Evolutionary genetics of the land snail Cepaea nemoralis in the Central Pyrenees

The pattern of life is a discontinuous one: both fossils and modem taxa show a clustering of forms separated by gaps. Evolutionary theory explains these gaps by relatively rapid transitions happening long ago, leaving few identifiable intermediates. One mechanism for this 'punctuated equilibrium' process suggests that these dramatic changes take place in small isolated populations at the edges of a species' range. This notion was tested in the land snail Cepaea nemoralis in the French and Spanish Pyrenees. Geographical patterns of genetic differentiation in enzyme polymorphisms ('molecular area effects') suggest that the population was once fractured into smaller units which, after a process of genetic disruption, recolonised their original range. The interface between two of these areas was mapped by an analysis of enzyme polymorphisms. Pure populations and their hybrids were then compared for adaptational differences (shell and body colour, thermal behaviour and mortality rates), and for reproductive isolation (as calculated by estimates of selection against hybrids from the enzyme dine width using field determinations of dispersal rate). No significant adaptive differentiation or reproductive barrier could be established between the two molecular areas. Instead, the same processes of adaptation seem to have taken place within the areas, in response to altitude and insolation. Furthermore, I investigated the possibility of a hybrid advantage effect taking place within the zone of hybridisation, where mean shell size is larger, resulting in more eggs being laid. Hybrid genotypes are thus likely to disperse faster than those of the pure types, dissipating the genetic differentiation between area effects more quickly than by random mixing. Present day selection is unlikely to determine the pattem of molecular area effects, as they do not coincide with geological, climatic, and vegetational discontinuities. Rather, they are relics of history, arising from the expansion of glacial refuges. The dynamics of this process were investigated by tracking vegetation patterns from ancient pollen cores, and by an analysis of mtDNA and enzyme variant distribution. The results of this study counter the expectations of the proposed mechanism of punctuated equilibrium. They are a reminder that, for most of the time, living organisms are robust to dramatic evolutionary change