A 40,000-year record of discontinuous evolution of island snails

The rate and pattern of phenotypic evolution of the endemic land-snail Mandarina chichijimana in the oceanic Bonin Islands through 40,000 years were examined by using radiocarbondated fossil specimens. There are drastic variations in rates and patterns of changes detected. Some characters show irregular fluctuation within a restricted range. Other characters show a rapid shift from stasis in one state to stasis in an alternative state. Because of the isolation, the limited distributions, and the presence of transitional forms, these changes are regarded as genuine evolutionary transformations within a lineage. These findings suggest that phyletic evolution is essentially irregular and punctuated.