THE POPULATION BIOLOGY OF THE WHITE-LIPPED LAND SNAIL, TRIODOPSIS ALBOLABRIS: GENETIC VARIABILITY.

Available evidence suggests that breeding in most terrestrial animal species occurs primarily within restricted, relatively small, local colonies rather than in large, panmictic populations (Diver, 1940; Dowdeswell, Fisher and Ford, 1949; Ehrlich and Raven, 1969; Johnson et al., 1969; Selander, 1970, and many others; also see Brussard and Ehrlich, 1970, for contrasting data). Since at present it seems that the geographical isolation of populations is the most common initial requisite for genetic divergence leading to reproductive isolation and speciation (Mayr, 1963; Dobzhansky, 1970), the study of the structure and isolation of individual populations, and the genetic implications of this structure, are important for understanding evolutionary processes. Our work on the breeding system and population structure of the eastern North American endemic land snail, Triodopsis albolabris, indicates that it is suitable for investigating this issue. Intensive markrecapture studies within populations (McCracken, 1976; McCracken and Brussard, unpubl.), and the extensive collections made in the present study, have confirmed that populations of this animal typically are small and isolated. Field data (McCracken, 1976; McCracken and Brussard, unpubl.) show that breeding within populations is panmictic, and that gene flow between populations is extremely low. Furthermore, populations are subjected to frequent changes in size and probably have high extinction rates. The genetically effective size of an average T. albolabris population is estimated at less than 100 individuals. It is also likely that populations are founded by only one or a few snails, which is facilitated by the ability of T. albolabris to store sperm, and to self-fertilize if never mated (McCracken and Brussard, unpubl.). Given this population structure, the genetic composition of populations may be strongly influenced by chance. In such small populations mutation pressure would be insignificant (Li, 1955) and drift and sampling error would be expected to override all but the strongest selective forces (Selander, 1975). As a result, a great deal of interpopulation heterogeneity should result, even between geographically proximate populations. Evolutionarily, this may be expected to lead to extensive race formation and speciation. Despite these expectations, T. albolabris is reported to have a large geographic range with little phenotypic variation throughout (Pilsbry, 1940; Vagvolgyi, 1968). A map of this range (Fig. 1), adapted from Vagvolgyi's (1968) taxonomic revision, shows that three subspecies are recognized in an area encompassing much of the eastern half of North America. The subspecific distinctions given are based on shell characteristics, and by these standards the present study includes collections of both T. a. albolabris and T. a. major. The third described subspecies, T. a. alleni, was not sampled. It is intriguing that an animal which we have observed to have such small local population size and patchy distribution should show so little morphological differentiation throughout this large area. The purpose of the present study was to investigate genotypic variability within and among populations of this animal over a broad geographical transect.