770 THE STRUCTURE OF EVOLUTIONARY THEORY
9 - 3A. The dissection of punctuation made possible by unusually high sedimentation rates.
Williamson's analysis of variation and central tendency during punctuation in the B. unicolor
lineage of Pleistocene fresh water pulmonate snails from the African rift valley. Each diagram
shows all the specimens from the entire sequence, with only those specimens for the relevant
interval depicted in black. A. Parental form before the punctuation with multivariate modal
morphology concentrated to the left of the range. B. Expanded variation throughout the range
during the time of the punctuation itself. C. Restricted variation again, but settling down upon
the morphology of a new taxon following the punctuation, as seen in the reduction of variation
with change in modal position towards the right side of the array. From Williamson, 1981.If this kind of unusual circumstance spreads a punctuational event of
speciation through a sufficient stratigraphic interval for resolution, another strategy
of research will sometimes permit the dissection of a punctuation in conventional
cases of full representation on a single bedding plane. Good-friend and Gould
(1996) documented such a case because they could establish