796 THE STRUCTURE OF EVOLUTIONARY THEORY
illustrations of punctuated equilibrium. These examples span the gamut of
taxonomies and ecologies, ranging from marine microfossils (Cronin, 1985, on
ostracodes); to "standard" macroscopic marine invertebrates (with Cheetham's
famous studies of bryozoans, 1986 and 1987, as classic and multiply documented
examples), to freshwater invertebrates (Williamson's 1981 work on multiple events
of speciation in African lake mollusks, where ancestral species reinvade upon
coalescence of lakes following periods of isolation that provided conditions for
speciation); to terrestrial vertebrates (Flynn, 1986, on rodents; Prothero and
Shubin, 1989, on horses). I shall discuss this important issue in more detail within
the forthcoming section on evidence for punctuated equilibrium (see pp. 822-874),
but I have been particularly (if parochially) gratified by the increasing application
of punctuated equilibrium to the resolution of hominid phylogeny. The criterion of
ancestral survival has been prominently featured in this literature, as by McHenry
(1994), who notes "ancestral species overlap in time with descendants in most
cases in hominid evolution, which is not what would be expected from gradual
transformations by anagenesis."
In any case, punctuated equilibrium can be adequately and generally
recognized by firm evidence linking observed punctuational patterns to branching
speciation as a cause. The theory of punctuated equilibrium is eminently testable
and has, indeed, passed such trials in cases now so numerous that a high relative
frequency for this important evolutionary phenomenon can no longer be denied
(see Gould and Eldredge, 1993).
CRITIQUES BASED ON DENYING EVENTS OF SPECIATION
AS THE PRIMARY LOCUS OF CHANGE
Once we overcome the problem of definability for species in the fossil record,
punctuated equilibrium still faces a major issue rooted in the crucial subject of
speciation. Punctuated equilibrium affirms, as a primary statement, that ordinary
biological speciation, when properly scaled into geological time, produces the
characteristic punctuational pattern of our fossil record. We must therefore be able
to defend the central implication that morphological change should be
preferentially associated with events of branching speciation. Our critics have
strongly argued that such a proposition cannot be justified by our best
understanding of evolutionary processes and mechanisms.
I believe that our critics have been correct in this argument, and that Eldredge
and I made a major error by advocating, in the original formulation of our theory, a
direct acceleration of evolutionary rate by the processes of speciation. This claim, I
now think, represents one of the two most important errors that we committed in
advocating punctuated equilibrium during the past 25 years. (The other error, as
discussed and corrected on pages 670-673, lay first in our failure to recognize the
phenomenon of species selection as distinct (by hierarchical reasoning) from
classical Darwinian organismic selection, and then (see Gould and Eldredge, 1977)
in our decision to advocate an overly broad and purely descriptive definition rather
than a properly limited meaning based on emergent characters or fitnesses—see
pages 656-670.)