832 THE STRUCTURE OF EVOLUTIONARY THEORY
changes, suggesting a pattern predicted by the model of punctuated equilibrium."
Whatley's (1985) study of the common and speciose ostracode genera
Poseidonamicus and Bradleya in Tertiary and Quaternary sediments of the
southwest Pacific also match the expectations of punctuated equilibrium
throughout. Whatley found some gradualism in size changes (a common pattern),
but only stasis for all defining features of shape and ornament. Whatley concludes
for Poseidonamicus (p. 108): "Although over some 55 million years, the ornament
of the genus underwent considerable change, several of its species remained
morphologically very stable over long periods of time: 10 to 15 million years being
not uncommon ... This would seem to be evidence of virtual stasis between
speciation events with respect to the evolution of the ornament of the various
species." In an interesting comment, relating stasis to the major prediction of
punctuated equilibrium for evolutionary trends—the stairstep rather than the ball-
up-the-inclined-plane model—Whatley writes (p. 109):
Although the morphological change from the ornate P. rudis to the smooth
P. nudus [I do love the rhyme as well] took place over a time span of more
than 50 million years and, therefore, from a generic standpoint represents a
very gradual change, it must be emphasized that the individual species
within this evolutionary series are effectively invariable with respect to
their ornament. Morphological change was abrupt and coincided with
speciation and further speciation was required to bring about yet further
ornamental change. Ornamental change is clearly saltatory, very abrupt,
and punctuated.Gingerich's (1974,1976) cases of gradualism in tooth size for several lower
Eocene mammalian lineages in the Big Horn Basin of Wyoming instituted the
empirical debate about punctuated equilibrium (see our response and critique in
Gould and Eldredge, 1977). The tracing of gradualistic sequences for densely
sampled series of small mammals (also based on dental evidence) then became an
important research program for French paleontologists (see Godinot, 1985; and
Chaline and Laurin, 1986, for sources more accessible to anglophonic readers).
Large mammals have also furnished evidence for gradual anagenesis within
species, as in Lister's study (1993a and b) of mammoths and moose—though he
acknowledges that small sample sizes preclude a rigorous distinction of this
interpretation from an alternative reading of several cladogenetic events, each
perhaps punctuated, and all leading in the same direction of change (Lister, 1993a,
p. 77).
But numerous examples of stasis in equally well-sampled strata have also
been documented for mammals (see pp. 854-870 for commentary on relative
frequencies). The rodent sequences that form the empirical basis for most
gradualistic studies of the French school have also yielded several examples of
stasis (Lich, 1990; Flynn, 1986). Summarizing his work on rhizomyid rodents from
the Miocene Siwalik deposits in Pakistan, Flynn (1986, p. 273) wrote: "Most early
rhizomyid species survive on the order of millions of years, with