Punctuated Equilibrium and the Validation of Macroevolutionary Theory 859
distribution throughout the full vertical range of species—compared with sharp
divisions between species—illustrates the strength and character of stasis in these
well-known fossil lineages.
Perhaps the most impressive and definitive study of pervasive stasis in
molluscan faunas has been presented by Stanley and Yang (1987) for Neogene
bivalves from the Western Atlantic region. They studied 24 variables (normalized
for shell size) in 19 lineages, for a total of more than 43,000 measurements.
Stanley and Yang followed a comprehensive sampling method, unbiased with
respect to likelihood of punctuation and stasis, and including all species within four
bivalve taxa (Lucinidae, Tellinacea, Veneridae and Arcticacea) with shells
sufficiently large and geometrically tractable (flat to only weakly convex) for their
measurement protocol, and with adequate numbers of well-preserved specimens
(almost always more than 20 per sample, with a minimum of 16) over a sufficient
range of time (at least 4 million years from early Pliocene to Recent).
9 - 23. Multivariate changes based on discriminant analysis of 10 characters throughout 14 to 20
temporal units in the evolution of seven molluscan genera in Miocene strata of Maryland. From
Kelly, 1984. Stasis prevails within a large majority of species. For most lineages where a
descendant replaces an ancestor, stair-step punctuation characterizes the transition. In a
particularly interesting case of three successional species, ancestral Anadara does seem to move
anagenetically towards the morphology of its descendant, which then remains quite stable. But
the third and uppermost species, arising punctuationally, returns virtually to the morphology of
the initial form.