Punctuated Equilibrium and the Validation of Macroevolutionary Theory 837
The mean values of Figure 9-16 record a conventional gradualistic sequence,
but the greater detail of Figure 9-17, illustrating the morphology of all specimens,
not only the means for each level, reveals fascinating details that suggest novel
interpretations. In short, the range of variation, after remaining stable during the
preceding period of morphological stasis, increased rapidly during the half million
year interval from 68.5 to 68.0 million years ago. The subsequent gradual trend
then developed within the envelope of this expanded range—a spread in variation
that had already reached its full extent at the onset of the gradualistic interval. In
other words, variation increased rapidly, and the gradual trend then unfurled into
the enlarged morphospace of this new range. In fact, as Kucera and Malmgren
point out, the upper endpoint of variation never expands after the initial surge, and
the trend in mean values records a loss of variation by removal of flattened shells
at the lower end.
I do not mention these details as a punctuational partisan trying to downgrade
this example of gradualism, or to reinterpret the trend as a "mere" consequence of a
punctuationally expanded range of variation. The gradual trend is both genuine and
well documented—but the mapping of variation into its space gives us new insight
into potential mechanisms of gradualism (while also imparting an important lesson
about the significance of variation, the perils of not recording such data, and the
potential for misreading patterns in expansion and contraction of variation as
conventionally directed trends in mean values—see Gould, 1996a). The gradual
trend to greater conicity in Contusotruncana probably warrants a conventional
selectionist explanation,