784 THE STRUCTURE OF EVOLUTIONARY THEORY
holds that evolution must be described as differential success in birth and death of
stable species, but allows that the causality behind reasons for differential success
might emerge from the conventional Darwinian level of struggling organisms
within successful populations—the effect hypothesis of Vrba (see p. 658). In this
version, we need a descriptive, but not a causal, account of macroevolution based
on species as individuals.
However, in the strong version, based on true species selection, the differ-
ential success of species arises from irreducible fitness defined by the interaction
of species-individuals with their environments. Chapter 8 presents an extensive
argument for the efficacy of true species selection at high relative frequency.
Validation of this argument would establish a genuinely causal and irreducible
theory of macroevolution. This difficult issue stands far from resolution, but
represents the most exciting potential for punctuated equilibrium as an impetus in
formulating a revised structure for evolutionary theory.
The Scientific Debate on Punctuated Equilibrium:
Critiques and Responses
CRITIQUES BASED ON THE DEFINABILITY OF
PALEONTOLOG1CAL SPECIESEmpirical affirmation
The issue of whether true biospecies (or entities operationally close enough to
biospecies) can be recognized in fossils has prompted long and intense debate in
paleontology (see Sylvester-Bradley, 1956, and other references previously cited),
and does not represent a new or special difficulty raised by punctuated equilibrium.
But given the reliance of punctuated equilibrium on speciation as the mechanism
behind the pattern, this old problem does legitimately assume a central place in
debates about our theory (as emphasized in all negative commentary, particularly
clearly by Turner, 1986, and in the book-length critiques of Levinton, 1988, and
Hoffman, 1989).
At least we may begin by exposing the canonical issue of the older literature
as a Scheinproblem (literally an "appearance problem" with no real content): the
logical impossibility of defining a species boundary within a gradualistic
continuum (see my previous discussion on p. 775). I think we may now accept that
the punctuational pattern exists at high relative frequency, and that few gradualistic
and anagenetic continua have been documented between fossil species. Turner's
(1986) sharp critique, for example (and I do accept his formulation, though not his
resolution), depicts the chief claims of punctuated equilibrium as a three-pronged
fork. He accepts the first tine—the existence of the punctuational pattern itself—as
sufficiently demonstrated by enough empirical cases in the fossil record. He
regards the third tine—macroevolutionary invocation of the theory to explain
trends by species sorting—as "an important extension of evolutionary theory into a
hitherto little explored territory" (1986, p. 206). But he then rejects the second