938 THE STRUCTURE OF EVOLUTIONARY THEORY
time. As a rationale for their study, the authors state an analogy to the lower level
phenomenon of punctuated equilibrium: "Just as the study of stasis within species
has facilitated understanding of morphologic changes associated with speciation,
we show that study of nonprogressive evolution offers valuable insight into how
the causes of trends interact and thereby produce complex evolutionary patterns
within clades, regardless of their overall direction."
The central theme of non-trending, identified by Budd and Coates for this
large clade of massive, reef-building corals, stands as an empirical pattern in any
case, but the (admittedly somewhat speculative) explanation proposed by the
authors also builds an interesting framework for regarding such a signal as
predictable and unsurprising, rather than anomalous. Their proposal also integrates
the two principal Darwinian critiques of this book by attributing a causal pattern
generated at the species level (the hierarchical expansion of my first theme) to the
effects of architectural or developmental constraint (the structuralist or internalist
perspective of my second theme) in channeling the possibilities and directions of
natural selection.
Budd and Coates (1992) propose that monstastraeid species vary within a
range set by minimal and maximal size of individual corallites on these large
colonies. Such a notion does not debar classical trending, for the clade could
originate in one small portion of the permitted range, and then strongly trend
towards the other domain. But Budd and Coates argue that Cretaceous
montestraeids already inhabited the full range, and that each end represented an
adaptive configuration continually available and exploited throughout the clade's
duration. Therefore, phenotypic evolution fluctuated between the two realized
potentials of a fully populated domain of workable solutions.
The authors argue that "large-corallite" species (3.5 to 8.0 mm in diameter)
maximize efficiency in removal of sediment, and tend to dominate in turbid waters;
while "small-corallite" species (2.0 to 3.5 mm in diameter) prevail in clearer waters
of the reef crest. Moreover, large-corallite species derive most nutrition by direct
carnivory, whereas small-corallite species tend to feed upon their own symbiotic
zooxanthellae. Budd and Coates then advance the claim—the more speculative
aspect of their scenario—that montastraeid species remain constrained with this
range by limitations at either end: an inability of still smaller corallites to develop
and function adequately, and a restriction in septal number and strength that would
not grant sufficient bio-mechanical support to still larger corallites.
These two arguments may validate and explain the basis of active non-
trending in a persistently vigorous and successful clade. For if such constraints
limit the range of corallite size, and if each end enjoys advantages in different
environments continuously available in major parts of the habitat, then evolution
might oscillate back and forth, with no persistent directional component,
throughout cladal history.
Budd and Coates document such a directionless oscillation within the clade's
developmental and adaptive boundaries during four successive divisions of
Cretaceous time. The transition from interval 1 to interval 2 featured