1318 THE STRUCTURE OF EVOLUTIONARY THEORY
Some of the specifically K-T patterns will strike no one as surprising, given the
context; but the readjustment of diversity under such a pronounced moment of
different rules still imposes a major signal upon the overall pattern of life's history.
For example, land plants reproducing by seeds, rhizomes, or any other mode of
propagation by bodies that can lay dormant under soil, tended to survive at higher
frequency (Wolfe, 1990). And Sheehan and Hansen (1986) found higher death rates
among animals directly linked in their feeding to a supply of living plants, whereas
feeders on dead plant material, scavengers and detritivores tended to fare better.
Jablonski's studies break more general conceptual ground in their broader scope
across several extinction events and in their search for commonalities across mass
dyings rather than reactions to a specific impact scenario of the K-T event. He argues
(in Jablonski and Bottjer, 1983, for example) that species-rich clades tend to increase
in numbers of taxa during background times (largely by species selection within
subclades whose species-individuals generate relatively more daughters, and
therefore already a macroevolutionary claim that cannot be extrapolated from
Darwinian organismal selection). However, these species-rich clades then tend to fail
differentially in mass extinction because the same properties that enhance the
capacity for speciation in background times—stenotopy and limited capacity for
dispersal, for example—make taxa more susceptible to removal in catastrophic
episodes.
In later extensions of the same theme, Jablonski (1986a, 1987) established the
best-documented case of genuine clade selection in evolution (working contrary, or at
least orthogonally, to species selection, and therefore atop the hierarchy of potential
levels at an apex that, at least in my judgment, probably operates only rarely in
nature—see pp. 712-714 for further discussion). He found no consistent relationship
between the properties of species within molluscan clades and the full clade's
propensity for survival through mass extinctions. However, the clade's entire
geographic range (but not the individual ranges of its component species) correlated
strongly and positively with survivorship through mass extinctions.
Jablonski (1996) then documented a further disconnect between
microevolutionary expectations in extrapolation, and macroevolutionary realities. In
his rich database of late Cretaceous mollusks, Jablonski could find no evidence for
the most venerable of all supposed generalities in trending—Cope's rule, or the
tendency of lineages to increase in body size (see pp. 902-905 for further discussion).
Nonetheless, good microevolutionary reasons—and data—can be cited for claiming a
general selective benefit for increased body size that should (ceteris paribus), in a
Darwinian world of extrapolation, yield Cope's rule in macroevolutionary extension.
The reasons for this extrapolationist failure may be formulated at several levels,
including forces operating in background times as well. But one important factor also
intervenes in the different rules of mass extinction, where susceptibility of clades
"does not appear to have been size selective" (Jablonski, 1996, p. 279). Jablonski
concludes, also citing his earlier work on geographic ranges (1996, p. 279):
"Survivorship [in mass extinction] appears to have hinged on other factors