Interpopulation variation in juvenile survival and growth rates of Lymnaea peregra (gastropoda: Pulmonata); temperature at recruitment as a selection pressure?

Three neighbouring populations of Lymnaea peregra had recruitment in the summer (June and July), but one population (Sheaf) had a second recruitment in September and October. We hypothesized that juveniles of the Sheaf population would be subject to selection under both 'summer' and 'winter' conditions, and thus should be more resistant to low-temperature stress than juveniles of the other populations. The hypothesis was supported by the findings that Sheaf juveniles survived and grew better over a wider range of temperatures (2, 10, 15 and 20°C )while juveniles of the other two populations were adapted only to higher temperatures (15 and 20°C). There was evidence that some of these traits were genetically fixed.