868 THE STRUCTURE OF EVOLUTIONARY THEORY
ever gathered for the domination of a total evolutionary pattern by punctuated
equilibrium.)
To recapitulate the major conclusions of these studies (see also Figure 9- 18
for the phylogeny of Metrarabdotos, with morphology expressed as multivariate
Euclidian distances between samples based on all canonical scores of a
discriminant analysis, and connecting nearest morphological neighbors in
stratigraphic sequence), Cheetham measured 46 characters in 17 species of
Metrarabdotos over a duration of 15 million years, with intense sampling for a 4.5
million year interval of Upper Miocene to Lower Pliocene sediments (3.5 to 8.0
million years ago) in the Dominican Republic. Cheetham (1986, p. 195) specified
the favorable features of Metrarabdotos on both geographic and phylogenetic
grounds: "The ascophoran genus Metrarabdotos is a favorable subject for detailed
analysis of evolutionary pattern because of its diversity and wide distribution
during much of Miocene and Pliocene time. Caribbean species... form an
apparently monophyletic subset within which phylogenetic relationships can be
inferred independently of evolutionary events in eastern Atlantic-Mediterranean
congeneric species groups."
Moreover, the unusual resolution for the detailed sampling interval permits "a
fine-scale comparison of successive populations similar to those made with
oceanic planktonic groups in deep-sea cores" (1986, p. 195), thus dispelling the
common argument of gradualists that stasis in metazoans from conventional
sediments must arise as an artifact of coarseness of resolution, while
9 - 27. From Geary, 1995. In the radiation of melanopsid gastropods in Middle to Late Miocene
beds (spanning 5-10 million years) of the isolated Pannonian Basin, Geary found one case of
gradualism where, after at least 7 million years of stasis, ancestral M. impressa transformed to
M. fossilis by directional increase in shell size and shouldering over a million year interval.
However, during the same time, at least six new melanopsid species arose by punctuation, as
also shown.