Rich:  Two of the concepts you refer to- the founder effect and genetic
drift, may account for  variation patterns in two interesting  Nova
Scotia populations i have observed: The Neptunea l. decemcostata on
Brown's Bank (off SW N.S.),  are far more often a beautiful shade of
violet, than any other population i have seen or heard of
(unfortunately, this pigmentation is nearly impossible to preserve- it
fades in alchohol, and on exposure to light.  Interestingly enough, it
seems to penetrate the entire thickness of the shell, instead of being
surficial).  Also, the frquency of albinism is considerably higher in
this population.  This unusual morphic disribution could have resulted
from the "founder effect": a die-off, following which the survivers
and/or reintroductions contained an unusually high percentage of the
apparently recessive albinistic and violet pigmentation alleles.  I know
of no environmental factors which could accout for these observations,
and am also surprised that such an interesting  occurance is seemingly
absent from the literature (i've looked!!).
        At a locality near Halifax, the Nucella lappilus is riotously variable-
far beyond what could be explained by "frequncy-dependant selection", in
my opinion.  Also, the relative frquencies of the population seem to
shift semi-ramdomly from year to year.              Both could be
exlained,at least partially, by genetic drift, enhanced by a natural
tendancy towards mutation in the genes controlling  pigmentation in the
species. (although the apparent yearly frequency shifts are more
probably the result of very incomplete viewing  of the population- each
time i visit, i probably see only 20-25% of the beasties in the local
gene pool.)
                                                                                -Ross M.
                                                                                Schooner Specimen Shells.