Changes in Populations j 133 



Polymorphic Land Snails 



The microevolution of polymorphic land snails of the genus Cepaea 

 has been studied in detail, mostly with the very variable species C. 

 nemoralis. The shell of this snail may be yellow, brown, or any shade 

 from pale fawn through pink and orange to red. The lip of the shell 

 may be black or dark brown (normally) or Dink or white (rarely), 

 and up to five black or dark brown (rarely transparent) longitudinal 

 bands may decorate it. The genetic basis of many of these characters 

 is fairly well understood, and fossil evidence shows that this poly- 

 morphism has existed since before the Neolithic. 



The snails have been studied extensively in Europe, where the fre- 

 quencies of different forms vary greatly from colony to colony. The 

 roles played by selection and drift in accounting for the intercolony 

 differences have been the subject of controversy. Lamotte in France 

 originally claimed that genetic drift accounted for the observed 

 diversity. However, Cain and Sheppard demonstrated that, at least 

 in some English colonies of the snails, selective forces were at work. 

 Near Oxford they found that the frequencies of the different kinds 

 of shells were correlated with the microhabitat of the snails (Fig. 

 7.4). Collections were made in six main types of habitats: downland 

 beech woods, oak woods, mixed deciduous woods, hedgerows, open 

 areas with long coarse herbage, and open areas with very short turf. 

 Analysis of the frequencies of color and pattern types in these sam- 

 ples showed that they were highly correlated with the color and 

 uniformity of the background. For instance, the percentages of 

 effectively unhanded shells in the five localities with the most and 

 the least uniform backgrounds ( uniformity decreases to the right in 

 each series) were as follows: 



The percentages of yellow shells in the five greenest and in the 

 five least green localities (greenness decreases to the right in each 

 series) were: 



This association between the frequencies of shell types and the 

 character of the habitat suggested that visual predators eating the 

 most conspicuous snails might be a selective agency causing differ- 

 ences among colonies. One such predator is the song thrush, Turdus 

 ericetorwn. In the summer of 1951 Cain and Sheppard studied a 

 colony of snails in a small hillside bog in Wytham Woods near 



