870 THE STRUCTURE OF EVOLUTIONARY THEORY
differences could be extrapolated from temporal variation within species (see
Cheetham and Jackson, 1995, p. 192, for an elaboration of this argument with
appropriate data).
The corroborative study of a second bryozoan genus, Stylopoma, from the
same beds yields an identical conclusion of overwhelming predominance—
indeed, exclusivity—for punctuated equilibrium. Jackson and Cheetham (1994)
used 12 morphological features to identify 19 species in this rarer genus. Despite
their more limited information, Cheetham and Jackson (1995, p. 195) found that
"temporal overlap between putative ancestor-descendant species pairs is even
greater than for Metrarabdotos, with 10 species surviving beyond the detailed
sampling interval more than 6 million years to the Holocene." Moreover, "no
evidence of morphologically intermediate forms" (Jackson and Cheetham, 1994, p.
420) has been found for any transition; all species origins are fully punctuational at
the scale of detailed sampling.
Finally, since Stylopoma provided Jackson and Cheetham's principal data for
the correspondence of genetically defined biospecies with morphologically
designated paleospecies (modern specimens of Metrarabdotos are much less
common and not so well suited for genetic work), this second study provides
strong additional support for punctuated equilibrium by coordinating several
potentially independent indicators of evolutionary change with rapid events of
branching speciation: "Moreover, the tight correlation between phenetic, cladistic,
and genetic distances among living Stylopoma species suggests that changes in all
three variables occurred together during speciation. All of these observations
support the punctuated equilibrium model of speciation."
I regard these empirical studies of relative frequencies as the strongest
evidence now available for the most important and revisionary claim made by the
theory of punctuated equilibrium: the overwhelming domination of evolutionary
patterns in geological time by events at the species level (or higher), and the
consequent need to explain macroevolution by patterns of sorting among species
rather than by extrapolated trends of anagenetic transformation within continuous
lineages.
Causal clues from differential patterns of relative frequencies
Once we set our focus of inquiry on determining the relative frequencies of
punctuated equilibrium in different times, places, environments, and taxa, we can
ask the classic question of natural history, a subject rooted in the concept of
variation: do we note characteristic differences in relative frequencies based on any
of these factors and, if so, can we draw any causal inferences (useful to
evolutionary theory) from these patterns. I have already raised this question in a
number of preceding contexts in this chapter, most extensively for the observed
higher frequency of gradualism in predominantly asexual oceanic protistan
lineages, where I argued that this unusual result may not record the greater
completeness of strata in oceanic cores (as traditional views have assumed), but
probably arises from interesting biological differences that have led us to look for a
truly underlying punctuational pattern at the wrong scale in this case (see pp. 803-
810).