Factors affecting gene-frequencies in some populations of Cepaea

This thesis describes a number of studies which are concerned with the genetics of natural populations of the polymorphic land snails of the genus Cepaea Held. Chapter 1 reports the results of breeding experiments with Cepaea hortensis (Müll.) and Cepaea nemoralis (L.). The mode of inheritance of some of the principal characters has been determined. In C. hortensis the dominance of pink to yellow shell colour is confirmed, and dark yellow is shown to be dominant to light yellow. The dominance of unbanded to banded is confirmed. This locus is probably linked to shell colour. Full pigmentation, partial pigmentation and absence of pigmentation of the bands form an allelic series with a dominance hierarchy increasing in that order. Fusion of bands is probably multifactorially controlled. Brown shell colour is associated with infertility. In C. nemoralis it is shown that the gene for absence of pigmentation of the bands may prevent the expression of genes controlling colour of the shell. Reddish body colour is dominant to gray. Chapter 2 describes a survey of population of C. nemoralis on sand dunes at Berrow in Somerset. The distribution of phenotypes at the present day is compared with that found in 1926 by Captain C. Diver and Professor A.E. Boycott. Clines in phenotype-frequency have persisted from 1926 to the present, but there have been consistent and significant changes in the proportions of two of the principal forms. The proportion of dark browns has nearly halved, and the proportion of 00300 has doubled since 1926. The changes are interpreted as the results of natural selection. In Chapter 3 the data for the frequencies of dark brown and 00300 in the Berrow collections have been used to calculate the selective value of these forms between 1926 and 1959-60. Assuming a generation time of three years, the dark brown phenotype has been at an overall disadvantage of 6.2% per generation, while the 00300 phenotype has enjoyed an overall selective advantage of 5.2%. Heterogeneity among the estimates of the selection coefficient of 00300 suggests that there are differences in the selective value of this phenotype in different parts of the area studied. In Chapter 4 the influence of the mating system on the effective population size of C. nemoralis is discussed. C. nemoralis is hermaphrodite with obligatory cross-fertilization. Sperm from several matings may be stored for more than a year. Breeding experiments show that the mean number of matings contributing to each brood is approximately two. Multiple mating and sperm storage serve to minimize the effects of population fluctuations and of increased variance of brood-size, which might otherwise reduce the effective size of populations. Chapter 5 is an analysis of the structure of a population of C. nemoralis at Wittenham Clumps, Berks. The standard deviation of dispersal per generation is estimated as 15 metres, and the area of the "neighbourhood" as 1180 sq. metres. Population density declined during the three years' study from about 2 per sq. metre to 0.25 per sq. metre. Predation in the colony is at a maximum in May and June. Differential selection by visual predators has been demonstrated. Thrushes tend to select more of the pink and brown snails; mammals select more yellow snails. Chapter 6 describes the populations of C. nemoralis inhabiting the Isles of Scilly. Although the populations of different islands are geographically isolated, they are, with few exceptions, phenotypically homogeneous. In comparison with populations from the Oxford region they appear to fall largely within a single habitat group. Strong evidence of natural selection in at least one population is provided by continuing disturbances of the Hardy-Weinberg equilibrium.