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Community assembly, niche conservatism and adaptive evolution in changing environments.
Ackerly, David*,1, Cornwell, Will1, Schwilk, Dylan1, 1 Stanford Unversity, Stanford, CA
ABSTRACT- The correspondence between phenotypic variation and environmental conditions (the 'fit' of organisms to their environment) reflects the adaptive value of plant functional traits. 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 environmental conditions. The corollary of this process is that traits related to habitat occupancy (e.g., environmental tolerances) are expected to be under stabilizing selection, leading to conservatism of niche parameters and related traits over evolutionary time. Theoretical support for this proposition is provided by habitat selection and community assembly theory. Examples of evolutionary trait conservatism in a phylogenetic, community and biogeographic context are presented, drawing on recent work on woody plants of coastal California. Based on Jackson and Overpeck's (2000) concept of the realized environment, we present 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 migrating in response to 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 multi-dimensional 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 adaptive evolution in heterogeneous and changing environments.
Key words: niche, community assembly, climate change, california flora