CLADISTIC INFERENCE AND EVOLUTIONARY SCENARIOS: LOCOMOTORY STRUCTURE, FUNCTION, AND PERFORMANCE IN WATER STRIDERS.

- A research methodology that aims to reveal how historical changes in environmental conditions (or selective regimes) have shaped the adaptive evolution of clades is applied to the adaptive evolution of water striders and their allies (Hemiptera-Heteroptera, Gerromorpha), a group of semiaquatic insects which includes species that are conspicuously adapted to life on the surface film of water. Based upon reconstructed phylogenies for the higher gerromorphan taxa, the hypothesis that the hygropetric zone is the ancestral one is confirmed for the Mesoveliidae, Hebridae and the clade comprising the Paraphrynoveliidae, Macroveliidae and Hydrometridae, but not for the Hermatobatidae and Veliidae. There is no support for the hypothesis that the intersection zone was a sort of transitional zone during the ecological evolution of pleustonic bugs. It is shown that the unique morphological and behavioural traits of the most derived members of this group evolved after inferred historical changes in environmental conditions and therefore qualify as adaptations in the sense ofGould and Vrba (1982),Coddington (1988) andBaum and Larson (1991). Other predictions about the adaptive evolution of gerromorphan bugs do not pass the cladistic test. The study illustrates that cladistic inference is a valuable tool in clarifying and sharpening retrospective explanations of complex evolutionary scenarios.