Punctuated Equilibrium and the Validation of Macroevolutionary Theory 821
an overwhelming predominance of speciation by cladogenesis as a cause of
phylogenetic patterning—thus affirming the predictions of punctuated equilibrium.
For globigerinids, the cladistic topology revealed 40 speciation events, 5 probably
anagenetic, 35 cladogenetic, and none bifurcating. Wagner and Erwin did not
present full tabulations for the globorotaliids, but stated (p. 105): "The results are
not presented here, but they were similar to those found for globigerinids:
cladogenesis is significantly more common than anagenesis, a positive association
exists between having long temporal ranges and leaving cladogenetic descendants,
and no such association exists for anagenetic ancestors."
For the lophospirid gastropods, they write (p. 106): "Our preferred cladogram
for lophospirids is rife with polytomies. Of the eleven polytomies, only
9 - 13. To my embarrassment, Wagner and Erwin (1995)—for I had not seen the obvious
implication that would have enormously helped my argument—showed how phylogenies based
upon iteration of several species from an unchanged parent stock (as Raup and Gould, 1974, had
generated, and Wagner and Erwin reproduced, at the top of this figure) must yield, in cladistic
representation, a polytomy. Thus, polytomies may provide evidence for punctuated equilibrium
and do not necessarily represent the "signature" of missing data needed to resolve the system into
dichotomies. If the ancestral form doesn't change throughout its geological range, all descendants
must in principle arise at a polytomous junction of a cladogram.