THE EVOLUTION OF WING POLYMORPHISM IN WATER STRIDERS (GERRIDAE) - A PHYLOGENETIC APPROACH

Water striders (Gerridae) exhibit wing length polymorphism that affects their flight ability. Gerrid populations may be either long-winged, wing dimorphic (permanent or seasonal), or short-winged. In bivoltine species, the wing morphism of the over-wintering (diapause) generation may differ from that of the summer (non-diapause) generation. Patterns of dispersal polymorphism, beside having adaptive value in an ecological context. also have unique evolutionary histories. Here, patterns of wing polymorphism in temperate gerrid species belonging to the genera Aquarius, Gerris, and Limnoporus are superimposed upon reconstructed phylogenies (cladograms) for these genera. For each of three characters describing wing morphism states, the most parsimonious sequence of changes was determined by character optimization. The results indicate that wing dimorphism and/or short-wingedness are the ancestral states in two of three genera, and that the short-winged morph has been lost to leave monomorphic long-wingedness in several species, at least in the diapause generation. It is further indicated that there are two different kinds of short-winged gerrids, probably as the result of different mechanisms of morph determination. I finally discuss how the phylogenetic approach can be extended to encompass other aspects of ecology and adaptation, such as habitat preferences and various life history traits.