Evolutionary patterns of niche acquisition in a montane community (Carabidae: Nebria)

Why do species live where they do? By analyzing the relationship between functional traits and habitat use in the context of evolutionary history, the processes which influence species distributions becomes clearer. The Holarctic ground beetle genus Nebria (Carabidae: Nebriini) has been extensively studied for such patterns. Nebria species are consistently associated with specific habitat types and a significant proportion of variation in habitat use can be explained by morphology. This pattern could be attributed to constraints due to ancestry (parallel evolution) or convergence on specific morphotypes in response to environmental conditions. Nebria species are the dominant arthropod where they are found, and previous work suggests that environmental filtering alone predicts species presence across spatial scales. Using museum specimens from the California Academy of Science and the environmental database Worldclim, the relationship between morphology, habitat use, and ancestry was assessed. Based on linear and shape measurements of elytra and pronota (ecomorphological traits) and antennal scape (scape length likely experiences sexual, but not ecological selection), only ecomorphological traits evolved faster than expected (Pagel’s lambda). Latitude and mean ground temperature were the environmental factors most closely associated with morphology. Based on likelihood ratio tests, the inclusion of ancestry does not improve models of environmental features regressed on morphology. Based on clustering analysis, an “alpine” morphotype has evolved through convergence. Convergence by repeated niche expansion is one mechanism that could generate highly diverse clades. This pattern is found in a number of arthropod groups, especially in insular habitats such as mountains and islands.