A CONTRIBUTION TO THE STUDY OF MORPHOLOGICAL AND BIOCHEMICAL DIFFERENTIATION IN FRENCH AND IBERIAN POPULATIONS OF CEPAEA-NEMORALIS

Twenty five populations of Cepaea nemoralis living in different geographic zones of the distribution area were studied for both morphological (shell colour and banding) and enzymatic polymorphism (14 loci). Allele frequency, phenotype and genotype distributions were analysed to provide information on population structure and genetic differentiation among populations. Although both series of characters indicated an important variability between the three regions studied (northern, southern French and Iberian areas), hierarchical F-statistic values showed the greatest geographic dependence of variations in shell polychromatism, especially for lip colour and banding models. A large proportion of the protein variability between zones resulted from the existence of alternative allozymes and gradual variations in allele frequency which moreover, were reinforced by the additional ''fast'' electromorphs which appear in Portuguese samples. Several kinds of factors might interfere on differentiation among populations. Whereas patterns of morph distribution reflected direct adapative responses to environmental components (climatic and microclimatic factors), most of the variation at enzyme loci emphasized the action of random processes such as historical and demographic events, strongly implicated in the Pyrenees. However, climatic selective pressures could also explain clinal variations in allele frequency from northern Britain to southern Portugal.