Punctuated Equilibrium and the Validation of Macroevolutionary Theory 855
based on previously published studies done for other reasons. Until quite recently
in paleontology, strong and pervasive biases equated evolution with gradual
change, and regarded stasis as "no data," and therefore not worth recording.
Tabulations of older literature will inevitably favor gradualism both because no
other style of evolution attracted study, and (even more problematically) because
paleontologists, expecting only gradualism, tended to misread other patterns in this
conventional light. Proper (and noncircular) testing—as in any statistical study—
requires that the items chosen for sampling display no bias (imposed by human
choice or preference) away from their relative frequencies in nature. When this
ideal cannot be realized in natural experiments, which necessarily lack the rigor of
laboratory controls, we should at least insist that unavoidable biases be directed
against the hypothesis under test.
Thus, one cannot achieve a reliable relative frequency for punctuated
equilibrium by tabulating cases from an existing literature, where strong biases in
favor of gradualism may reasonably be suspected (or, to put the issue more
accurately, virtually guaranteed). May I simply restate Tony Hallam's comment to
me on why evolutionary studies of mollusks in English Liassic beds have
concentrated with near exclusivity on Gryphaea (which, ironically, does not, after
all, display the kind or direction of gradualism that initiated this literature in
Trueman's famous (1922) paper—see Hallam, 1968; Gould, 1972; Jones and
Gould, 1999): "Why hasn't anyone ever examined any of the 100 or so other
molluscan species, many with equally good records, in the same strata?" Hallam
then answered his own rhetorical question: "Because they seem to show stasis, and
were therefore regarded as uninteresting "(see Johnson's (1985) affirmation of this
stasis).
As an example of major differences between adequate and biased modes of
sampling, two contrasting studies were presented at the North American
Paleontological Convention, Boulder, Colorado, 1986. Barnovsky calculated the
relative frequency of punctuated equilibrium vs. anagenetic transformation for
Pleistocene mammals based exclusively on previously published reports in the
literature. The two modes were supported at close to equal frequency. * Prothero
then reported his field study for all mammalian lineages in Oligocene rocks of the
Big Badlands of South Dakota. (See pages 861-865 for a full discussion of
Prothero's refined and extended results—an even more impressive validation of
punctuation equilibrium by well-established relative frequencies.) Nearly all
lineages remained in stasis, and all new forms entered the record with geological
abruptness. Prothero found very few cases of gradual anagenesis. Of course the
differences might be real; perhaps the Pleistocene
- Interestingly, Barnosky's (1987) published version of his oral presentation refined his
conclusion and tabulated a strong majority for punctuated equilibrium, even when compiled
from an existing literature biased by previous traditions for ignoring stasis as non-data, and
favoring apparent cases of gradualism. In his compendium for Quaternary mammals,
Barnosky (1987) found punctuated equilibrium "supported twice as often as phyletic
gradualism... The majority of species considered exhibit most of their morphological
change near a speciation event, and most species seem to be discrete entities."