In fossil material from two welis from Java, one section from East Java and two sections from Madura, specimens belonging to the extinet genusLepidocyclina (Foraminifera) were examined. Attention was paid to the embryonic part of the megalospheric forms exclusively. The size of the protoconch and deuteroconch was measured, but in particular also their spatial relation as determined by the extent to which the latter encloses the former, or the extent to which the dividing-wall between them is curved (“degree of curvature”). This parameter was found to increase persistently from 10% in the Middle Oligocene to 67% in the Middle Miocene. The rate of evolution showed to be inconstant, changing from very slow to very fast to slow again and to fast again. The first period of rapid change coincides with the time (Middle Oligocene) during which the genus migrated from America to Europe and to the Far East and Australia. The second phase of rapid evolution took place from the beginning of the Miocene up to the moment of dying out of the genus. The progressive change of this parameter is considered from the point of view of genetic variation and selection.
KeywordsGenetic Variation Rapid Evolution Rapid Change Spatial Relation Progressive Change
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