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 ye ars were examined by using radiocarbon-dated fossil specimens. There a re 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 a n alternative state. Because of the isolation, the limited distributio ns, and the presence of transitional forms, these changes are regarded as genuine evolutionary transformations within a lineage. These findi ngs suggest that phyletic evolution is essentially irregular and punct uated.