Genetic Variation in a Pacific Island Land Snail: Population History Versus Current Drift and Selection

Previous studies of Partula land snails from the Society Islands, French Polynesia, have shown that there can be striking differences in shell shape, colour and banding pattern between nearby populations, even in the absence of any obvious geographical barriers to the movement of snails, or environmental gradients. Elsewhere, there may be relative uniformity over large distances. Analysis of a mitochondrial gene from Partula taeniata (Mörch) shows a similar pattern. The relative frequencies of two mitochondrial haplotypes change abruptly over small distances, seemingly independent of the environment. Although the transition roughly coincides with clines in the frequencies of some morphological characteristics, it appears to be unrelated to others. It is likely that many of the differences accumulated while populations were isolated from one another, through the effects of random genetic drift and selection. Isolation of populations may have occurred as a result of demographic changes, or during the process of colonization if occasional long-distance migrants establish populations ahead of the main invading front. Current genetic drift, even without restrictions to gene flow, may contribute to genetic patchiness on a small scale, although it is likely that conspicuous characteristics such as shell colours and banding patterns are also influenced by selection.