940 THE STRUCTURE OF EVOLUTIONARY THEORY
class of circumstances from those that regulate the ordinary pace of flux and
speciation during the long-term geological history of most clades and
morphotypes. In other words, a timekeeper with a metronome beating at an
appropriate frequency for discerning the units and causes of evolution at each scale
of nature's hierarchy might recognize an episodically (and rarely) pulsating, rather
than an equably flowing, tempo as the dominant signal of change in all realms.
For example, Erdtmann (1986, p. 139) proposed that the active cladogenesis
of early Ordovician planktic graptolites (an extinct subphylum of colonial
organisms close to the chordate lineage) "operated on two levels: gradualistic
change involving species-level and intergeneric clades, and punctualistic
(anagenetic) changes operating on supergeneric levels." He linked the rapid and
extensive morphogenetic innovations of the punctuational mode, involving such
basic features of colonial form as loss of bithecae and reduction in number of
stipes, to major environmental changes marked by rapid eustatic shifts in sea level.
Moreover, these punctuational innovations arise by an astogenetic mode (a term
for the ontogeny of colonies) different from the developmental basis of most
smaller changes that mark the flux of speciation during "normal" geological
intervals. The punctuational innovations that produce new developmental patterns
begin at the proximal end of the colony—that is, they affect the early ontogeny of
the initial organisms of the developing aggregate, thereby pervading the life cycles
of both the organism and the colony, and strongly affecting the global phenotype
of the entire structure. The lower-level changes (smaller variations within existing
developmental themes), on the other hand, tend to begin at the colony's distal
end—that is, they arise late in the ontogeny of older organisms in the colony (for
new organisms of the colony arise proximally, pushing older organisms to
progressively more distal positions), and then proceed to earlier phylogenetic
expression in both the colony's astogeny and the individual organism's ontogenies.
These lower-level changes therefore affect only a small, and astogenetically late,
portion of the colony's form—hence their much more limited capacity for yielding
major morphological change.
This case provides an interesting astogenetic analog to the common claim that
heterochronic changes in the early ontogeny of organisms gain a distinctive status
among evolutionary mechanisms in their potential for rendering substantial
phenotypic change (at a punctuational tempo) with minimal genetic alteration. In
fact, the lability inherent in early ontogenetic changes of rate and regulation
undergirds most theorizing about qualitatively different categories of evolutionary
outcomes based on similar underlying magnitudes of raw genetic alteration—the
most promising basis for a dominant punctuational tempo in the history of
morphological innovation in evolution.
By linking constraints of preferred developmental channels with a
punctuational tempo that precludes accumulative incremental selection as the sole
cause of extensive evolutionary change, this familiar argument unites the two
central themes of this book—hierarchical models of evolutionary mechanics with
structuralist accounts of evolutionary stasis and directionality. Several