North/South differentiation in the distribution of Cepaea nemoralis in Spain

Abstract

Sixteen enzymatic loci of Cepaea nemoralis were studied in an area of 180 km long by 40 km wide in northern Spain. Two new polymorphic loci DIA and GLO, and two monomorphics, ARYL and β-GAL are described.The analysis of principal components groups the populations in two geographical areas, in the north and south of the sampling area. There is also an intermediate area between them. Heterozygosity in the northern area is significantly higher than in the southern one. Heterozygosity seems to diminish in unfavourable conditions (greater dryness or greater altitude), which is in accordance with the niche width variation hypothesis. There seems to exist an association between alleles of GLO and PGM and altitude.The geographical areas created by the biochemical polymorphism coincide with those of the visible polymorphism, which in turn correspond to climatic zones. The northern area, according to visible and biochemical polymorphism is located in the so-called “Green Iberia”, and the southern area in “Brown Iberia”. The intermediate area corresponds to the area of climatic transition between them. These areas could correspond to “geographical races” of Cepaea nemoralis.Climatic selection may be acting upon some loci such as PGM and GLO while in others random drift could be the cause of heterogeneity.