-- bivalve <[log in to unmask]> wrote:
>Mostly what we need to know is if the evolution of mollusks agrees with the Gradualism Theory stated by Charles Darwin or if it agrees with Punctuated Equilibria stated by Steven Jay Gould or if it depends on what type of mollusk it is.<

Actually, Darwin was not strictly committed to gradualism.

Within mollusks, both gradual and pucntuated patterns are found.  In the original paper on punctuated equilibrium, Gould and Eldridge used Gould's work on land snails from Bermuda as an example of punctuated patterns.  Likewise, Stanley and Yang found several examples of stasis (long intervals of no change, as expected in a punctuated pattern) in bivalves.  I looked at Macrocallista nimbosa and M. maculata from eastern North America and found that M. maculata showed almost no change from the Miocene to the Recent (a punctuated pattern), whereas M. nimbosa was the modern end of a gradual pattern of change in time and space from the lower Pliocene to the Recent.  Jurassic oysters in the genus Gryphaea had been an old classic example of gradual change, but I think some recent studies have raised questions about details of that pattern.  Pliocene scallops in the genus Chesapecten show a punctuated pattern, in which C. jeffersonius remains constant through the lowermost units and then passes through a brief transition to become C. madisonius, which remains constant in form until its extinction.  C. septenarius also split off from jeffersonius and then remained consistent for the rest of its existence.

In general, gradual change suggests a very easy environment, where small changes do not have much impact on survival.  Stasis (no change) suggests a more difficult environment, such as high competition, where changing from what already works is likely to be detrimental.  Punctuated changes suggest a changing environment or a sudden inovation that results in the establishment of a new standard form.

David,
So much seems to depend upon who is setting the limits.  The Fasciolaria species that I have been studying through the Neogene of the Western Atlantic can be shown to exhibit both gradualism and punctuation, depending on how small a time frame you want to look at.  F. rhomboidea has a fairly definite beginning and a definite extinction point.  In between, it is variable, but without a clear pattern of change.  The very similar F. apicina succeeded this species (succession due to the extreme conditions of glaciation.)  It is even more variable, with varieties in many areas and different varieties coexisting.  It also has an apparent clinal relationship with 2 lined forms found most commonly further north, and three lined forms found further south.  After the next glaciation, the two lined forms recolonized southern Florida (again, very variable within and between populations) and may have survived the next glaciation, but only in the offshore Yucatan.  The three lined form did, and is F. lilium of the Yucatan. So there are big jumps caused by extremes in environmental conditions, and little, but generally fairly random changes, except where changing conditions favored one part of the population, but not the other.
The fact that people generally agree that there is a succession of species implies punctualism.  Otherwise there would be one species that simply varied in time, like a clinal relation in space.   But if you insist on long periods of time for stasis, you may not find it always.