Tiers of Time and Trials of Extrapolationism 1319
such as broad geographic range, which shows little or no correlation with body size in
marine invertebrates. The analyses presented here reinforce the view that
macroevolutionary patterns need not be simple extensions of those seen at the level of
individual organisms over microevolutionary time."
I would also make the further observation that these discordances between
background and mass rules (the "alternation" of regimes in Jablonski's terminology)
must play a major role in the most general patterning of life's history, absent which
the tree of earthly life would have grown in a markedly different shape. (In this most
crucial sense, of course, I reassert the primary theme of this book that
macroevolutionary theory matters profoundly.) For if different rules did not impose
their signals at levels above microevolutionary extrapolation, the powerful themes of
Darwin's world would push through to completion in life's phylogeny. For example,
the species-rich clades of background times, with their dual advantages in organismic
and species-level selection, would eventually eliminate the species-poor clades
entirely if a still higher-level component of advantage for at least some species-poor
clades did not "kick in" during episodes of different rules in mass extinction. And if
the general, albeit slight, statistical edge of larger body size scaled straight up from
local populations to the global biota at geological scales, what would guarantee a
world enriched with all the little shrews and hummingbirds of our delight, not to
mention the continued existence of short people, including the author of this book.
These generalities enter the corpus of macroevolutionary theory, but the
different rules of mass extinction must still work through the specificities of various
causes that provoke the rare, but potent, catastrophes of planetary history (with
impact at the K-T boundary as the only firmly established case so far). Thus, some of
the most interesting, if hypothetical, invocations have been proposed as explanations
for specific and otherwise puzzling results of the K-T event. I have long been
intrigued, for example, by the striking pattern of differential extinction in the oceanic
plankton—with 73 percent of coccolithophorid genera, 85 percent of radiolarians,
and 92 percent of forams failing to survive, while diatoms suffered only a 23 percent
loss of genera.
Kitchell, Clark and Gombos (1986) made the interesting argument, later
supported by direct data of Griffis and Chapman (1988) on survivorship of
phytoplankton in conditions of prolonged darkness, that the differential success of
diatoms probably bears no relationship to any notion of cosmic "betterness" or
general "progress," but may only record the fortuity in exaptive use (under the
different rules of K-T darkness) of adaptations evolved for ordinary, short-term
microevolutionary advantages in background times (and not, obviously, in
anticipation of any additional edge in forthcoming catastrophes!) Kitchell et al.
(1986) argue that most diatom species have evolved mechanisms of dormancy
(formation of resting spores, for example), permitting these photosynthetic organisms
to survive extended periods of darkness, including several polar months per year for
species living at high latitudes. Moreover, since diatoms build their skeletons of
silica, which they can extract most readily in oceanic zones of, upwelling that can be
uncertain in placement