Persistence of the ecological niche in pond damselflies underlies a stable adaptive zone despite varying selection

Following the development of regression-based methods to estimate natural and sexual selection, evolutionary biologists have quantified the strength, mode and direction of selection in natural populations. Although this approach has been successful, its limitations include lack of replication across species, compromising the generality of the inferences beyond microevolutionary time scales. Here, we carried out a comparative study of selection on wing shape and body size across multiple populations of two closely related and ecologically similar pond damselflies: Enallagma cyathigerum and Ischnura elegans (Odonata: Coenagrionidae). We found weak stabilizing selection on wing shape in both sexes, and no evidence that selection on this trait differed between the species. In contrast, selection on body size was curvilinear in males and directional in females, and they differed in form (males) and intensity (females) between these two species. By analyzing selection on the fine-grained spatial scale, we found that selection on male body size was shaped by the local mating system, and the relationship between mating system characteristics and directional selection was remarkably consistent across these species. Finally, we present a graphical model that links contemporary selection and macroevolution. Based on this model, we conclude that the persistence in ecological modes of life in pond damselflies offers a plausible explanation for why varying selection in nature may still result in a stable adaptive zone lasting millions of years.