MEASURING PHENOTYPIC SELECTION ON AN ADAPTATION - LAMELLAE OF DAMSELFLIES EXPERIENCING DRAGONFLY PREDATION

Previous studies suggest that the evolution of increased caudal lamell ae size to increase swimming speed was an adaptation of Enallagma dams elflies for coexisting with large, predatory dragonflies in fishless l akes. To test whether dragonfly predation still exerts selection press ures for increased lamellae size, I performed a field experiment in wh ich I manipulated the abilities of dragonfly larvae to inflict mortali ty on Enallagma boreale larvae and compared differences in lamellae si ze and shape between treatments. In cages where dragonflies were free to forage on damselflies, surviving E. boreale larvae had lamellae tha t were larger in lateral surface area, and that were wider relative to their length, as compared with larvae recovered from treatments in wh ich dragonflies were not permitted to forage on damselflies. Selection differentials of about 0.25 phenotypic standard deviation units were measured for both of these characters. These results indicate that dra gonfly predation still exerts significant selection pressures on damse lfly antipredator adaptations. The results of this study are discussed in the context of studies of adaptation.