998 THE STRUCTURE OF EVOLUTIONARY THEORY
from personal conversation, that she initially felt quite skeptical about the importance
of punctuated equilibrium—so her generous treatment records the judgment of a
critical observer, not a partisan.) I reproduce below most of Curtis and Barnes's
(1985, pp. 556-557) section on "Punctuated Equilibria," the closing topic in their
chapter on "evolution." If these authors could be so fair and accurate, then textbooks
can achieve excellence as a genre, and punctuated equilibrium lies safely within the
domain of the understandable, the informative, and the interesting:
Although the fossil record documents many important stages in evolutionary
history, there are numerous gaps... Many more fossils have, of course, been
discovered in the 100 years since Darwin's death. Nevertheless, fewer
examples of gradual change within forms have been found than might have
been expected. Until recently, the discrepancy between the model of slow
phyletic change and the poor documentation of such change in much of the
fossil record has been ascribed to the imperfection of the fossil record itself.
About a decade ago, two young scientists, Niles Eldredge of the American
Museum of Natural History and Stephen Jay Gould of Harvard University,
ventured the radical proposal that perhaps the fossil record is not so imperfect
after all. Both Eldredge and Gould have backgrounds in geology and
invertebrate paleontology, and both were impressed with the fact that there
was very little evidence of phyletic change in the fossil species they studied.
Typically, a species would appear abruptly in the fossil strata, last 5 million to
10 million years, and disappear, apparently9 - 39. An excellent textbook figure of punctuated equilibrium from Price, 1996. He includes, in
a way that had never occurred to me or Eldredge, both spatial and temporal dimensions to
show how allopatric speciation yields both stasis and punctuation in the fossil record. His own
caption reads: "A general scenario for the punctuated equilibrium concept of evolutionary
change. Visualize a species change pattern that appears when time is measured vertically
through a stratigraphic section of rock, and ponder how many such rock sections would be
needed to reveal at least the distribution of species in the central population."