776 THE STRUCTURE OF EVOLUTIONARY THEORY
9 - 5. Typical textbook illustration of evolution by continuous anagenetic transformation of an
unbranched population through time. This textbook labels the figure explicitly and exclusively as
its icon of "evolution" itself, not of gradualism or any other subcategory of evolutionary change.
From the standard paleontological textbook of my student generation, Moore, Lalicker, and
Fischer, 1953.with a new Linnaean name (as species B), then where should we place the
breakpoint between A and B? Any boundary must be arbitrary—if only by the
illogic of the unavoidable implication that the last parental generation of species A
could not, in principle, breed with its own immediate offspring in species B. (We
may abhor human incest for social reasons, but we can scarcely deny the biological
possibility—hence the perceived societal need for a taboo.) This problem
generated a large, tedious, and fruitless literature, primarily because the issue
always remained available, unresolved and therefore ripe for yet another go-round
whenever a paleontologist needed to deliver a general address and couldn't think of
anything else to say.
Punctuated equilibrium took a radically different approach by admitting
unresolvability under the stated assumptions, but then denying the focal empirical
premise that new species usually (or even often) arise by gradualistic anagenesis.
Instead, Eldredge and I argued that the vast majority of species originate by
splitting, and that the standard tempo of speciation, when expressed in geological
time, features origin in a geological moment followed by long persistence in stasis.
Thus, the classic and endlessly fretted "species problem in paleontology"
disappears because species act as well-defined Darwinian individuals, not as
arbitrary subdivisions of a continuum. Species then gain definability because they
almost always arise by speciation (that is, by splitting, or geographic isolation of a
daughter population followed by genetic differentiation from the parental
population), not by anagenesis (or transformation of the entire mass of an ancestral
species). To be sure, a new species must pass through a short period of ambiguity
during its initial differentiation from an ancestral population, but, in the proper
scaling of macroevolutionary time, this period passes so quickly (almost always in
the