Punctuated Equilibrium and the Validation of Macroevolutionary Theory 819
9 - 12B. Redrawing and simplification of these data in the excellent paleontological textbook of
Raup and Stanley (1971). The bottom icon, showing an early mode and a right skew, has become
canonical in textbooks. The data are firm and fascinating, but the interpretation has general been
faulty as a result of gradualistic and anagenetic assumptions. Lineages did not stagnate in any
anagenetic sense; rather, species diversity became so dramatically lowered (and has always
stayed so—only three genera of lungfishes remain extant today) that speciational processes have
never again had enough fuel to power further extensive phyletic change.not be affirmed merely by showing that realized evolutionary patterns must record
speciation and cannot be rendered by anagenetic, end-to-end stacking. Even the
most committed anagenetic gradualists never denied the importance and
prevalence of speciation. They hold, rather, that speciation generally occurs in the
gradualistic mode—as two cases of divergence at characteristic rates for
unbranching lineages—and not, as supporters of punctuated equilibrium maintain,
as geologically momentary bursts representing the budding of descendant
populations from unchanged, and usually persisting, ancestral species in stasis.
Thus, the best possible test for punctuated equilibrium must distinguish between
the expectations of bifurcating vs. cladogenetic models of speciation.
I am embarrassed to say that neither I nor my colleagues working on the
validation of punctuated equilibrium ever conceptualized the simple and obvious