Community assembly, niche conservatism, and adaptive evolution in changing environments

The widespread correspondence between phenotypic variation and environmental conditions, the "fit" of organisms to their environment, reflects the adaptive value of plant functional traits. Several processes contribute to these patterns: plasticity, ecological sorting, and adaptive evolution. This article addresses the importance of ecological sorting processes (community assembly, migration, habitat tracking, etc.) as primary causes of functional trait distributions at the local and landscape level. In relatively saturated communities, plants will establish and regenerate in environments to which they are well adapted, so their distributions, and the distributions of associated functional traits, will reflect the distribution of optimal or near-optimal environmental conditions in space and time. The predicted evolutionary corollary of this process is that traits related to habitat occupancy, e.g., environmental tolerances, will be under stabilizing selection. This process contributes to the widely observed pattern of phylogenetic niche conservatism, i.e., ecological and phenotypic similarities of closely related species. Evidence for niche conservatism in plants is reviewed. Based on Jackson and Overpeck's concept of the realized environment, I propose three scenarios in which a species' distributional responses to environmental conditions will lead to a "mismatch" between its environmental tolerances and the environments it occupies, thus creating opportunities for adaptive evolution: (1) the colonization of "environmental islands" (habitats that are discontinuous in niche space) that require large adaptive shifts in tolerance of one or more environmental factors; (2) the persistence of "trailing-edge" populations in species tracking changing climate, if barriers to dispersal of competitors prevent competitive exclusion in the deteriorating conditions; and (3) responses to changes in the realized environment in multidimensional niche space, in which species are predicted to track environmental factors for which they exhibit narrow tolerances and exhibit adaptive evolutionary response along axes where they exhibit greater niche breadth. These three scenarios provide a conceptual framework that emphasizes the role of ecological sorting processes and stabilizing selection as the context for understanding opportunities for adaptive evolution in heterogeneous and changing environments.