The Structure of Evolutionary Theory

822 THE STRUCTURE OF EVOLUTIONARY THEORY

two do not include plesiomorphic species. Thus, nine may represent hard
polytomies." Of 42 implied speciation events, a maximum of six may have been
anagenetic, while only one may represent a bifurcation. Again, cladogenetic
speciation, the expectation of punctuated equilibrium, dominates the phylogenetic
pattern.
Wagner and Erwin's overall conclusion accords fully with patterns expected
in phylogenies built primarily—one might say overwhelmingly—by punctuated
equilibrium (Wagner and Erwin, p. 110):

• Cladogenesis is significantly more common than anagenesis.


• Species with longer temporal and geographic ranges are more likely to
  leave descendants via cladogenesis or the factors contributing to wider
  temporal and geographic ranges also contribute to the likelihood of
  cladogenetic evolution.


• If anagenesis occurs, it only applies to species with restricted temporal
  and geographic ranges.


• Bifurcation accounts for a negligible amount of speciation.

We cannot often obtain well-resolved species level phylogenies from
paleontological data, and inferences from higher taxa will probably remain too
murky and insecure to permit general use of such models for testing hypotheses
explicitly based on the evolutionary behavior of species. Still, other data sets do
exist in fair absolute abundance (while representing a low percentage of the total
number of potential lineages in life's history). Studies like Wagner and Erwin's can
be replicated and extended for many taxa—and such a strategy can provide
powerful tests for the relative importance of punctuated equilibrium in the history
of life and the generation of phylogenetic patterns. The first tests have been highly
favorable, but we have scarcely any idea what an extended effort might teach us
about the basic modalities of macroevolution.

Sources of Data for Testing Punctuated Equilibrium

PREAMBLE

Punctuated equilibrium has generated a fruitful and far ranging, if sometimes
acrimonious, debate within evolutionary theory (see appendix to this chapter).
While we feel much pride (mixed with occasional frustration) for the role that
punctuated equilibrium has played in instigating such extensive rethinking about
the definitions and causes of macroevolution, we take even more pleasure in the
volume of empirical study provoked by the theory of punctuated equilibrium, and
pursued by paleontologists throughout the world. These carefully documented case
studies (both pro and con) build a framework of proof for the value of punctuated
equilibrium, as illustrated by the most important of all scientific criteria—
operational utility. Such cases have been featured in numerous symposia and books
dedicated to the empirical