Punctuated Equilibrium and the Validation of Macroevolutionary Theory 753
(1968, p. 70) wrote from long practical experience: "During my work as an oil
paleontologist, I had the opportunity to study sections meeting these rigid
requirements [of continuous sedimentation and sufficient span of time]. As an
ardent student of evolution, moreover, I was continually on the watch for evidence
of evolutionary change... The great majority of species do not show any
appreciable evolutionary change at all. These species appear in the section (first
occurrence) without obvious ancestors in underlying beds, are stable once
established, and disappear higher up without leaving any descendants."
Echoing the hopes and disappointments of many paleontologists (including
both Eldredge and me), who trained themselves in statistical methods primarily to
find the "subtle" cases of gradualism that had eluded traditional, subjective
observation, Reyment (1975, p. 665) wrote: "The occurrences of long sequences
within species are common in boreholes and it is possible to exploit the statistical
properties of such sequences in detailed biostratigraphy. It is noteworthy that
gradual, directed transitions from one species to another do not seem to exist in
borehole samples of microorganisms."
Moving to a metazoan group generally regarded as relatively "simple" in
form, and especially prominent in the fossil record, particularly in Paleozoic strata,
Roberts (1981, p. 123) concluded from many years of studying Australian
Carboniferous brachiopods: "There is no evidence of 'gradualistic' evolutionary
processes affecting brachiopod species either within or between zones, and the
succession of faunas can be regarded as 'punctuated.'"
Johnson (1975), inspired by Ziegler's (1966) documentation of one putatively
gradualistic sequence in the brachiopod Eocoelia, decided to search for others—
and found only examples of punctuation and stasis throughout the Paleozoic
record. He wrote (1975, p. 657):
After completion of Ziegler's paper we talked a number of times about the
possibilities of duplicating his efforts with other fossils and in other times.
It was a heady prospect ... In subsequent years many workers have
attempted to seek out and define lineages of brachiopod species and other
megafossils in the lower and middle Paleozoic with little success. My
conclusion, subjective in many ways, is that speciation of brachiopods in
the mid-Paleozoic via a phyletic mode has been rare. Rather, it is probable
that most new brachiopod species of this age originated by allopatric
speciation.Derek Ager, the world leader in studying later Mesozoic brachiopods,
summed up his lifelong effort in several papers towards the end of his career. He
wrote (1973, p. 20): "In twenty years work on the Mesozoic Brachiopods, I have
found plenty of relationships, but few if any evolving lineages ... What it seems to
mean is that evolution did not normally proceed by a process of gradual change of
one species into another over long periods of time." Ten years later (1983, p. 563),
Ager reiterated: "The general picture seems to fit in with the Gouldian doctrine of
'hardly ever' [that is, documentation of gradualism only very rarely]. Certainly
there is no evidence in the group as a whole