Two settlements of the periwinkle Littorina sitkana from the Vostok Gu
lf of the Sea of Japan were compared for their genotypic composition a
t five polymorphic enzyme loci: esterase D, glucosephosphate isomerase
, inorganic pyrophosphatase, and nonspecific esterase (two loci). The
settlements were located four km apart on the opposite coasts of a sma
ll gulf. As Littorina sitkana is a sedentary animal without a pelagic
developmental stage, this distance prevents genetic exchange between t
he settlements, which can thus be regarded as separate demes. In each
settlement, snails of two different age classes (juveniles and older a
nimals) and those differing in shell morphology (with and without shel
l sculpturing) were compared. Genetic differences were found between t
he settlements at all five loci and between the age groups in each set
tlement at three loci. The age changes were shown to have the same tre
nd for both settlements and both groups differing in shell sculpturing
. The latter groups did not exhibit genetic differences within a settl
ement. The greatest difference between settlements was demonstrated at
the loci that displayed age differences. The most plausible explanati
on for the observed intrademe age differences is natural selection. Th
e interdeme genetic variation appears to result from the combined acti
on of selection and genetic drift.