Punctuated Equilibrium and the Validation of Macroevolutionary Theory 777
unresolvable geological moment of a single bedding plane), that operational
definability encounters no threat.
Of course, gradualists did not deny that speciation often occurs by branching.
They just didn't grant this process of splitting any formative role in the
accumulation of macroevolutionary change for three reasons. First, they conceived
speciation only as an engine for generating diversity, not as an agent for changing
average form within a clade (that is, for the key macroevolutionary phenomenon of
trends—see quotes of Huxley and Ayala, and Mayr's response, on p. 563). Trends
arose by anagenesis (see Fig. 9 - 6 ), and speciation only served the subsidiary (if
essential) function of iterating a favorable feature, initially evolved by anagenesis,
into more than one taxon—thus providing a hedge against extinction.
Second, they granted little quantitative weight to the role of speciation (splitting as
opposed to anagenesis) in the totality of evolutionary change. In a famous estimate
that became canonical, Simpson (1944) stated that about 10 percent of
evolutionary change occurred by speciation, and 90 percent by anagenesis.
Third, when gradualists portrayed speciation at all (see Fig. 9-7), they
depicted the process as two events of anagenesis proceeding at characteristically
slow rates. Thus, they identified nothing distinctively different about change by
speciation. Some contingency of history, they argued, splits a population into two
separate units, and each proceeds along its ordinary anagenetic way. Punctuated
equilibrium, on the other hand, proposes that the geological tempo of speciation
differs radically from gradualistic anagenesis. (We also argue, of course, that such
anagenesis rarely occurs at all!)
The theory of punctuated equilibrium therefore began as a faithful response to
Schopf's original charge to Eldredge and me: to show how standard
9 - 6. A standard illustration from Simpson (1944), showing that all trends, and all stability for
that matter, originate primarily in the anagenetic mode—that is, by change during the lifetime of
individual species, with branching serving primarily to diversify and iterate the favorable designs
originated by anagenesis, and thus to prevent extinction of the lineage.