traits that
enhance speciation rates, then a trend spreading such organismic features through a
clade may arise by positive sorting of species rather than by general adaptive
advantage of the phenotype itself. The organismal phenotypes may enjoy no
general advantage at all, and may only produce adaptation to relatively ephemeral
habitats within the clade's potential range. In fact, both fossorial rodent species (for
reasons of small population size) and nonplanktonic molluscan species (for limited
geographic ranges)