844 THE STRUCTURE OF EVOLUTIONARY THEORY
Caribbean range, during a short interval (0.6 to 2.5 percent) of ancestral persistence
in stasis. Following the descendant's origin, ancestors continued to thrive in central
areas of the range.
In a common pattern found in many taxa, punctuated equilibrium can be
confirmed, even in local sections, and even when ancestors do not occur in the
same strata as their descendants. Frequently, a population from an ancestral species
of known and widespread geographic range branches punctuationally to a
descendant that maintains exclusive occupancy of the range for a time, but then
becomes extinct. The ancestor subsequently reinvades the range, thus establishing
earlier coexistence during the descendant's geological tenure. For example,
Bergstrom and Levi-Setti (1978) documented the threefold reappearance of the
Middle Cambrian trilobite Paradoxides davidis
9 - 18. The best-documented, indeed already canonical, example of punctuated equilibrium as an
invariant pattern for an entire clade across its full geographic range—the research of Cheetham
on Tertiary and Quaternary Caribbean species of Metrarabdotos. Each point depicts a
multivariate centroid based on all characters, not just a single feature. All species express stasis,
several for extended periods and a large number of samples. Ancestors persist after the origin of
descendants in 7 of 9 cases where Cheetham felt confident enough to assert a claim for direct
filiation.