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 the lack of replication across species, which compromises 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 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, with differences in form (males) and intensity (females) between the species. By analyzing selection in each local pond, we found that selection on male body size was shaped by various population characteristics that reflect local mating systems. Furthermore, 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 with macroevolution. Based on this model, we conclude that the persistence of ecological modes of life in pond damselflies explains why varying selection in nature may still result in a stable adaptive zone lasting millions of years.