BLANCKENHORN, W.U.; University of Zurich, Zoological Museum: Bergmann’s rule, the converse and countergradients: Clinal body size variation in dung flies and water striders.

At least four different evolutionary patterns or hypotheses (co)exist concerning clinal variation in body size and associated life history traits: (1) Bergmann’s rule (body size increases with latitude, a temperature effect); (2) the converse Bergmann rule (body size decreases with latitude, a season length effect); (3) the beneficial acclimation hypothesis (animals perform maximal at the temperature they are adapted or acclimated to); and (4) countergradient variation (populations of a given species compensate environmental limitations at higher latitudes by evolving faster growth and larger body sizes compared to their low latitude conspecifics). I discuss the differences and commonalities of these patterns, and argue that they are not mutually exclusive because they are driven by different environmental causes and proximate mechanisms. Nevertheless, they can potentially even cancel each other. I shall illustrate this with data for the wide-spread yellow dung fly, Scathophaga stercoraria (Diptera: Scathophagidae) and the water strider Aquarius remigis (Hemiptera: Gerridae).