Special environmental conditions developed in the Tarfayan Basin of southwestern Morocco during the Cenomanian to Coniacian, presumably in response to eustatic changes of sea-level. The passage of V. rotunda to V. rahhalii took place over approximately 2 × 105 years, the appearance of the new species being heralded by the development of a new morph displaying some of the properties of the descendant species. This morph coexisted with the normal morph of V. rotunda. Using methods of quantitative genetics it is shown that speciation could have occurred either by weak selection or by random genetic drift in a deme of moderate size. The phenotype is regionally stable, hence the observed morphological changes in the lineage are of genetic origin. The morphological trend is regional.
- © 1982 The Micropalaeontological Society