842 THE STRUCTURE OF EVOLUTIONARY THEORY
in declining to count them as proven support for punctuated equilibrium. Most
paleontologists recognize and follow this recommended practice. For example, to
cite three titles from opposite ends of the conventional taxonomic spectrum,
Sorhannus (1990) could not determine whether the punctuational origin of the
diatom Rhizosolenia praebergonii from ancestral R. bergonii 2.9 million years ago
in the Indian Ocean occurred in situ or by migration from the central Pacific. He
entitled his article: "Punctuational morphological change in a Neogene diatom
lineage: 'local' evolution or migration?" Schankler (1981), as previously reported,
attributed punctuational patterns of Eocene condylarth Phenacodus to probable
migration, and called his paper: "Local extinction and ecological re-entry of early
Eocene mammals." And Flynn (1986) documented an excellent case of ancestral
survival in Miocene rodents from Pakistan (in a group frequently cited for high
relative frequencies of gradualism), but couldn't distinguish evolution in situ from
migration as the cause of observed cladogenesis. He therefore only cited the literal
pattern itself in his title: "Species longevity, stasis, and stairsteps in rhizomyid
rodents."
Among affirmations of punctuated equilibrium by the criterion of ancestral
survival, and ordering my discussion along a conventional taxonomic spectrum
(for no reason beyond antiquated custom), Wei and Kennett's classic study (1988)
illustrates how geographic data can be integrated with vertical sequences to resolve
evolutionary modes not deducible from data of single sections. These authors
showed that the upper Miocene planktonic foram Globorotalia (Globoconella)
conomiozea terminalis evolved gradually into G. (G.) sphericomiozea during a 0.2
million-year interval in central parts of its geographic range.
At the same time, intensification of the Tasman Front (Subtropical
Divergence) separated peripheral populations of the warm subtropics from the
central stock. The isolated population then branched rapidly into a new species, G.
(G.) pliozea, in less than 0.01 million years, or 5 percent of the time taken for
anagenetic transformation of the ancestral stock at the center of its range. The
anagenetic trend proceeded in a direction (loss of keel and development of a more
conical test) opposite to the morphological innovations (flattened test and more
pronounced keel) of the allopatrically speciating peripheral form. The new species,
following its punctuational origin, persisted in stasis for more than a million years.
About halfway through this interval, a descendant of the central stock migrated
into the warm subtropical region of G. (G.) pliozea. The two species then coexisted
for half a million years without apparent intermixing, and with no interruption of
stasis.
The rich data of microfossils from oceanic cores, often providing good
resolution for both geographic and temporal variation, have also documented
punctuational speciation (usually allopatric) with ancestral survival in several other
cases. Cronin (1985) correlated the punctuational origin of six species in the
ostracode Puriana with changes in oceanographic circulation engendered by the
Pliocene rise of the Isthmus of Panama. Cronin comments