860 THE STRUCTURE OF EVOLUTIONARY THEORY
Two additional features enhance the methodological value of this study: first,
these species belong to the best known and most intensely studied of all molluscan
faunas; secondly, all species either still exist (12 of 19 cases) or can be compared
with a close living relative (almost surely the immediate descendant in 4 cases, and
perhaps directly filiated in the other 3). Thus, in the most important innovation of
this study, temporal variation can be directly scaled against current geographic
variation of the same species, or of a close relative. In testing whether temporal
fluctuations exceed the limits of stasis, comparison with the range of geographic
variation among current populations of the same species should serve as our best
anchor and standard.
Using eigenshape analysis for multivariate representation of shell form,
Stanley and Yang first compared variation among modern populations for each
species with differences between these modern populations and early Pliocene
(circa 4 million years old) samples of the same species. In a convincing
demonstration of stasis properly scaled to realized intraspecific variation, Figure 9-
24 (from Stanley and Yang, 1987, p. 124) shows histograms for overlap of
eigenshape areas in comparing modern geographic variation with 4 million year
distances between early Pliocene and modern samples. The temporal mode slightly
exceeds the geographic value, but the ranges overlap completely, and the
difference in central tendency is very small. The authors conclude (p. 113) that
"with minor exceptions, the distribution of morphologic distances between 4
million year old and Recent populations resembled the distribution of distances
between conspecific Recent populations." "Approximate morphological stasis has
been the rule for the taxa considered" (p. 124).
Stanley and Yang then extended their study (for species with available data)
back to Miocene samples up to 17 million years old. Even for this extended
duration, they found the same pattern of mild fluctuation, rarely extending outside
the range of modern geographic variation, and with no