Palaeontological documentation of speciation in Cenozoic molluscs from Turkana Basin

A recently discovered series of mollusc faunas from the late Cenozoic of the eastern Turkana Basin constitutes one of the best documented metazoan fossil sequences. Evolutionary patterns in all lineages conform to the ‘punctuated equilibrium’ model; no ‘gradualistic’ morphological trends occur. These faunas provide the first fine-scaled palaeontological resolution of events during speciation: fundamental phenotypic transformation of both sexual and asexual taxa occurs rapidly, in comparatively large populations, and is accompanied by a significant elevation of phenotypic variance. This increase in variance reflects extreme developmental instability in the transitional populations.