Disruptive selection by predation offsets stabilizing selection on shell morphology in the land snail Iberus g. gualtieranus

This work analyses the selection on shell morphology (height and width) in an arid-dwelling land snail with a flattened shell, Iberus g. gualtieranus. The findings show absence of selection, but more detailed analyses, separating the effect of different selective agents, show that there is disruptive selection on shell height caused by predation (by black rats, Rattus rattus). Nevertheless, this disruptive selection was balanced by stabilizing selection caused by other unknown mortality sources with the same strength. The two selective forces acting in opposite directions resulted in an absence of appreciable selection on shell height. This study suggests that it is important to analyse the effect of different sources of selection acting simultaneously on a trait, in order to attain a precise picture of the selective patterns, especially when the whole selection is close to zero, as this does not necessarily imply the absence of selection on the trait.