The variation of Cepaea nemoralis in three Pyrenean valleys

Abstract

Samples of Cepaea nemoralis (L) have been made in three Pyrenean valleys to study factors influencing variation in the shell colour and banding polymorphism. Botanical evidence has been used to draw conclusions about the climate and habitat of each site. In the Jueu valley, north of the Pyrenean watershed (sampled from 750 to 1900 m. a.sl.), morph frequencies of C. nemoralis vary in such a way as to suggest the operation of climatic selection. The frequencies of yellow and unbanded shells increase with altitude, and are at or near fixation in all populations subject to sub-alpine climatic conditions (over 1450 m.). Frequencies of white-lipped shells vary with altitude in a more complex way, banded white-lipped shells being frequent at the base of the valley, and unbanded ones at the head ; all white-lipped shells are rare in the intermediate altitudes. Woodland samples in the Jueu have higher frequencies of pink and unbanded shells than those from open habitats at comparable altitudes, suggesting the operation of visual selection for crypsis. In the valley of the Noguera Ribagorzana, south of the watershed (sampled from 750 to 1760 m.), there are also altitudinal and habitat influences on morph frequencies. Pink and banded shells occur at high frequencies only in the montane zone (above 1300 m.) while in the Mediterranean zone (750–900 m.) there are very high frequencies of yellow and effectively unbanded shells. Between these extremes there is variation at the colour locus with habitat, brown shells occurring at high frequencies in the denser and more shaded habitats. The third valley sampled, that of the Noguera de Tort, joins the Noguera Ribagorzana, and contains a similar range of habitats and vegetation zones, but does not show the same pattern of variation in morph frequencies. There is some variation with altitude, but the pattern is not nearly as clear as in the other two valleys. In spite of the evidence that climatic selection is acting on C. nemoralis populations in the Pyrenees, there is little consistency in morph frequencies between populations from the same vegetation zone in different valleys. It is suggested that consideration of variations in climate within zones and of the past history of the populations is necessary to account for the variation observed.