730 THE STRUCTURE OF EVOLUTIONARY THEORY
any general or global advantage for nonplanktotrophic organisms. (In fact,
planktotrophic species might hold a small advantage in species selection for
longevity, and the trend to nonplanktotrophy might still arise by directional
speciation under this potent constraint.)
I strongly suspect that trends driven by structural constraints within large
systems, and not by adaptational advantages to organisms, pervade evolution, but
have been missed because we focus on means or extremes in a distribution and not
on the full range of variation as a more telling "reality" (see Gould, 1996a, for an
entire book on this subject, written for popular readers; and Gould, 1988b, for a
technical account). The vaunted trend to increasing complexity in the history of
life, for example, only records the small and extending tail of an increasingly right-
skewed distribution through time—but with a strong and persistent bacterial mode
that has never altered during life's entire 3.5 billion year history, leaving this planet
now, as always, in the Age of Bacteria (see pp. 897-901 for a further development
of this example). This extending right tail may record little more than the
constraint of life's origin right next to the lower bound of preservable complexity
in the fossil record. Only one direction—towards greater complexity—remained
open to "invasion," and a small number of species dribble in that direction through
time, thus extending the right tail of the skewed distribution. * But no evidence
now exists to support an argument that higher complexity should be construed as a
"good thing in general" (in adaptive terms, or otherwise), either at the organismal
or species level. In fact, the few studies based on patterns of speciation in clades
where founding members lie far from any upper or lower structural boundary, and
therefore impose no constraint upon either decreasing or increasing complexity,
show no trend at all towards increasing complexity. Approximately equal numbers
of species arise with less complex and with more complex phenotypes than their
ancestor (see McShea, 1993,
- Examples of this sort illustrate the important point that drives of directional
speciation do not necessarily require a differential number of speciation events along the
route of the trend. A directional bias may also arise if numbers of speciation events occur
with equal frequency in either direction, but the average phenotypic magnitude of the
trending half exceeds the amount of change in the half oriented away from the trend. Such
cases may be common when a founding lineage lies near a boundary, and amounts of change
become severely constrained in one direction. Thus, for the bacterial mode of life, for
example, we may easily imagine (data for an adequate test do not exist, so far as I know) that
as many speciation events yield a less complex as a more complex descendant. But so little
room exists between the mode and the lower limit that changes to reduced complexity cannot
depart far from the ancestral state, while an open range to the right of the mode permits a far
greater magnitude of change in the direction of greater complexity. For an actual example,
Wagner (1996) documented a general trend to increasing spire height in Paleozoic
gastropods, but found an equal frequency of speciation events towards lower-spired and
higher-spired daughters. The trend, however, records a bias in amounts of change. For some
reason, gastropods that become high spired also experience a marked reduction in the
amount of change per speciation event, even though they continue to produce equal numbers
of daughters in both directions—whereas low-spired ancestors generate much higher average
change per speciation event. The mean spire height of the entire clade therefore increases.